Java

多线程增加效率 并行, 就是多个任务同时进行, 多核cpu同时执行 并发, 多个任务轮流进行, cpu一个一个的执行 证明 JVM 是多线程的 垃圾回收和main线程间隔执行 public class Demo1_Thread { public static void main(String[] args) { for(int i=0; i < 10000000 ;i++){ new Demo(); } for(int i=0; i < 10000 ; i++){ System.out.println("主线程"); } } } class Demo{ @Override protected void finalize() throws Throwable { System.out.println("垃圾被回收了"); } } 继承Thread 开启一个线程 实际run() 方法 start() 开启线程 public class Demo2_ExtendsThread { public static void main(String[] args) { MyThread mt = new MyThread(); // mt.run(); mt.start(); for (int i=0; i <10000 ; i++){ System.out.println(i+"bbbbbbbbbbbbb"); } } } class MyThread extends Thread{ @Override public void run() { for(int i=0; i < 10000 ;i++){ System.out.println(i+"aaaaaaaaaaaaaaa"); } } } 实际 Runnable 接口的类 实际Runnable类, 类作为参数传递给线程, start() 开启线程 public class Demo3_Runnable { public static void main(String[] args) { MyRunnable d = new MyRunnable(); Thread t = new Thread(d); t.start(); for(int i=0; i < 10000 ;i++){ System.out.println(i+"bbbbbbbbbbbbbbb"); } } } class MyRunnable implements Runnable{ @Override public void run() { for(int i=0; i < 10000 ;i++){ System.out.println(i+"aaa"); } } } 多线程的2种区别 继承 Thread: 由于子类重写了Thread类的run()方法, 当调用start()时, 直接找子类的run()方法 实际 Runnable: 构造函数中传入了Runnable的引用, 成员变量记住了它, start()调用run() 方法时内部判断成员变量Runnable的引用是否为空, 不为空看的Runnable的run(), 运行时是子类的run()方法. 继承Thread实现简单, 方便使用, 有父类则不能使用这种方法 有父类时可以实际Runnable 接口实现多线程, 对继承Thread的补充, 使用时必须获取线程对象, 使用复杂 匿名内类实现多线程的方法 public class Demo4_NoNameThread { public static void main(String[] args) { new Thread(){ @Override public void run() { for(int i=0; i<100000 ;i++){ System.out.println("aaaaaa"); } } }.start(); Thread t = new Thread(new Runnable() { @Override public void run() { for(int i=0;i<10000;i++){ System.out.println("bbbbbbbbbbbbbbbbbbb"); } } }); t.start(); } } 匿名内部类设置线程名字 public class Demo5_ThreadName { public static void main(String[] args) { // demo1(); Thread t = new Thread(){ @Override public void run() { // this.setName("小明"); System.out.println(this.getName()+"aaaaa"); } }; t.setName("李明"); t.start(); new Thread(){ @Override public void run() { this.setName("小明"); System.out.println(this.getName()+"aaaaa"); } }.start(); } private static void demo1() { new Thread("小明"){ @Override public void run() { System.out.println(this.getName()+ "aaaaa"); } }.start(); new Thread("小红"){ @Override public void run() { System.out.println(this.getName()+ "bbbbbb"); } }.start(); } } 也可调用setName更改名称 获取本类的引用Thread.currentThread() ...

递归练习, 计算文件夹大小 import java.io.File; import java.util.Scanner; public class Demo1_FolderSize { public static void main(String[] args) { Scanner sc = new Scanner(System.in); String strFolder = null; long size = 0; while (true){ if( sc.hasNext()){ strFolder = sc.nextLine(); break; } } if( strFolder.equals(null)){ System.out.println("不能为空"); } else { File folder = new File(strFolder); if(folder.isFile()){ size = folder.length(); } else if(folder.isDirectory()){ size = getSize(folder); } else if(! folder.exists()){ System.out.println("文件夹不存在"); } } System.out.println(size); } public static long getSize(File folder){ long len = 0; File [] files = folder.listFiles(); for(File subFile:files){ if(subFile.isFile()){ len = len + subFile.length(); } else{ len = len + getSize(subFile); } } return len; } } 递归删除文件夹 删除文件内的文件 删除文件夹本身 import java.io.File; import java.util.Scanner; public class Demo2_DeleteFolder { public static void main(String[] args) { Scanner sc = new Scanner(System.in); String strFolder = null; strFolder = getDir(sc); if( strFolder.equals(null)){ System.out.println("不能为空"); } else { File folder = new File(strFolder); if(folder.isFile()){ folder.delete(); } else if(folder.isDirectory()){ deleteFolder(folder); } else if(! folder.exists()){ System.out.println("文件夹不存在"); } } } private static void deleteFolder(File folder) { File[] files = folder.listFiles(); for (File f : files){ if(f.isFile()){ f.delete(); } else if(f.isDirectory()){ deleteFolder(f); f.delete(); } } folder.delete(); } public static String getDir(Scanner sc) { String strFolder; while (true){ if( sc.hasNext()){ strFolder = sc.nextLine(); break; } } return strFolder; } } 复制文件到指定文件夹下 源文件夹, 目标文件夹 在目标文件夹中创建源文件夹 遍历源文件夹, 文件则调用stream 复制到目标文件夹/源文件夹, 文件夹则递归调用 import java.io.*; public class Demo3_MvFolder { public static void main(String[] args) throws IOException { File folderSrc = new File("test"); File folderDes = new File("des"); copyFile3(folderSrc, folderDes); } public static void copyFile3(File folderSrc, File folderDes) throws IOException { if(! folderDes.isDirectory()){ System.out.println("请复制到文件夹中"); return; } File newFolder = new File(folderDes, folderSrc.getName()); newFolder.mkdir(); for(File f : folderSrc.listFiles()){ if( f.isFile()){ fileStream(f.getAbsolutePath(), newFolder.getAbsolutePath()+"/"+f.getName()); } else{ copyFile3(f, newFolder); } } } public static void copyFile2(File folderSrc, File folderDes) throws IOException{ if(! folderDes.isDirectory()){ System.out.println("请复制到文件夹中"); return; } File tempFolder = new File(folderDes.getAbsolutePath()+"/"+folderSrc.getName()); if( folderSrc.isDirectory()){ tempFolder.mkdir(); } for( File f : folderSrc.listFiles()){ if( f.isFile()){ fileStream(f.getAbsolutePath(), tempFolder.getAbsolutePath()+"/"+f.getName()); } else { copyFile2(f, tempFolder); } } } public static void copyFile(File folderSrc, File folderDes) throws IOException { if(! folderDes.isDirectory()){ System.out.println("请复制到文件夹中"); return; } if( folderSrc.isDirectory()){ new File(folderDes.getAbsolutePath()+"/"+folderSrc).mkdir(); } for(File f: folderSrc.listFiles()){ if ( f.isFile()){ fileStream(f.getAbsolutePath(), (folderDes.getAbsolutePath()+"/"+f.getPath())); } else if( f.isDirectory()){ copyFile(f, folderDes); } } } public static void fileStream(String src, String desc) throws IOException{ FileInputStream fis = new FileInputStream(src); int len; byte[] arr = new byte[1024*100]; FileOutputStream fos = new FileOutputStream(desc); while ( (len = fis.read(arr)) != -1){ fos.write(arr,0,len); } fis.close(); fos.close(); } } 打印文件夹的内文件, 保持层级 import java.io.File; public class Demo4_PrintFolder { public static void main(String[] args) { File folder = new File("test"); int lev = 0; printFolder(folder, lev); } public static void printFolder(File folder , int lev){ System.out.println(folder.getPath()); for ( File f : folder.listFiles()){ for (int i=0;i<=lev;i++){ System.out.print("\t"); } if(f.isFile()){ System.out.println(f.getPath()); } else { // lev不能用lev++或++lev, 改变了lev值 printFolder(f, lev+1); } } } } 斐波那契数列 public class Demo5_TwoZi { public static void main(String[] args) { System.out.println(twoZi(6)); } public static int twoZi(int month){ int sum = 0; if(month > 2){ sum = twoZi(month-1) + twoZi(month-2); } else if(month == 2){ sum = 1; } else if(1 == month){ sum = 1; } return sum; } } 1000的阶乘, 其中所有0和尾部0的个数 import java.math.BigInteger; public class Demo6_1000JieChen { public static void main(String[] args) { BigInteger bi = new BigInteger("1"); for(int i=1;i<=1000;i++) { BigInteger tmepBi = new BigInteger(i + ""); bi = bi.multiply(tmepBi); } int sum = 0; String str = String.valueOf(bi); for(int i=0;i<str.length();i++){ if('0'== (str.charAt(i))){ sum++; } } int num = 0; for(int j=str.length()-1;j>=0;j--){ if( '0' == str.charAt(j)){ num++; } else { break; } } System.out.println(sum); System.out.println(num); } } 约瑟夫环 import java.util.ArrayList; import java.util.Iterator; public class Demo7_LuckYue { public static void main(String[] args) { int num = 8; System.out.println(getLuck(num)); System.out.println(getLuck2(num)); } public static int getLuck2(int num) { ArrayList<Integer> arrayList = new ArrayList<>(); for(int i=1;i<= num;i++){ arrayList.add(i); } int sum = 1; for(int i=0; arrayList.size()!=1;i++){ if( i == arrayList.size() ){ i = 0; } if(0 == sum % 3){ arrayList.remove(i--); } sum ++; } return arrayList.get(0); } public static int getLuck(int num){ ArrayList<Integer> arrayList = new ArrayList<>(); for(int i=1;i<= num;i++){ arrayList.add(i); } int sum = 0; while (arrayList.size() > 1){ Iterator<Integer> iterator = arrayList.iterator() ; while (iterator.hasNext()){ Integer i = iterator.next(); // next()和remove方法依赖关系 sum++; if(0 == sum % 3){ iterator.remove(); } } } return arrayList.get(0); } }

序列流 SequenceInputStream 关闭时会关闭所有流 public class Demo1_Stream { public static void main(String[] args) throws IOException { FileInputStream fr1 = new FileInputStream("read1.md"); FileInputStream fr2 = new FileInputStream("read2.md"); SequenceInputStream sis = new SequenceInputStream(fr1,fr2); FileOutputStream fos = new FileOutputStream("read3.md"); int b ; while ( ( b = sis.read()) != -1){ fos.write(b); } sis.close(); fos.close(); } public static void demo1() throws IOException { FileInputStream fr1 = new FileInputStream("read1.md"); FileOutputStream fos = new FileOutputStream("read3.md"); int b; while ((b = fr1.read()) != -1) { fos.write(b); } fr1.close(); FileInputStream fr2 = new FileInputStream("read2.md"); while ((b = fr2.read()) != -1) { fos.write(b); } fr2.close(); fos.close(); } } IO 流整合多个 public class Demo1_Stream { public static void main(String[] args) throws IOException { Vector<FileInputStream> v = new Vector<>(); FileInputStream fr1 = new FileInputStream("read1.md"); FileInputStream fr2 = new FileInputStream("read2.md"); v.add(fr1); v.add(fr2); Enumeration<FileInputStream> en = v.elements(); SequenceInputStream sis = new SequenceInputStream(en); FileOutputStream fos = new FileOutputStream("read3.md"); int b ; while ( (b = sis.read()) != -1){ fos.write(b); } sis.close(); fos.close(); } } ByteArrayOutputStream 获取数据 ByteArrayOutputStream 的 toString 可直接转为默认或指定的字符 import java.io.ByteArrayOutputStream; import java.io.FileInputStream; import java.io.IOException; public class Deom2_ByteArrayOutputStream { public static void main(String[] args) throws IOException { FileInputStream fi = new FileInputStream("read1.md"); ByteArrayOutputStream baos = new ByteArrayOutputStream(); int b ; while ( (b = fi.read()) != -1){ baos.write(b); } fi.close(); System.out.println(baos.toString()); System.out.println(baos.toString("utf-8")); } private static void Demo1() throws IOException { FileInputStream fi = new FileInputStream("read1.md"); byte[] arr = new byte[3]; int len; while ( ( len = fi.read(arr)) != -1){ System.out.println(new String(arr,0,len)); } fi.close(); } } 内存输出 import java.io.ByteArrayOutputStream; import java.io.FileInputStream; import java.io.IOException; public class Demo3_Test { public static void main(String[] args) throws IOException { FileInputStream fis = new FileInputStream("read1.md"); byte[] arr = new byte[5]; int len; ByteArrayOutputStream bos = new ByteArrayOutputStream(); while ( (len = fis.read(arr)) != -1){ bos.write(arr,0,len); } fis.close(); System.out.println(bos.toString()); bos.close(); } } 随机读和存 RandomAccessFile import java.io.IOException; import java.io.RandomAccessFile; public class Demo4_RandomAccessFile { public static void main(String[] args) throws IOException { // Demo1(); Demo2(); } public static void Demo2() throws IOException{ RandomAccessFile raf = new RandomAccessFile("random.txt","rw"); int b; while ( (b = raf.read()) != -1){ System.out.println(b); } raf.close(); } private static void Demo1() throws IOException { RandomAccessFile raf = new RandomAccessFile("random.txt","rw"); raf.write(97); raf.seek(10); raf.write(98); raf.close(); } } 对象操作流 ObjecOutputStream 输出对象流, 序列化 import com.lizicai.bean.Person; import java.io.FileOutputStream; import java.io.IOException; import java.io.ObjectOutputStream; public class Demo5_ObjectOutputStream { public static void main(String[] args) throws IOException { Person p1 = new Person("小明", 23); Person p2 = new Person("小红", 23); ObjectOutputStream oos = new ObjectOutputStream( new FileOutputStream("person.txt")); oos.writeObject(p1); oos.writeObject(p2); oos.close(); } } 读取对象流 import com.lizicai.bean.Person; import java.io.*; public class Deom6_ObjectInputStream { public static void main(String[] args) throws IOException, ClassNotFoundException { ObjectInputStream oos = new ObjectInputStream(new FileInputStream("person.txt")); Person p1 = (Person) oos.readObject(); System.out.println(p1.getName()+p1.getAge()); Person p2 = (Person) oos.readObject(); System.out.println(p2.getName()+p2.getAge()); } } 对象操作优化 对象放入集合中, 集合输出到流 import com.lizicai.bean.Person; import java.io.*; import java.util.ArrayList; public class Demo7_OptimeObjectStream { public static void main(String[] args) throws IOException, ClassNotFoundException { ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("hashSetPerson.txt")); ArrayList<Person> arrayList = new ArrayList<>(); arrayList.add(new Person("小明",25)); arrayList.add(new Person("小红",23)); arrayList.add(new Person("小王",26)); oos.writeObject(arrayList); oos.close(); ObjectInputStream ois = new ObjectInputStream(new FileInputStream("hashSetPerson.txt")); ArrayList<Person> personArrayList = ( ArrayList<Person> ) ois.readObject(); for(Person p : personArrayList){ System.out.println(p.getName() + p.getAge()); } } } IO 流增加 id 号 更改Person属性时更改serialVersionUID 可以准备知道原来Persion和现在Persion属性区别 import java.io.Serializable; public class Person implements Serializable { private static final long serialVersionUID= 1L; private String name; private int age; public Person(){} public Person(String name, int age) { this.name = name; this.age = age; } public void setName(String name) { this.name = name; } public String getName() { return name; } public void setAge(int age) { this.age = age; } public int getAge() { return age; } } IO 基本数据流, DataInputStream DataOutputStream import java.io.*; public class Demo8_DataInputStream { public static void main(String[] args) throws IOException { Demo1(); DataOutputStream dos = new DataOutputStream(new FileOutputStream("outData.txt")); dos.writeInt(997); dos.writeInt(998); dos.writeInt(999); dos.close(); DataInputStream dis = new DataInputStream(new FileInputStream("outData.txt")); int x = dis.readInt(); int y = dis.readInt(); int z = dis.readInt(); System.out.println(x); System.out.println(y); System.out.println(z); dis.close(); } private static void Demo1() throws IOException { FileOutputStream fos = new FileOutputStream("output.txt"); fos.write(997); fos.close(); } private static void Demo2() throws IOException { FileInputStream fis = new FileInputStream("output.txt"); int b = fis.read(); fis.close(); } } 打印流的概述和特点 PrintStream 和 PrintWriter 分别是打印的字节流和字符流 只操作数据目的 自动刷出作用不大 import java.io.*; public class Demo9_PrintStream { public static void main(String[] args) throws IOException { // Demo2(); PrintWriter pw = new PrintWriter(new FileOutputStream("printWriter.txt"), true); pw.println(97); } private static void Demo2() throws FileNotFoundException { PrintWriter pw = new PrintWriter("printWriter.txt"); pw.println(97); pw.write(97); pw.flush(); pw.close(); } private static void demo1() { PrintStream p = System.out; p.println("Hello World!"); p.write(97); p.close(); } } 标准输入输出流概述和输出语句 标准输入只有1个, 未关联到文件, 不用关闭, 关闭后无法打开了 标准输出流未关联文件, 也不用关闭 import java.io.FileInputStream; import java.io.IOException; import java.io.InputStream; import java.io.PrintStream; public class Demo10_SystemInOut { public static void main(String[] args) throws IOException { // demo1(); System.setIn(new FileInputStream("systemin.txt")); System.setOut(new PrintStream("systemout.txt")); InputStream is = System.in; PrintStream ps = System.out; int b; while ( ( b = is.read()) != -1){ ps.write(b); } is.close(); ps.close(); } private static void demo1() throws IOException { InputStream is = System.in; int x = is.read(); System.out.println(x); is.close(); InputStream is2 = System.in; int y = is2.read(); System.out.println(y); } } 练习, 复制文件, 增加使用数组, 增加复制速度 import java.io.*; public class Demo11_SystemInOutTest { public static void main(String[] args) throws IOException { System.setIn(new FileInputStream("systemin.txt")); System.setOut(new PrintStream("systemout.txt")); InputStream is = System.in; PrintStream ps = System.out; byte [] arr = new byte[1024]; int len; while ( ( len = is.read(arr)) != -1){ ps.write(arr,0,len); } is.close(); ps.close(); } } 2种键盘录入方式 import java.util.Scanner; public class Demo12_SystemIn { public static void main(String[] args) throws IOException { // demo1(); Scanner sc = new Scanner(System.in); String s = null; if(sc.hasNext()){ s = sc.nextLine(); } System.out.println(s); } private static void demo1() throws IOException { BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); String line = br.readLine(); System.out.println(line); } } Properties的概述和作为Map集合的使用 Object setProperty(String key, String value) 添加键值对 Enumeration<?> propertyNames() 获取 key值枚举 String getProperty(String key) 根据key 值获取 value import java.util.Enumeration; import java.util.Properties; public class Demo13_Properties { public static void main(String[] args) { // demo1(); Properties pp = new Properties(); pp.setProperty("name", "小明"); pp.setProperty("tel", "12310001000"); System.out.println(pp); Enumeration <String> en = (Enumeration<String>) pp.propertyNames(); while ( en.hasMoreElements()){ String key = en.nextElement(); System.out.println(key+pp.getProperty(key)); } } private static void demo1() { Properties p = new Properties(); p.put("intA", 12); System.out.println(p); } } Properties 读和存 void load(InputStream inStream) throws IOException 读取文件 void store(OutputStream out, String comments) throws IOException 写到文件, comments 加入注释 import java.io.FileInputStream; import java.io.FileOutputStream; import java.io.IOException; import java.util.Enumeration; import java.util.Properties; public class Demo13_Properties { public static void main(String[] args) throws IOException { Properties pp = new Properties(); System.out.println(pp); pp.load(new FileInputStream("config.properties")); System.out.println(pp); pp.setProperty("qq","123"); System.out.println(pp); pp.store(new FileOutputStream("config.properties"), "utf-8"); } }

FileReader 读取文件 import java.io.FileReader; import java.io.IOException; public class Demo1_FileReader { public static void main(String[] args) throws IOException { // Demo1(); FileReader fr = new FileReader("m.txt"); int b ; while ( (b = fr.read()) != -1){ System.out.println((char)b); } fr.close(); } private static void Demo1() throws IOException { FileReader fr = new FileReader("m.txt"); int x = fr.read(); System.out.println(x); System.out.println((char)x); } } FileWriter 字符写入文件 import java.io.FileWriter; import java.io.IOException; public class Demo2_FileWriter { public static void main(String[] args) throws IOException { FileWriter fw = new FileWriter("output.txt"); String s = "这只羊叫Leon"; fw.write(s); fw.flush(); fw.close(); } } 字符复制 import java.io.FileReader; import java.io.FileWriter; import java.io.IOException; public class Demo3_ReaderWriter { public static void main(String[] args) throws IOException { FileReader fr = new FileReader("input.txt"); FileWriter fw = new FileWriter("output.txt"); int b ; while ( (b = fr.read()) != -1){ fw.write(b); } fr.close(); fw.close(); } } 什么情况使用使用字符流 字符流也能复制文件, 但不推荐, 字节->字符, 字符->字节 多一步转换 程序需要读取一段文本, 或都需要写入一段文本的时候可以使用字符流 读取的时候按字符读出, 不会出现半个中文 写出时可直接字符串写出, 不用转换为字节数组 图片类文件不能使用字符流复制, 丢失数据 自定义数组复制 import java.io.FileReader; import java.io.FileWriter; import java.io.IOException; public class Demo4_ReaderWriter { public static void main(String[] args) throws IOException { FileReader fr = new FileReader("input.txt"); FileWriter fw = new FileWriter("output.txt"); char[] arr = new char[1024]; int len; while ( (len = fr.read(arr)) != -1){ fw.write(arr, 0 , len); } fr.close(); fw.close(); } } BufferedReader 和 BufferedWriter 读取写入字符 import java.io.*; public class Demo5_BufferedReader { public static void main(String[] args) throws IOException { BufferedReader br = new BufferedReader(new FileReader("input.txt")); BufferedWriter bw = new BufferedWriter(new FileWriter("output.txt")); int b ; while ( (b=br.read()) != -1){ bw.write(b); } br.close(); bw.close(); } } BufferedReader readLine()方法, BufferedWriter 的newLine方法 newLine 没有放在write(line)后, 并写在循环内, 这样写多写一个空行 import java.io.*; public class Demo6_ReadLine { public static void main(String[] args) throws IOException { BufferedReader br = new BufferedReader(new FileReader("input.txt")); BufferedWriter bw = new BufferedWriter(new FileWriter("output.txt")); String line ; if( (line = br.readLine() ) != null ){ bw.write(line); } while ( (line = br.readLine()) != null){ bw.newLine(); bw.write(line); } br.close(); bw.close(); } } 练习, 读取行, 并行数反向输出 流, 最好晚开早关 import java.io.*; import java.util.ArrayList; public class Demo7_ReverseLine { public static void main(String[] args) throws IOException { ArrayList<String> arrayList = new ArrayList<>(); String str ; BufferedReader br = new BufferedReader(new FileReader("input.txt")); while( ( str=br.readLine()) != null){ arrayList.add(str); } br.close(); BufferedWriter bw = new BufferedWriter(new FileWriter("output.txt")); for(int i= arrayList.size()-1;i>=0 ;i--){ bw.write(arrayList.get(i)); if( i == 0 ){ break; } bw.newLine(); } bw.close(); } } LineNumberReader import java.io.FileReader; import java.io.IOException; import java.io.LineNumberReader; public class Demo8_LineNumberReader { public static void main(String[] args) throws IOException { LineNumberReader lnr = new LineNumberReader(new FileReader("input.txt")); String line; lnr.setLineNumber(3); while ( (line = lnr.readLine()) != null){ System.out.println(lnr.getLineNumber() + " "+line); } lnr.close(); } } 装饰设计模式 使用被装饰的类, 对类中方法升级增加内容 好处 耦合性不强, 被装饰类的变化与装饰类的变化无关 public class Demo9_Wrap { public static void main(String[] args) { JiuCai j = new JiuCai(new Student()); j.code(); } } class Student { private String name; private int age; public void code(){ System.out.println("C"); System.out.println("Java"); } } interface Coder{ public void code(); } class JiuCai implements Coder{ JiuCai (Student stu){ this.stu = stu; } private Student stu; @Override public void code() { stu.code(); System.out.println("JavaEE"); System.out.println("SprintBoot"); } } 使用指定被码表读写字符 utf8格式写到gbk中, 乱码 ...

Java 的 IO 流 字节流, 字节流可以操作任何数据, 计算机中存储是字节流 InputStream OutputStream 字符流, 只能操作纯字符数据, 比较方便 Reader Writer 使用IO流要导入包, 使用时要进行异常处理, 使用后要释放资源 FileInputStream 读取一个文件 import java.io.FileInputStream; import java.io.IOException; public class Demo1_FileInputStream { public static void main(String[] args) throws IOException { FileInputStream fps = new FileInputStream("test.txt"); int x; while ((x = fps.read() ) != -1){ System.out.println(x); } fps.close(); } } read() 为什么接收的int类型 因为字节输入流可以操作任意类型的文件,比如图片音频等, 这些文件底层都是以二进制形式的存储的,如果每次读取都返回byte, 有可能在读到中间的时候遇到111111111,那么这11111111是byte类型的-1, 我们的程序是遇到-1就会停止不读了,后面的数据就读不到了,所以在读取的时候用int类型接收, 如果11111111会在其前面补上24个0凑足4个字节,那么byte类型的-1就变成int类型的255了这样可以保证整个数据读完, 而结束标记的-1就是int类型 字节输出流 write() void write(int b) throws IOException 在输出时int 去掉前3个byte字节, 写入第4个byte到文件中, 这个写是覆盖 import java.io.FileOutputStream; import java.io.IOException; public class Deom2_FileOutputStream { public static void main(String[] args) throws IOException { FileOutputStream fps = new FileOutputStream("test.txt"); fps.write(100); fps.write(101); fps.write(102); fps.close(); } } 字节输出流 追加 FileOutputStream(File file, boolean append) throws FileNotFoundException 输出字节流是否选择追加 import java.io.FileOutputStream; import java.io.IOException; public class Demo3_FileOutputStream { public static void main(String[] args) throws IOException { FileOutputStream fps = new FileOutputStream("test.txt", true); fps.write(100); fps.write(101); fps.close(); } } 复制文件 byte字节读取, byte复制 import java.io.FileInputStream; import java.io.FileOutputStream; import java.io.IOException; public class Demo4_FileInOutStream { public static void main(String[] args) throws IOException { FileInputStream fis = new FileInputStream("test.txt"); FileOutputStream fos = new FileOutputStream("copy.txt"); int b = 0; while ( (b = fis.read() )!= -1){ fos.write(b); } fis.close(); fos.close(); } } FileInputStream 的 available方法 int available() throws IOException 返回输入流的长度 import java.io.FileInputStream; import java.io.FileOutputStream; import java.io.IOException; public class Demo5_FileInOutStream { public static void main(String[] args) throws IOException { FileInputStream fis = new FileInputStream("test.txt"); FileOutputStream fos = new FileOutputStream("copy.txt"); int len = fis.available(); byte[] arr = new byte[fis.available()]; fis.read(arr); fos.write(arr); fis.close(); fos.close(); } } 通过数组 byte[] 读取写入 import java.io.FileInputStream; import java.io.FileOutputStream; import java.io.IOException; public class Demo6_FileInOutStream { public static void main(String[] args) throws IOException { FileInputStream fis = new FileInputStream("test.txt"); FileOutputStream fos = new FileOutputStream("copy.txt"); byte[] arr = new byte[1024 * 1024]; int b = 0; while ( (b =fis.read(arr)) != -1 ){ fos.write(arr,0,b); } fis.close(); fos.close(); } private static void errSimple() throws IOException { FileInputStream fis = new FileInputStream("test.txt"); FileOutputStream fos = new FileOutputStream("copy.txt"); byte[] arr = new byte[3]; int b = 0; while ( (b =fis.read(arr)) != -1 ){ fos.write(arr); } fis.close(); fos.close(); } } BufferedInputStream BufferedOutputStream 原码先读取1024*8个字节到内存, 复制给 BufferedOutputStream 1024*8 然后才写入文件 只需要关闭BufferedInputStream BufferedOutputStream 就能关闭所有流 import java.io.*; public class Demo7_BufferInputStream { public static void main(String[] args) throws IOException { FileInputStream fis = new FileInputStream("test.txt"); FileOutputStream fos = new FileOutputStream("copy.txt"); BufferedInputStream bis = new BufferedInputStream(fis); BufferedOutputStream bos = new BufferedOutputStream(fos); int b ; while( ( b = bis.read() ) != -1){ bos.write(b); } // 只需关闭 BufferedInputStream BufferedOutputStream bis.close(); bos.close(); } } flush和close方法的区别 flush 刷新缓冲区, 缓冲区写到文件中, 刷完可以继续写. close 关闭前 就会刷新缓冲区 将缓冲区内容存到文件中 import java.io.*; public class Demo8_FlushClose { public static void main(String[] args) throws IOException { FileInputStream fis = new FileInputStream("test.txt"); FileOutputStream fos = new FileOutputStream("copy.txt"); BufferedInputStream bis = new BufferedInputStream(fis); BufferedOutputStream bos = new BufferedOutputStream(fos); int b ; while ( (b = bis.read()) != -1 ){ bos.write(b); bos.flush(); } bis.close(); bos.close(); } } 字节流读取中文有乱码 UTF-8中英文1个字节, 中文3个字节, 按字节读取都会读出乱码 import java.io.FileInputStream; import java.io.FileOutputStream; import java.io.IOException; import java.nio.charset.StandardCharsets; public class Demo9_CharIO { public static void main(String[] args) throws IOException { // Demo1(); FileOutputStream fos = new FileOutputStream("chinese.txt"); fos.write("你好啊, 世界!".getBytes(StandardCharsets.UTF_8)); fos.write("\r".getBytes(StandardCharsets.UTF_8)); fos.close(); } private static void Demo1() throws IOException { FileInputStream fis = new FileInputStream("chinese.txt"); byte[] arr = new byte[3]; String s; int len ; while ( ( len=fis.read(arr) ) != -1){ System.out.println(new String(arr,0,len)); } } } 异常处理 流初始化null try 关闭流 能关闭一个就关闭一个 import java.io.FileInputStream; import java.io.FileOutputStream; import java.io.IOException; public class Demo10_IOException { public static void main(String[] args) throws IOException{ FileInputStream fis = null; FileOutputStream fos = null; try { fis = new FileInputStream("test.txt"); fos = new FileOutputStream("copy.txt"); } finally { try{ if(null != fis){ fis.close(); } } finally { if(null != fos){ fos.close(); } } } } } 图片加密解密 加密把输出的字节异或一个数, 解密时再异或这个数 import java.io.*; public class Demo12_Encrypt { public static void main(String[] args) throws IOException { String src = "beauty.png"; String dest = "copy.png"; String dest2 = "copy1.png"; // extracted(src, dest); extracted(dest, dest2); } private static void extracted( String src, String dest) throws IOException { BufferedInputStream fis = new BufferedInputStream(new FileInputStream(src)); BufferedOutputStream fos = new BufferedOutputStream(new FileOutputStream(dest)); int b ; while ( (b = fis.read()) != -1){ fos.write(b ^ 123 ); } fis.close(); fos.close(); } } 输入文件复制到当前路径下 import java.io.*; import java.util.Scanner; public class Demo13_CopyFile { public static void main(String[] args) throws IOException { Scanner sc = new Scanner(System.in); String src = null; if(sc.hasNext()){ src = sc.nextLine(); } BufferedInputStream fis =null; int b ; BufferedOutputStream fos = null; if(src != null){ File file = new File(src); fis = new BufferedInputStream(new FileInputStream(src)); fos = new BufferedOutputStream(new FileOutputStream("./"+file.getName())); if( file.isFile()){ while ( (b = fis.read()) != -1){ fos.write(b); } } else if( file.isDirectory()){ System.out.println("文件夹无法复制"); } } fis.close(); fos.close(); } } 练习, 录入输入字符到文件中 import java.io.BufferedOutputStream; import java.io.FileOutputStream; import java.io.IOException; import java.nio.charset.StandardCharsets; import java.util.Scanner; public class Demo14_InputKey { public static void main(String[] args) throws IOException { Scanner sc = new Scanner(System.in); BufferedOutputStream bos = new BufferedOutputStream(new FileOutputStream("test.txt", true)); String input = null; while ( sc.hasNext()){ input = sc.nextLine(); if( "quit".equals(input)){ break; } else{ bos.write(input.getBytes(StandardCharsets.UTF_8)); bos.write("\n".getBytes(StandardCharsets.UTF_8)); } } bos.close(); } }

File 相对路径文件或目录 绝对路径文件或目录 File(String pathname) 文件路径或目录 File(String parent, String child) 父文件路径, 和子文件名 File(File parent, String child) 父文件路径file, 和子文件名 import java.io.File; public class Demo1_File { public static void main(String[] args) { // Demo1(); // Demo2(); File parentFile = new File("/usr/local/etc/nginx/"); String child = "nginx.conf"; File file = new File(parentFile, child); } private static void Demo2() { String parent = "/usr/local/etc/nginx/"; String child = "nginx.conf"; File file = new File(parent,child); System.out.println(file.exists()); } // a.c 与 src 在同级目录下 private static void Demo1() { File file = new File("/usr/local/etc/nginx/nginx.conf"); System.out.println(file.exists()); File file2 = new File("a.c"); System.out.println(file2.exists()); } } File 的方法 boolean createNewFile() throws IOException 如果没有则创建true, 有则不创建false. boolean mkdir() 无则创建文件夹, 有则不创建false boolean mkdirs() 无则创建多级文件夹, 有则不创建false import java.io.File; import java.io.IOException; public class Demo2_FileMethod { public static void main(String[] args) throws IOException { File file = new File("aaa.c"); System.out.println(file.createNewFile()); File file2 = new File("test"); System.out.println(file2.mkdir()); File file3 = new File("test/aaa"); System.out.println(file3.mkdirs()); } } boolean renameTo(File dest) 就是mv命令 boolean delete() 删除文件或删除空文件夹, 文件夹内有文件或文件夹(空的也算)则删除不了 import java.io.File; import java.io.IOException; public class Demo3_FileMethod { public static void main(String[] args) throws IOException { File file1 = new File("aaa.c"); File file2 = new File("bbb.c"); System.out.println(file1.renameTo(file2)); System.out.println(file2.delete()); File file3 = new File("test/aaa"); System.out.println(file3.delete()); File file4 = new File("test/ccc"); System.out.println(file4.mkdirs()); file4.delete(); File file5 = new File( "test"); System.out.println(file5.delete()); } } boolean isDirectory() 判断是否文件夹 boolean isFile() 判断是否文件 boolean exists() 判断是否存在 boolean canRead() 判断是否可读权限 boolean canWrite() 判断是否可写权限 boolean isHidden() 判断是否隐藏 import java.io.File; import java.io.IOException; public class Demo4_FileMethod { public static void main(String[] args) throws IOException { File file1 = new File("test"); System.out.println(file1.isDirectory()); File file2 = new File("test.c"); System.out.println(file1.isFile()); System.out.println(file2.isFile()); File file3 = new File("aaa.c"); System.out.println(file3.exists()); System.out.println(file3.createNewFile()); System.out.println(file3.exists()); File file4 = new File("ccc.c"); System.out.println(file4.createNewFile()); System.out.println(file4.canRead()); File file5 = new File("ddd.c"); System.out.println(file5.createNewFile()); System.out.println(file5.canWrite()); File file6 = new File("root.c"); System.out.println(file6.setReadable(false)); System.out.println(file6.canRead()); System.out.println(file6.canWrite()); File file7 = new File("test.c"); System.out.println(file7.isHidden()); File file8 = new File(".idea"); System.out.println(file8.isHidden()); } } String getAbsolutePath() String getPath() String getName() long length() long lastModified() String[] list() File[] listFiles() import java.io.File; import java.text.SimpleDateFormat; import java.util.Date; public class Demo5_FileMethod { public static void main(String[] args) { File file = new File("aaa.c"); File file2 = new File("/Users/test/IdeaProjects/day19/"); System.out.println(file.getAbsoluteFile()); System.out.println(file2.getAbsoluteFile()); System.out.println(file.getPath()); System.out.println(file2.getPath()); System.out.println(file.getName()); System.out.println(file2.getName()); System.out.println(file.length()); System.out.println(file2.length()); System.out.println(file.lastModified()); System.out.println(file2.lastModified()); Date date = new Date(file.lastModified()); SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); System.out.println(simpleDateFormat.format(date)); String [] arr = file.list(); // for(String s: arr){ // System.out.println(s); // } String [] arr2 = file2.list(); for(String s: arr2){ System.out.println(s); } File[] list1 = file.listFiles(); // for(File f : list1){ // System.out.println(f); // } File[] list2 = file2.listFiles(); for(File f : list2){ System.out.println(f); } } } 练习 查找文件夹下所有的.c结尾的文件 遍历所有的文件, 碰到文件夹则进入遍历 文件名字正则匹配, 匹配输出 或使用String endsWith 匹配 import java.io.File; public class Demo6_FileTest { public static void main(String[] args) { File file = new File("/Users/test/workplace/test"); File file2 = new File("/Users/test/workplace/test"); File[] arrFile = file.listFiles(); File[] arrFile2 = file2.listFiles(); String regex = ".*[\\.][c]$"; String strEnd = ".c"; fileFind(arrFile, strEnd); fileFindRegex(arrFile2, regex); } public static void fileFind(File[] file, String strEnd){ for(File f : file){ if(f.isDirectory()){ fileFind(f.listFiles(), strEnd); } else{ if( f.getName().endsWith(strEnd) ){ System.out.println(f.getName()); } } } return ; } public static void fileFindRegex(File[] file, String regex){ for(File f : file){ if(f.isDirectory()){ fileFindRegex(f.listFiles(), regex); } else{ if( f.getName().matches(regex) ){ System.out.println(f.getName()); } } } return ; } } String[] list(FilenameFilter filter) 实现FilenameFilter 接口, 返回匹配的数组 原码 ...

Java 的异常类 异常的体系 Throwable Error Exception RuntimeException 如果程序没有处理异常, JVM 自行处理, 把异常的名称和信息, 打印在控制台上. 异常的2种处理方式 try catch finally try catch try catch finally try finally throws public class Demo1_Exception { public static void main(String[] args) { Demo1 demo1 = new Demo1(); int x; try{ x = demo1.div(10,0); } catch (ArithmeticException e){ System.out.println(e.getClass()); System.out.println("test"); }finally { System.out.println("最后执行了吗"); } } } class Demo1 { public int div (int a, int b){ return a/b; } } public class Demo2_Exception { public static void main(String[] args) { int a = 0; int b = 10; int[] arr = {1, 2, 3}; int x ; arr = null; try { // x = b / a; System.out.println(arr[10]); } catch (ArithmeticException e){ System.out.println(e.getClass()); } catch (IndexOutOfBoundsException e){ System.out.println(e.getClass()); } catch (Exception e){ System.out.println("有异常"); } } } public class Demo3_Exception { public static void main(String[] args) { try { System.out.println(10/0); } catch (ArithmeticException | IndexOutOfBoundsException e){ System.out.println("Exception"); } } } 异常throwable 的方法 String getMessage() 获取异常信息 String toString() 获取异常类和异常信息 void printStackTrace() 获取异常类和异常信息, 异常在程序出现的位置 public class Demo3_Exception { public static void main(String[] args) { try { System.out.println(10/0); } catch (ArithmeticException | IndexOutOfBoundsException e){ System.out.println(e.getMessage()); System.out.println(e.toString()); e.printStackTrace(); } } } 方法上的2种异常 RuntimeException 运行时异常, 不需要在方法向上抛出, 使用的时候也不需要在使用的方法上向上抛出 非RuntimeException 的异常, 必须在方法上向上抛出, 使用方法时也必须向上抛出异常 import lombok.Getter; import lombok.Setter; public class Person { public Person(){} public Person(String name, int age){ this.name = name; this.age = age; } @Getter @Setter private String name; @Getter private int age; public void setAge(int age) { if(age < 1 || age > 150){ throw new RuntimeException ("年龄非法"); } this.age = age; } } import lombok.Getter; import lombok.Setter; public class Person { public Person(){} public Person(String name, int age){ this.name = name; this.age = age; } @Getter @Setter private String name; @Getter private int age; public void setAge(int age) throws Exception{ if(age < 1 || age > 150){ throw new Exception ("年龄非法"); } this.age = age; } } public class Demo4_Exception { public static void main(String[] args) throws Exception{ Person p = new Person(); p.setAge(-12); System.out.println("MM"); } } throws 和 throw 区别 throws 用在方法声明后, 跟的是异常类名 跟以跟多个类名, 用逗号隔开 表示抛出异常, 由该方法的调用者处理 throw 用在方法内, 跟的是异常对象名 只能抛出一个异常对象名 表示抛出异常, 由方法体内的语句处理 finally finally 的特点 被finally 控制的语句一定会执行 特殊情况: 在执行finally 前 JVM 退出了(如System.exit(0)) finally 的作用 用于释放资源, 在IO 操作和数据库操作中会见到 return 的区别 return 执行后, 如果有finally 则执行finally 类 public class Demo5_Exception { public static void main(String[] args) { try { System.out.println(1/0); } catch (Exception e){ System.out.println("异常"); } finally { System.out.println("最后一定执行"); } } } final finally finalize 区别 final final 可以修饰类, 但不能被继承 修饰方法不能被重写 修饰变量只能赋值一次 finally try catch finally 体系中的一个语句, 不能单独使用 finalize 当垃圾回收器确定不存在该对象的更多引用时, 对象回收器则调用此方法 public class Demo6_Exception { public static void main(String[] args) { System.out.println(demo1()); } public static int demo1(){ int x = 10; try { x = 20; System.out.println(1/0); return x; } catch (Exception e){ x = 30; return x; } finally { x = 40; } } } x 会返回30, return 把x=30装箱返回回去 ...

Map 将键映射到值的对象 一个映射不能包含重复的键 每个键最多只能映射到一个值 Map 与 Collection 接口的不同 Map 是双列的, Collection 是单列的 Map 是键唯一, Collection 的子体系Set是唯一的 Map 集合的数据结构值针对键有效, 跟值无关; Collection集合的数据结构是针对元素有效 Map 方法 V put(K key, V value) 添加键和值, 成功返回null, 覆盖重复key值, 则返回被覆盖的Value值 V remove(Object key) 通过key删除元素, 删除成功则返回Value值 boolean containsKey(Object key) Map 中是否包含Key值 boolean containsValue(Object value) Map 中是否包含Value值 boolean isEmpty() Map 是否为空 clear() 清空Map import java.util.Collection; import java.util.HashMap; import java.util.Map; public class Demo1_Map { public static void main(String[] args) { // Demo1(); Map<String, Integer> hashMap = new HashMap<>(); hashMap.put("小明", 18); hashMap.put("小红", 19); hashMap.put("小王", 20); System.out.println(hashMap); Integer i = hashMap.remove("小王"); System.out.println(i); System.out.println(hashMap); System.out.println(hashMap.containsKey("小明")); System.out.println(hashMap.containsValue(19)); System.out.println(hashMap.isEmpty()); Collection<Integer> v = hashMap.values(); System.out.println(v); hashMap.clear(); System.out.println(hashMap.size()); } private static void Demo1() { Map<String,Integer> map = new HashMap<>(); Integer i1 = map.put("s1",12); Integer i2 = map.put("s2",22); Integer i3 = map.put("s3",33); Integer i4 = map.put("s3",34); Integer i5 = map.put("s5",35); Integer i6 = map.put("s6",35); System.out.println(map); System.out.println(i4); System.out.println(i5); System.out.println(i6); } } Map 根据键获取值 import java.util.HashMap; public class Demo2_Iterator { public static void main(String[] args) { HashMap<String, Integer> hashMap = new HashMap<>(); hashMap.put("a", 100); hashMap.put("b",90); hashMap.put("c",98); for(String s:hashMap.keySet()){ System.out.println(s + "="+ hashMap.get(s)); } } } Map 的键值对, 来获取Map中的Key 与 Value import java.util.HashMap; import java.util.Map; public class Demo3_Iterator { public static void main(String[] args) { HashMap<String, Integer> hashMap = new HashMap<>(); hashMap.put("a", 90); hashMap.put("b",95); hashMap.put("c",98); for(Map.Entry<String,Integer> single: hashMap.entrySet()){ System.out.println(single.getKey() + " "+ single.getValue()); } } } HashMap 存入Student类和String时, 重写 hashCode 和 equals方法 import lombok.Getter; import lombok.Setter; import java.util.Objects; public class Student { public Student(){} public Student(String name, int age){ this.name = name; this.age = age; } @Getter @Setter private String name; @Getter @Setter private int age; @Override public String toString() { return "Student "+this.name+this.age; } @Override public int hashCode() { return Objects.hash(name, age); } @Override public boolean equals(Object obj) { Student s = (Student) obj; return this.name == s.name && this.age == s.age; } } import com.lizicai.bean.Student; import java.util.HashMap; public class Demo4_HashMap { public static void main(String[] args) { HashMap<Student, String> hashMap = new HashMap<>(); hashMap.put(new Student("小明",23), "上海"); hashMap.put(new Student("小明",23), "北京"); hashMap.put(new Student("小王",26), "北京"); hashMap.put(new Student("莱昂纳多",28), "美国"); System.out.println(hashMap); } } LinkedHashMap import java.util.LinkedHashMap; public class Demo5_LinkedHashMap { public static void main(String[] args) { LinkedHashMap<String,Integer> linkedHashMap = new LinkedHashMap<>(); linkedHashMap.put("a", 20); linkedHashMap.put("b", 20); linkedHashMap.put("c", 20); System.out.println(linkedHashMap); } } TreeMap 必须实现 Comparable 接口, 2种方式 方式一在bean类中实现Comparable 接口 import lombok.Getter; import lombok.Setter; import java.util.Objects; public class Student implements Comparable<Student>{ public Student(){} public Student(String name, int age){ this.name = name; this.age = age; } @Getter @Setter private String name; @Getter @Setter private int age; @Override public String toString() { return "Student "+this.name+this.age; } @Override public int hashCode() { return Objects.hash(name, age); } @Override public boolean equals(Object obj) { Student s = (Student) obj; return this.name == s.name && this.age == s.age; } @Override public int compareTo(Student o) { int numName = this.name.compareTo(o.name); int numAge = numName == 0 ? this.age-o.age : numName; return numAge; } } public class Demo6_TreeMap { public static void main(String[] args) { // Demo1(); TreeMap<Student, String> treeMap = new TreeMap<>(); treeMap.put(new Student("ab",23), "上海"); treeMap.put(new Student("ab",23), "北京"); treeMap.put(new Student("ac",26), "北京"); treeMap.put(new Student("abc",28), "美国"); System.out.println(treeMap); } } 方式二, 在创建TreeMap时实现 Comparator 接口 public static void main(String[] args) { TreeMap<Student, String> treeMap = new TreeMap<>(new Comparator<Student>() { @Override public int compare(Student o1, Student o2) { int numName = o1.getName().compareTo(o2.getName()); int numAge = numName == 0 ? o1.getAge()-o2.getAge() : numName; return numAge; } }); treeMap.put(new Student("ab",23), "上海"); treeMap.put(new Student("ab",23), "北京"); treeMap.put(new Student("ac",26), "北京"); treeMap.put(new Student("abc",28), "美国"); System.out.println(treeMap); } 练习, 字符串 aaabbbccccfgkj 统计字母出现次数 字母在Map里Key 没有, 由存入key,1, 有则存key, value+1 import java.util.HashMap; import java.util.Map; public class Demo7_HashMap { public static void main(String[] args) { HashMap<Character, Integer> hashMap = new HashMap<>(); String str = "aaabbbbccccd"; char[] cArray = str.toCharArray(); for(char c : cArray){ if( ! hashMap.containsKey(c)){ hashMap.put(c,1); } else { hashMap.put(c, hashMap.get(c)+1); } } for(Map.Entry<Character,Integer> ci:hashMap.entrySet()){ System.out.println(ci.getKey() + "=" + ci.getValue() ); } } } 练习 HashMap 嵌套 HashMap import lombok.Getter; import lombok.Setter; import java.util.Objects; public class Student implements Comparable<Student>{ public Student(){} public Student(String name, int age){ this.name = name; this.age = age; } @Getter @Setter private String name; @Getter @Setter private int age; @Override public String toString() { return "Student "+this.name+this.age; } @Override public int hashCode() { return Objects.hash(name, age); } @Override public boolean equals(Object obj) { Student s = (Student) obj; return this.name == s.name && this.age == s.age; } import com.lizicai.bean.Student; import java.util.HashMap; import java.util.Map; public class Demo8_HashMapHashMap { public static void main(String[] args) { HashMap<Student, String> hashMap1 = new HashMap<>(); hashMap1.put(new Student("小明",23), "上海"); hashMap1.put(new Student("小王",26), "北京"); hashMap1.put(new Student("莱昂纳多",28), "美国"); HashMap<Student, String> hashMap2 = new HashMap<>(); hashMap2.put(new Student("小明",23), "上海"); hashMap2.put(new Student("小王",26), "北京"); hashMap2.put(new Student("莱昂纳多",29), "美国"); HashMap<HashMap<Student,String>,String > hashMap = new HashMap<>(); hashMap.put(hashMap1,"班级1"); hashMap.put(hashMap2,"班级2"); for(Map.Entry<HashMap<Student,String>, String> HH : hashMap.entrySet()){ for(Map.Entry<Student,String> HHS : HH.getKey().entrySet()){ System.out.println(HHS.getKey() + HHS.getValue()+HH.getValue()); } } System.out.println(hashMap); } } HashMap 和 Hashtable 区别 共同点 都是双链集合 区别 HashMap 是线程不安全的, 效率高JDK 1.2 版本 Hashtable 是线程安全的, 效率低, JDK 1.0 版本 HashMap 可以存储null 键 和 null 值 Hashtable 不可以存储null 键 及 null 值 import java.util.HashMap; import java.util.Hashtable; public class Demo9_HashMapHashtable { public static void main(String[] args) { HashMap<String, Integer> hashMap = new HashMap<>(); hashMap.put(null, 10); hashMap.put("a", null); System.out.println(hashMap); Hashtable<String, Integer> hashtable = new Hashtable<>(); hashtable.put(null, 10); hashtable.put("a", null); System.out.println(hashtable); } } Collections 中的方法 static <T extends Comparable<? super T» void sort(List list) 排序 static int binarySearch(List<? extends Comparable<? super T» list, T key) 二分查找 static <T extends Object & Comparable<? super T» T max(Collection<? extends T> coll) 返回最大值 static void reverse(List<?> list) 反转列表 static void shuffle(List<?> list) list 序列洗牌 private static void Demo5() { ArrayList<String> arrayList = new ArrayList<>(); arrayList.add("d"); arrayList.add("a"); arrayList.add("a"); arrayList.add("b"); arrayList.add("c"); arrayList.add("f"); System.out.println(arrayList); Collections.shuffle(arrayList); System.out.println(arrayList); } private static void Demo4() { ArrayList<String> arrayList = new ArrayList<>(); arrayList.add("d"); arrayList.add("a"); arrayList.add("a"); arrayList.add("b"); arrayList.add("c"); arrayList.add("f"); System.out.println(arrayList); Collections.reverse(arrayList); System.out.println(arrayList); } private static void Demo3() { ArrayList<String> arrayList = new ArrayList<>(); arrayList.add("d"); arrayList.add("a"); arrayList.add("a"); arrayList.add("b"); arrayList.add("c"); arrayList.add("f"); String max = Collections.max(arrayList); System.out.println(max); } private static void Demo2() { ArrayList<String> arrayList = new ArrayList<>(); arrayList.add("d"); arrayList.add("a"); arrayList.add("a"); arrayList.add("b"); arrayList.add("c"); arrayList.add("a"); Collections.sort(arrayList); System.out.println(arrayList); int index = Collections.binarySearch(arrayList, "b"); System.out.println(index); System.out.println(Collections.binarySearch(arrayList,"o")); } private static void Demo1() { ArrayList<String> arrayList = new ArrayList<>(); arrayList.add("d"); arrayList.add("a"); arrayList.add("a"); arrayList.add("b"); arrayList.add("c"); arrayList.add("a"); System.out.println(arrayList); Collections.sort(arrayList); System.out.println(arrayList); } 练习, 54 张牌, 每人17张, 剩下3张, 打印每个人牌和底牌 import java.util.ArrayList; import java.util.Collections; import java.util.Scanner; public class Demo2_Collections { public static void main(String[] args) { String[] nu = {"2", "3", "4", "5", "6", "7", "8", "9", "10", "J", "Q", "K", "A"}; String[] color = { "♠️", "♥️", "♦️", "♣️"}; ArrayList<String> arrayList = new ArrayList<>(); for(String nuStr : nu){ for(String colorStr : color){ arrayList.add(nuStr + colorStr); } } arrayList.add("Big King"); arrayList.add("Small King"); for(int i=0;i<3;i++){ Collections.shuffle(arrayList); } System.out.println(arrayList); ArrayList<String> list1 = new ArrayList<>(); ArrayList<String> list2 = new ArrayList<>(); ArrayList<String> list3 = new ArrayList<>(); ArrayList<String> list4 = new ArrayList<>(); Scanner sc = new Scanner(System.in); for(int i=0;i<53;i++){ if(i >= 50 ){ list4.add(arrayList.get(i)); } else if( i % 3 == 0){ list1.add(arrayList.get(i)); } else if( i % 3 == 1) { list2.add(arrayList.get(i)); } else if( i % 3 ==2 ){ list3.add(arrayList.get(i)); } } System.out.println(list4); System.out.println(list1); System.out.println(list2); System.out.println(list3); } } 练习使用 HashMap 模拟一副牌 用HashMap 的 Key 生成arraylist来洗牌 import java.util.ArrayList; import java.util.Collections; import java.util.HashMap; import java.util.TreeMap; public class Demo3_Collections { public static void main(String[] args) { HashMap<Integer, String> poker = new HashMap<>(); String[] nu = { "3", "4", "5", "6", "7", "8", "9", "10", "J", "Q", "K", "A","2"}; String[] color = { "♠️", "♥️", "♦️", "♣️"}; int count = 0; ArrayList<Integer> pokerIndex = new ArrayList<>(); for(String nuStr : nu){ for(String colorStr : color){ poker.put(count, (colorStr + nuStr)); pokerIndex.add(count); count++; } } poker.put(count, "Small King"); pokerIndex.add(count); count++; poker.put(count, "Big King"); pokerIndex.add(count); TreeMap<Integer, String> gaojin = new TreeMap<>(); TreeMap<Integer, String> jp = new TreeMap<>(); TreeMap<Integer, String> tiger = new TreeMap<>(); TreeMap<Integer, String> dipai = new TreeMap<>(); System.out.println(pokerIndex.size()); Collections.shuffle(pokerIndex); System.out.println(pokerIndex); for(int i=0;i< pokerIndex.size(); i++){ int index = pokerIndex.get(i); if(i > 50){ dipai.put( index , poker.get( index)); } else if ( i % 3 == 0){ gaojin.put( index, poker.get(index )); } else if (i % 3 == 1){ jp.put( index , poker.get( index )); } else if (i % 3 == 2){ tiger.put( index, poker.get( index )); } } System.out.println(gaojin); System.out.println(jp); System.out.println(tiger); System.out.println(dipai); } } ? Super E 情况 TreeSet TreeMap 会调用 import lombok.Getter; import lombok.Setter; import java.util.Objects; public class Student implements Comparable<Student>{ public Student(){} public Student(String name, int age){ this.name = name; this.age = age; } @Getter @Setter private String name; @Getter @Setter private int age; @Override public String toString() { return "Student "+this.name+this.age; } @Override public int hashCode() { return Objects.hash(name, age); } @Override public boolean equals(Object obj) { Student s = (Student) obj; return this.name == s.name && this.age == s.age; } @Override public int compareTo(Student o) { int numName = this.name.compareTo(o.name); int numAge = numName == 0 ? this.age-o.age : numName; return numAge; } } public class BaseStudent extends Student{ public BaseStudent(){} public BaseStudent(String name, int age){ super(name, age); } } import com.lizicai.bean.BaseStudent; import com.lizicai.bean.Student; import java.util.ArrayList; import java.util.Comparator; import java.util.TreeSet; public class Demo4_Generic { public static void main(String[] args) { // Demo1(); TreeSet<Student> treeSet = new TreeSet<>(new CompareByAge()); treeSet.add(new Student("小明",22)); treeSet.add(new Student("小红",22)); treeSet.add(new Student("小王",22)); System.out.println(treeSet); TreeSet<BaseStudent> treeSet2 = new TreeSet<>(new CompareByAge()); treeSet2.add(new BaseStudent("小明",22)); treeSet2.add(new BaseStudent("小红",22)); treeSet2.add(new BaseStudent("小王",22)); System.out.println(treeSet2); } private static void Demo1() { ArrayList<Student> arrayList = new ArrayList<>(); arrayList.add(new Student("小明",22)); arrayList.add(new Student("小红",22)); arrayList.add(new Student("小王",22)); ArrayList<BaseStudent> arrayList2 = new ArrayList<>(); arrayList2.add(new BaseStudent("小明",22)); arrayList2.add(new BaseStudent("小红",22)); arrayList2.add(new BaseStudent("小王",22)); arrayList.addAll(arrayList2); System.out.println(arrayList); } } class CompareByAge implements Comparator<Student> { @Override public int compare(Student o1, Student o2) { int num = o1.getAge() - o2.getAge(); int numName = num == 0 ? o1.getName().compareTo(o2.getName()): num; return numName; } } Collection List 存取有序, 有索引, 可重复 ArrayList 底层数组实现的, 线程不安全, 查找修改快, 增删慢 LinkedList 底层是链表实现的, 线程不安全, 查找慢, 增删快 Vetor 底层数组实现的, 线程安全, 查找修改增删都慢 如果查找修改多用ArrayList, 增和删多用LinkedList, 如果都多用ArrayList ...