Java | 李子菜Lizicai

Java的Set类 Li.038

Set 无序不可重复 import java.util.HashSet; public class Demo1_HashSet { public static void main(String[] args) { HashSet<String> hs = new HashSet<>(); boolean b1 = hs.add("a"); boolean b2 = hs.add("a"); System.out.println(hs); System.out.println(b1+" "+b2); for(String s:hs){ System.out.println(s); } } } HashSet 的重写hashCode() 方法, hashCode值一致时, 调用equals方法使用HashSet 存取元素时, 这个元素必须重写 hashCode 和 equals方法 import lombok.Getter; import lombok.Setter; import java.util.Objects; public class Person { public Person(){} public Person(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 "Person" + this.getName() + this.getAge(); } @Override public boolean equals(Object obj) { Person p = (Person) obj; return this.getName() == p.getName() && this.getAge() == p.getAge(); } @Override public int hashCode() { return Objects.hash(name, age); } } public static int hashCode(Object a[]) { if (a == null) return 0; int result = 1; for (Object element : a) result = 31 * result + (element == null ? 0 : element.hashCode()); return result; } LinkedHashSet 底层链表实现的是Set对象中唯一一个保证怎么存就怎么取的集合对象 HashSet的子类, 也是保证元素是唯一的 import java.util.LinkedHashSet; public class Demo1_LinkedHashSet { public static void main(String[] args) { LinkedHashSet <String> linkedHashSet = new LinkedHashSet<>(); linkedHashSet.add("a"); linkedHashSet.add("a"); linkedHashSet.add("b"); linkedHashSet.add("b"); linkedHashSet.add("b"); linkedHashSet.add("c"); System.out.println(linkedHashSet); } } HashSet 练习, 放入集合10个[1,20]的整数 import java.util.HashSet; import java.util.Random; public class Demo1_Test { public static void main(String[] args) { Random r = new Random(); HashSet <Integer> hashSet = new HashSet<>(); while(hashSet.size() < 10 ){ Integer i = r.nextInt(20) + 1; hashSet.add(i); } System.out.println(hashSet); } } HashSet 练习, 输入一串字符串, 去掉重复字符 import java.util.HashSet; import java.util.Scanner; public class Demo2_Test { public static void main(String[] args) { HashSet<Character> hs = new HashSet<>(); Scanner sc = new Scanner(System.in); String s = null; if(sc.hasNext()){ s = sc.nextLine(); } char[] charArray= s.toCharArray(); // for(Character c:charArray){ for(char c:charArray){ hs.add(c); } System.out.println(hs); } } HashSet 练习, list 去重, 并返回元素list import java.util.ArrayList; import java.util.HashSet; import java.util.LinkedHashSet; import java.util.List; public class Demo3_Test { public static void main(String[] args) { List<String> list = new ArrayList<>(); list.add("a"); list.add("b"); list.add("b"); list.add("c"); list.add("c"); list.add("c"); HashSet <String> hs = new LinkedHashSet<>(); // deleteCopyElement(list, hs); deleteCopyE(list); // System.out.println(hs); System.out.println(list); } public static void deleteCopyElement(List<String> list, HashSet<String> hashSet){ hashSet.addAll(list); list.clear(); list.addAll(hashSet); } public static void deleteCopyE(List<String> list){ LinkedHashSet<String> hs = new LinkedHashSet<>(); hs.addAll(list); list.clear(); list.addAll(hs); } } TreeSet 集合, 对集合进行排序的, 元素唯一保证元素唯一集合排序 import java.util.TreeSet; public class Demo1_TreeSet { public static void main(String[] args) { TreeSet <Integer> ts = new TreeSet<>(); ts.add(1); ts.add(1); ts.add(2); ts.add(2); ts.add(3); ts.add(3); System.out.println(ts); } } TreeSet , 类实现Comparable接口 compareTo 方法返回0, 则集合中只用一个元素 compareTo 方法返回正数, 集合怎么存怎么取 compareTo 方法返回负数, 集合倒序存储 import lombok.Getter; import lombok.Setter; import java.util.Objects; public class Person implements Comparable <Person>{ public Person(){} public Person(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 "Person" + this.getName() + this.getAge(); } @Override public boolean equals(Object o) { if (this == o) { return true; } if (!(o instanceof Person)) { return false; } Person person = (Person) o; return getAge() == person.getAge() && Objects.equals(getName(), person.getName()); } @Override public int hashCode() { return Objects.hash(name, age); } @Override public int compareTo(Person p) { int num = this.age - p.age; return num == 0 ? this.name.compareTo(p.name) : num; } } public class Demo1_TreeSet { public static void main(String[] args) { TreeSet<Person> ts = new TreeSet<>(); ts.add(new Person("小明", 20)); ts.add(new Person("小红", 19)); ts.add(new Person("小张", 21)); ts.add(new Person("小张", 21)); System.out.println(ts); } } TreeSet 如果使用Person 的name排序, 则需要实现Compareable 接口, compareTo方法 @Override public int compareTo(Person p) { int num = this.name.compareTo(p.name); return num == 0 ? this.age-p.age : num; } public class Demo1_TreeSet { public static void main(String[] args) { TreeSet<Person> ts = new TreeSet<>(); ts.add(new Person("a", 20)); ts.add(new Person("a", 19)); ts.add(new Person("ab", 19)); ts.add(new Person("ba", 21)); ts.add(new Person("ca", 21)); System.out.println(ts); } } TreeSet 使用Person 的name长度比较 @Override public int compareTo(Person p){ int length = this.name.length()- p.name.length(); int num = length == 0 ? this.name.compareTo(p.name) : length; return num == 0 ? this.age - p.age : num; } TreeSet 实现Comparator接口, 在创建TreeSet时调用自动定义String 长度作为比较 class Compare implements Comparator<String>{ @Override public int compare(String o1, String o2) { int length = o1.length() - o2.length(); int num = length == 0 ? o1.compareTo(o2) : length; return num; } } public class Demo4_TreeSet { public static void main(String[] args) { TreeSet<String> ts = new TreeSet<>(new Compare()); ts.add("asdfsdf"); ts.add("z"); System.out.println(ts); } } 练习, ArrayList 重复无序的数据, 如何按字典排序, 且不可去除重复数据 import java.util.ArrayList; import java.util.Comparator; import java.util.TreeSet; public class Demo5_TreeSet { public static void main(String[] args) { ArrayList<String> aList = new ArrayList<>(); aList.add("aaa"); aList.add("aaa"); aList.add("aaa"); aList.add("bbb"); aList.add("bbb"); aList.add("bbb"); aList.add("bbb"); aList.add("aaa"); aList.add("aaa"); aList.add("zzz"); aList.add("zzz"); aList.add("zzz"); sortArrayList(aList); System.out.println(aList); } public static void sortArrayList(ArrayList<String> list){ // 实际Comparator // list添加到tree(new comparator)中 // 清空list // list.addAll(TreeSet) TreeSet<String> ts = new TreeSet<>(new StrComparator()); ts.addAll(list); list.clear(); list.addAll(ts); } } class StrComparator implements Comparator<String> { @Override public int compare(String o1, String o2) { int num = o1.compareTo(o2); if(0 == num){ num ++; } return num; } } 练习, 输入字符hello 输出ehllo import java.util.Comparator; import java.util.Scanner; import java.util.TreeSet; public class Demo6_TreeSet { public static void main(String[] args) { Scanner sc = new Scanner(System.in); String s = null; if(sc.hasNext()){ s = sc.nextLine(); } char[] chaArray = s.toCharArray(); // Arrays.sort(chaArray); // System.out.println(chaArray); TreeSet<Character> ts = new TreeSet<>(new Comparator<Character>() { @Override public int compare(Character o1, Character o2) { int num = o1.compareTo(o2); return num == 0? 1 :num; } }); for(Character c:chaArray){ ts.add(c); } for(Character c:ts){ System.out.print(c); } } } 练习, 输入数字并倒序输入quit 退出 import java.util.Comparator; import java.util.Scanner; import java.util.TreeSet; public class Demo7_TreeSet { public static void main(String[] args) { Scanner sc = new Scanner(System.in); TreeSet<Integer> ts = new TreeSet<>(new Comparator<Integer>() { @Override public int compare(Integer o1, Integer o2) { int num = o2.compareTo(o1); return num == 0 ? 1 :num ; } }); while (sc.hasNext()){ String s = sc.next(); if( "quit".equals(s)){ break; } else{ ts.add(Integer.parseInt(s)); } } for(Integer i:ts){ System.out.print(i+" "); } } } 练习录入5个学生信息(姓名,语文成绩,数学成绩,英语成绩) 按总成绩从高到低输入到控制台 import lombok.AllArgsConstructor; import lombok.Getter; import lombok.NoArgsConstructor; import lombok.Setter; @AllArgsConstructor @NoArgsConstructor public class Student implements Comparable<Student>{ @Getter @Setter private String name; @Getter @Setter private int motherTongue; @Getter @Setter private int math; @Getter @Setter private int english; @Override public int compareTo(Student o) { int num = (o.motherTongue + o.math + o.english) - (this.motherTongue + this.math + this.english); int num2 = num == 0 ? o.motherTongue - this.motherTongue : num; int num3 = num2 == 0 ? o.math - this.math : num2; int num4 = num3 == 0 ? o.english - this.english : num3; int num5 = num4 == 0 ? o.name.compareTo(this.name) : num4 ; if (0 == num5){ num5 = -1; } return num5; } @Override public String toString() { return "Student."+this.name +".语文"+this.motherTongue+".数学"+this.math+".英语" + this.english; } } import com.lizicai.bean.Student; import java.util.Scanner; import java.util.TreeSet; public class Demo8_TreeSet { public static void main(String[] args) { Scanner sc = new Scanner(System.in); TreeSet<Student> ts = new TreeSet<>(); for(int i=0; i<5;i++){ String str = null; if(sc.hasNext()){ str = sc.nextLine(); } String[] strArray = str.split(","); ts.add(new Student(strArray[0], Integer.parseInt(strArray[1]), Integer.parseInt(strArray[2]), Integer.parseInt(strArray[3]))); } System.out.println(ts); } }

Java的List类 Li.037

List 练习Person 集合去重 @Data @AllArgsConstructor @NoArgsConstructor public class Person { private String name; private int age; @Override public boolean equals(Object obj) { Person p = (Person) obj; boolean d = false; if(this.getAge() == p.getAge() && this.getName() == p.getName()){ d = true; } else{ d = false; } return d; } } import java.util.ArrayList; import java.util.Iterator; import java.util.List; public class Demo2_ArrayList { public static void main(String[] args) { List list = new ArrayList(); list.add(new Person("小明", 23)); list.add(new Person("小明", 23)); list.add(new Person("小红", 22)); list.add(new Person("小王", 25)); List list2 = new ArrayList(); deleteMu(list, list2); System.out.println(list2); } public static void deleteMu(List srcList, List destList){ Iterator iTsrc = srcList.iterator(); while (iTsrc.hasNext()){ Person p = (Person) iTsrc.next(); if( !destList.contains(p) ){ destList.add(p); } } return ; } } LinkedList 方法 void addFirst(E e) void addLast(E e) 在头增加或在尾增加 public E getFirst() public E getLast() 获取第1个或最后1个 public E removeFirst() public E removeLast() 删除第1个或最后1个 public E get(int index) 获取指定位置的元素 import java.util.LinkedList; public class Demo1_LinkedList { public static void main(String[] args) { LinkedList ll = new LinkedList(); ll.add("a"); ll.add("b"); ll.add("c"); ll.add("d"); ll.addFirst(0); ll.addLast(10); System.out.println(ll); System.out.println(ll.getFirst()); System.out.println(ll.getLast()); ll.removeFirst(); System.out.println(ll); ll.removeLast(); System.out.println(ll); System.out.println(ll.get(0)); } } LinkedList 模拟栈 import lombok.Data; import java.util.LinkedList; @Data public class Stank { private LinkedList ll = new LinkedList(); public void in(Object obj){ ll.addLast(obj); } public void out(){ ll.removeLast(); } public void outAll(){ while (! ll.isEmpty()){ ll.removeLast(); } } public boolean isEmpty(){ if(ll.isEmpty()){ return true; } return false; } } import com.lizicai.bean.Stank; public class Demo3_LinkedList { public static void main(String[] args) { Stank s = new Stank(); s.in("a"); s.in("b"); s.in("c"); s.out(); System.out.println(s.getLl()); s.outAll(); System.out.println(s.getLl()); } } 泛型 generic 泛型的好处提高安全性, 把运行的错误转移到编译时校验省去强转的麻烦前后泛型一致, 或者后面泛型是前端泛型的子类 ...

Java的Collection集合一 Li.036

Collection 和数组区别数组可以存储基本数据类型, 也可以存储引用数据类型集合可以存储引用数据类型, 也能存基本数据类型, 存储时会自动装箱数组的长度固定, 不可以自由增加集合长度可变, 可以自由增加 List Set 集合 List 有序集合, 存取位置一致有索引可存储重复数据 Set 无序集合, 存取位置不一致无索引不可存储重复数据 List 体系 ArrayList LinkedList Vector ArrayList 底层是数组, 查询快, 增删慢线程不安全, 效率高 LinkedList 链表实现, 查询慢, 增删快线程不安全, 效率高 Vection 数组实现, 查询快, 增删慢线程安全, 效率低 Set 体系 HashSet TreeSet HashSet 哈希算法 TreeSet 二叉树算法 ArrayList 的 boolean add(E e) 方法一直返回true, Set 集合存取重复元素时, 则返回false boolean remove(Object o) 一次删除一个对象 int size() 打印集合对象数 void clear() 清空集合 boolean isEmpty() 判断集合是否为空 public class Demo2_Collection { public static void main(String[] args) { Collection collection = new ArrayList(); boolean b1 = collection.add("abc"); boolean b2 = collection.add(true); boolean b3 = collection.add(100); boolean b4 = collection.add(new Student("小红",18)); boolean b5 = collection.add("abc"); System.out.println(b1); System.out.println(b2); System.out.println(b3); System.out.println(b4); System.out.println(b5); System.out.println(collection); System.out.println(collection.contains("abc")); collection.remove("abc"); System.out.println(collection); collection.remove("abc"); System.out.println(collection.size()); System.out.println(collection); System.out.println(collection.isEmpty()); collection.clear(); System.out.println(collection.isEmpty()); System.out.println(collection); } } Object[] toArray() 集合转换成数组 import com.lizicai.bean.Student; import java.util.ArrayList; import java.util.Collection; public class Demo3_Collection { public static void main(String[] args) { Collection c = new ArrayList(); c .add( new Student("小明", 24)); c .add( new Student("小红", 22)); c .add( new Student("小张", 30)); Object [] oArray = c.toArray(); for(Object o:oArray){ if( o instanceof Student){ Student s = (Student) o; System.out.println(s.getName()+s.getAge()); } else{ System.out.println(o); } } } } List 中带有All的方法 boolean addAll(Collection<? extends E> c) 添加另一个集合 boolean removeAll(Collection<?> c) 移除集合中所有与C集合一样的元素 boolean containsAll(Collection<?> c) 返回结果c集合全部是调用集合内元素的判断结果 boolean retainAll(Collection<?> c) 调用集合和c集合取交集, 调用集合改变则是true, 没改是false private static void Demo4() { Collection c = new ArrayList(); c.add("a"); c.add("b"); c.add("c"); Collection c2 = new ArrayList(); c2.add("a"); c2.add("b"); System.out.println(c.retainAll(c2)); System.out.println(c); } private static void Demo3() { Collection c = new ArrayList(); c.add("a"); c.add("b"); c.add("b"); c.add("c"); Collection c2 = new ArrayList(); c2.add("a"); c2.add("b"); c2.add("c"); System.out.println(c.containsAll(c2)); System.out.println(c); System.out.println(c2); } private static void Demo2() { Collection c = new ArrayList(); c.add("a"); c.add("b"); c.add("b"); c.add("c"); Collection c2 = new ArrayList(); c2.add("a"); c2.add("b"); c2.add("cc"); c.removeAll(c2); System.out.println(c); } private static void Demo1() { Collection c = new ArrayList(); c.add("a"); c.add("b"); c.add("c"); Collection c2 = new ArrayList(); c2.add("aa"); c2.add("bb"); c2.add("cc"); c.addAll(c2); c2.add(c); System.out.println(c); System.out.println(c2); } 集合的迭代器遍历 import com.lizicai.bean.Student; import java.util.ArrayList; import java.util.Collection; import java.util.Iterator; public class Demo1_Iterator { public static void main(String[] args) { Collection c = new ArrayList(); c.add(new Student("小明", 24)); c.add(new Student("小红", 22)); c.add(new Student("小张", 30)); Iterator it = c.iterator(); while (it.hasNext()){ Object obj = it.next(); if( obj instanceof Student){ Student s = (Student)obj; System.out.println(s.getName()+s.getAge()); } else { System.out.println(obj); } } } } List 集合 void add(int index, E element) E remove(int index) E get(int index) E set(int index, E element) import java.util.ArrayList; import java.util.List; public class Demo1_List { public static void main(String[] args) { List list = new ArrayList(); list.add("a"); list.add("b"); list.add("c"); list.add(0,"d"); System.out.println(list); list.remove(0); System.out.println(list); list.set(2,"w"); System.out.println(list); System.out.println(list.get(2)); } } List 遍历的2种方式 for循环 iterator迭代器 public class Demo2_List { public static void main(String[] args) { List list = new ArrayList(); list.add("a"); list.add("b"); list.add("c"); for(int i=0;i<list.size();i++){ System.out.println(list.get(i)); } Iterator it = list.iterator(); while (it.hasNext()){ System.out.println(it.next()); } } } List 的 ListIterator 可以在遍历的时候添加元素, List不能在遍历的时候添加元素 import java.util.ArrayList; import java.util.Iterator; import java.util.List; import java.util.ListIterator; public class Demo3_List { public static void main(String[] args) { List list = new ArrayList(); list.add("a"); list.add("ab"); list.add("abc"); list.add("Hello"); // Iterator li = list.iterator(); // while (li.hasNext()){ // String s = (String)li.next(); // if("Hello".equals(s)){ // list.add("World"); // } // System.out.println(s); // } ListIterator li = list.listIterator(); while (li.hasNext()){ String s = (String)li.next(); if("Hello".equals(s)){ li.add("World"); } } System.out.println(list); } } Vector import java.util.Enumeration; import java.util.Vector; public class Demo1_Vector { public static void main(String[] args) { Vector v = new Vector(); v.addElement("a"); v.addElement("b"); v.addElement("c"); v.addElement("d"); Enumeration en = v.elements(); while (en.hasMoreElements()){ System.out.println(en.nextElement()); } } }

Java的正则表达式 Li.035

Java 的正则表达式Regex java.util.regex.Pattern 有正则表达示例 /* [abc] a, b, or c (simple class) abc中任意一个字符 [^abc] Any character except a, b, or c (negation) 非abc的一个字符 [a-zA-Z] a through z or A through Z, inclusive (range) 匹配a-z A-Z 大小写字符 [a-d[m-p]] a through d, or m through p: [a-dm-p] (union) 匹配a-d 和匹配 m-p 字符, 匹配则true [a-z&&[def]] d, e, or f (intersection) 匹配def 与 a-z的并集是 def [a-z&&[^bc]] a through z, except for b and c: [ad-z] (subtraction), 非bc 和 a-z取并集(匹配a d-z字符) [a-z&&[^m-p]] a through z, and not m through p: [a-lq-z](subtraction), 非m-p字符与a-z取并集, 并集是a-l和q-z */ private static void Demo7() { String regex = "[a-z&&[^m-p]]"; System.out.println("a".matches(regex)); System.out.println("n".matches(regex)); } private static void Demo6() { String regex = "[a-z&&[^bc]]"; System.out.println("a".matches(regex)); System.out.println("b".matches(regex)); } private static void Demo5() { String regex = "[a-z&&[def]]"; System.out.println("m".matches(regex)); System.out.println("a".matches(regex)); System.out.println("d".matches(regex)); } private static void Demo4() { String regex = "[a-d[x-z]]"; System.out.println("m".matches(regex)); System.out.println("a".matches(regex)); System.out.println("x".matches(regex)); } private static void Demo3() { String regex = "[a-zA-Z]"; System.out.println("a".matches(regex)); System.out.println("*".matches(regex)); } private static void Demo2() { String regex = "[^abc]"; System.out.println("a".matches(regex)); System.out.println("d".matches(regex)); System.out.println("10".matches(regex)); } private static void Demo1() { String regex = "[abc]"; System.out.println("a".matches(regex)); System.out.println("d".matches(regex)); } 正则预先定义的字符 public class Demo4_Regex { public static void main(String[] args) { /* . Any character (may or may not match line terminators) 匹配任何字符 \d A digit: [0-9] 0-9字符, 其他字符则false \D A non-digit: [^0-9] 非 0-9 字符, 0-9字符则false \h A horizontal whitespace character: [ \t\xA0\u1680\u180e\u2000-\u200a\u202f\u205f\u3000] 水平空白字符 \H A non-horizontal whitespace character: [^\h] 非水平空白字符 \s A whitespace character: [ \t\n\x0B\f\r] 空白字符 \S A non-whitespace character: [^\s] 非空白字符 \v A vertical whitespace character: [\n\x0B\f\r\x85\u2028\u2029] 垂直空白字符 \V A non-vertical whitespace character: [^\v] 非垂直空白字符 \w A word character: [a-zA-Z_0-9] 大小写数字下划线 \W A non-word character: [^\w] 除大小写数字下划线以外字符 */ } private static void Demo5() { // 字符 \w a-zA-Z0-9_ \W, 除去a-zA-Z0-9_的字符 String regex = "\\w"; System.out.println("s".matches(regex)); System.out.println("_".matches(regex)); System.out.println("0".matches(regex)); System.out.println("&".matches(regex)); String regex2 = "\\W"; System.out.println("s".matches(regex2)); System.out.println("_".matches(regex2)); System.out.println("0".matches(regex2)); System.out.println("&".matches(regex2)); } private static void Demo4() { String regex = "\\v"; System.out.println(" ".matches(regex)); System.out.println("\t".matches(regex)); System.out.println("\n".matches(regex)); System.out.println("a".matches(regex)); String regex2 = "\\V"; System.out.println(" ".matches(regex2)); System.out.println("\t".matches(regex2)); System.out.println("\n".matches(regex2)); System.out.println("a".matches(regex2)); } private static void Demo3() { String regex = "\\s"; System.out.println(" ".matches(regex)); System.out.println("\t".matches(regex)); System.out.println("\n".matches(regex)); System.out.println("a".matches(regex)); String regex2 = "\\S"; System.out.println(" ".matches(regex2)); System.out.println("\t".matches(regex2)); System.out.println("\n".matches(regex2)); System.out.println("a".matches(regex2)); } private static void Demo2() { String regex = "\\h"; String regex2 = "\\H"; System.out.println("a".matches(regex)); System.out.println(" ".matches(regex)); System.out.println("\t".matches(regex)); System.out.println("\n".matches(regex)); System.out.println("a".matches(regex2)); System.out.println(" ".matches(regex2)); System.out.println("\t".matches(regex2)); System.out.println("\n".matches(regex2)); } private static void Demo1() { String regex = "\\d"; System.out.println("a".matches(regex)); System.out.println("9".matches(regex)); String regex2 = "\\D"; System.out.println("a".matches(regex2)); System.out.println("9".matches(regex2)); } private static void Demo() { String regex = "."; System.out.println("a".matches(regex)); System.out.println("%".matches(regex)); System.out.println("9".matches(regex)); } } 正则匹配出现次数 public class Demo5_Regex { public static void main(String[] args) { /* X? X, once or not at all X出现一次或一次也没有, 但是不能出现其他内容 X* X, zero or more times X出现0次或>0次, 但是不能出现其它内容 X+ X, one or more times X出现1次或>1次, 但是不能出现其它内容 X{n} X, exactly n times X准确出现n次, 不多不少, 但是不能出现其它内容 X{n,} X, at least n times X出现次数至少n次, 但是不能出现其它内容 X{n,m} X, at least n but not more than m times X出现次数至少n次, 不超过m次, 记[n, m)次 */ } private static void Demo6() { String regex = "[abc]{2,4}"; System.out.println("a".matches(regex)); System.out.println("ab".matches(regex)); System.out.println("aba".matches(regex)); System.out.println("abca".matches(regex)); System.out.println("abcabc".matches(regex)); System.out.println(" ".matches(regex)); } private static void Demo5() { String regex = "[abc]{5,}"; System.out.println("abca".matches(regex)); System.out.println("abcab".matches(regex)); System.out.println("abcabc".matches(regex)); System.out.println("dkfjkf".matches(regex)); } private static void Demo4() { String regex = "[abc]{5}"; System.out.println("abca".matches(regex)); System.out.println("abccc".matches(regex)); System.out.println("abcabc".matches(regex)); System.out.println("abccd".matches(regex)); System.out.println("d".matches(regex)); System.out.println(" ".matches(regex)); } private static void Demo3() { String regex = "[abc]+"; System.out.println("a".matches(regex)); System.out.println("abc".matches(regex)); System.out.println("abcabccc".matches(regex)); System.out.println("abcd".matches(regex)); System.out.println("".matches(regex)); System.out.println("d".matches(regex)); } private static void Demo2() { String regex = "[abc]*"; System.out.println("".matches(regex)); System.out.println("a".matches(regex)); System.out.println("b".matches(regex)); System.out.println("bb".matches(regex)); System.out.println("abc".matches(regex)); System.out.println(" ".matches(regex)); System.out.println("s".matches(regex)); } private static void Demo1() { String regex = "[abc]?"; System.out.println("".matches(regex)); System.out.println("a".matches(regex)); System.out.println("b".matches(regex)); System.out.println(" ".matches(regex)); System.out.println("bb".matches(regex)); System.out.println("s".matches(regex)); } } String 中的 split 方法 String[] split(String regex) 匹配正则分隔成字符串数组 // 空格或问号出现1次或以上, 分隔 public class Demo6_Regex { public static void main(String[] args) { String str = "What is a youth?"; String regex = "[\\s\\?]+"; String [] strArray = str.split(regex); for(String s:strArray){ System.out.println(s); } } } 练习字符串的数组, 经过排序后, 再转换成字符串字符串, 分隔成字符串组字符串组转换为数组数组排序数组重新转换为字符串 public class Demo1_Split { public static void main(String[] args) { String str = "1 10 2 30 100 40"; String regex = "[\\s]+"; String [] strArray = str.split(regex); int [] a = new int[strArray.length]; for(int i=0;i<strArray.length;i++){ a[i] = Integer.parseInt(strArray[i]); } StringBuffer sb = new StringBuffer(); Arrays.sort(a); if(a.length <1){ sb = sb.append("{}"); } else{ sb.append("{"); for (int i=0;i<a.length;i++){ if(i == a.length-1){ sb.append(a[i] + "}"); break; } sb.append(a[i] + " "); } } System.out.println(sb); } } 正则匹配, 替换字符 public class Demo2_Replace { public static void main(String[] args) { String str = "What2is2a233youth?"; String regex = "[\\d]"; String res = str.replaceAll(regex,""); System.out.println(res); String regex2 = "[\\d]"; String res2 = str.replaceAll(regex2," "); System.out.println(res2); String regex3 = "[\\d]+"; String res3 = str.replaceAll(regex3," "); System.out.println(res3); } } 正则匹配, 组捕获 ((A)(B(C))) , 组是以左括号为一组, 所有共有 4 组 ((A)(B(C))) (A) (B(C)) (C) 正则匹配, 组的练习 public class Demo4_Replace { public static void main(String[] args) { // Demo1(); String str = "我...我...我...我....想想........学编...编...程....程...程程.."; String regex = "[\\.]+"; String str2 = str.replaceAll(regex, ""); System.out.println(str2); String regex2 = "(.)\\1+"; String str3 = str2.replaceAll(regex2, "$1"); System.out.println(str3); } private static void Demo1() { String str = "abchhhhkdjflllljkklqqqq"; String regex = "(.)\\1+"; String[] strArray = str.split(regex); for(String s:strArray){ System.out.println(s); } } } 正则匹配, Pattern 和 Matcher, 练习获取手机号考虑手机号第2位一直在增加, 以1开头11位即认为正确手机号 public static void main(String[] args) { String str = "我用过的手机号12310001000, 上一个手机号是12310001001, 目前在用的12310001002"; String regex = "[1][\\d]{10}"; Pattern p = Pattern.compile(regex); Matcher m = p.matcher(str); while (m.find()){ System.out.println(m.group()); } } Math 类 public class Demo1_Math { public static void main(String[] args) { System.out.println(Math.PI); System.out.println(Math.abs(-120)); // 向上取整 (0,1] System.out.println(Math.ceil(23.8)); System.out.println(Math.ceil(23.1)); // 向下取整 [0,1) System.out.println(Math.floor(23.8)); System.out.println(Math.floor(24.0)); System.out.println(Math.max(23,30)); System.out.println(Math.pow(3,3)); // 包含 [0.0,1.0) System.out.println(Math.random()); System.out.println(Math.sqrt(2)); } } Random 随机数 Random 是伪随机数, 生成的随机数由随机因子决定的 nextInt(100) 可以生成[0,100) 的随机整数 import java.util.Random; public class Demo1_Random { public static void main(String[] args) { Random r = new Random(); for(int i=0;i<10;i++){ int a = r.nextInt(100); System.out.println(a); } Random r2 = new Random(1000); int a1 = r2.nextInt(); int a2 = r2.nextInt(); System.out.println(a1); System.out.println(a2); // 执行多少次 a1 和 a2 固定 } } System 类 System.in 输入流, Scanner 录收键盘 System.out 输出控制台 System.gc() 垃圾回收 System.currentTimeMillis() 到1971年的毫秒, 2次时间差就能知道程序运行时间 exit() 退出 0 正常终止, 非0 异常终止 arraycopy(Object src, int srcPos, Object dest, int destPos, int length) 数组复制, int str 对象等都可复制 public class Demo1_System { public static void main(String[] args) { Scanner sc = new Scanner(System.in); String str = sc.nextLine(); System.out.println("jkdjf"); System.out.println(System.currentTimeMillis()); long start = System.currentTimeMillis(); for(int i=0;i<10000000;i++){ System.out.print(""); StringBuffer sf = new StringBuffer(); } long end = System.currentTimeMillis(); System.out.println(end-start); for(int i=0;i<10;i++){ new Test(); System.gc(); } int [] src = {1, 2, 3, 4, 5}; int [] dest = new int[8]; System.arraycopy(src, 0, dest,0,src.length); for(int i:dest){ System.out.println(i); } } } class Test{ Test(){} @Override public void finalize(){ System.out.println("垃圾回收"); } } BigInteger 类 import java.math.BigInteger; public class Demo1_BigInteger { public static void main(String[] args) { BigInteger bi1 = new BigInteger("100"); BigInteger bi2 = new BigInteger("3"); System.out.println(bi1.add(bi2)); System.out.println(bi1.subtract(bi2)); System.out.println(bi1.multiply(bi2)); System.out.println(bi1.divide(bi2)); BigInteger [] bArray; bArray = bi1.divideAndRemainder(bi2); for(BigInteger b:bArray){ System.out.println(b); } } } BigDecimal, 金融运算使用 static BigDecimal valueOf(double val) 由double 转为 BigDecimal 构造函数时BigDecimal(String val), string 转为 BigDecimal public class Demo1_BigDecimal { public static void main(String[] args) { System.out.println(2.0 - 1.1 ); BigDecimal bd1 = new BigDecimal(2.0); BigDecimal bd2 = new BigDecimal(1.1); System.out.println(bd1.subtract(bd2)); BigDecimal bd3 = new BigDecimal("2.0"); BigDecimal bd4 = new BigDecimal("1.1"); System.out.println(bd3.subtract(bd4)); BigDecimal bd5 = BigDecimal.valueOf(2.0); BigDecimal bd6 = BigDecimal.valueOf(1.1); System.out.println(bd5.subtract(bd6)); } } Date 类 System.currentTimeMillis date.getTime() 都是与GMT 1970 01 01 00:00:00 的毫秒值 public class Demo1_Date { public static void main(String[] args) { // Demo1(); Date date = new Date(); System.out.println(date.getTime()); System.out.println(System.currentTimeMillis()); date.setTime(2000); System.out.println(date); } private static void Demo1() { Date date = new Date(); System.out.println(date); Date date2 = new Date(0); System.out.println(date2); } } SimpleDateFormat Date 特定格式输出字符串转为Date import java.text.ParseException; import java.text.SimpleDateFormat; import java.util.Date; public class Demo1_DateFormat { public static void main(String[] args) throws ParseException { // Demo1(); SimpleDateFormat sdf = new SimpleDateFormat("yyyy年MM月dd日 HH:mm:ss E"); String strDate = "2021年08月18日 23:44:59 星期三"; Date date = sdf.parse(strDate); System.out.println(date); } private static void Demo1() { Date date = new Date(); SimpleDateFormat sdf = new SimpleDateFormat("yyyy年MM月dd日 HH:mm:ss E"); System.out.println(sdf.format(date)); } } SimpleDateFormat 练习, 从生日到现在多少天了 import java.text.ParseException; import java.text.SimpleDateFormat; import java.util.Date; public class Demo2_DateFormat { public static void main(String[] args) throws ParseException { String strDate = "1991-01-01"; Date date = new Date(); SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd"); Date birthday = sdf.parse(strDate); long days = date.getTime() - birthday.getTime(); System.out.println(days/1000/3600/24); } } Calendar 类 Calendar 的获取月份时的0对应1月, 7 对应8月 Calendar 获取的1对应的是星期天, DAY_OF_WEEK add(int field, int amount) 给年月日时分秒增减 set(int field, int value) 给年月日时分秒设置数值 set(int year, int month, int date) 设置年月日 import java.text.SimpleDateFormat; import java.util.Calendar; public class Demo1_Calendar { public static void main(String[] args) { Calendar cal = Calendar.getInstance(); String month = setTwoNu(cal.get(Calendar.MARCH) + 1); System.out.println(month); String week = getWeek(cal.get(Calendar.DAY_OF_WEEK)); System.out.println(week); System.out.println(cal.get(Calendar.DAY_OF_MONTH)); System.out.println(cal.getTime()); SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss E"); System.out.println(sdf.format(cal.getTime())); } public static String getWeek(int week){ String [] strArray = {"", "星期日", "星期一","星期二","星期三","星期四","星期五","星期六"}; return strArray[week]; } public static String setTwoNu(int nu){ String s = String.valueOf(nu); if( nu <10 ){ s = "0" + nu; } return s; } } import java.util.Calendar; public class Demo2_Calendar { public static void main(String[] args) { Calendar calendar = Calendar.getInstance(); calendar.add(Calendar.YEAR, 10); System.out.println(calendar.get(Calendar.YEAR)); System.out.println(calendar.get(Calendar.MONTH)); calendar.set(Calendar.MONTH, 10); System.out.println(calendar.get(Calendar.MONTH)); calendar.set(2021,9,01); System.out.println(calendar.get(Calendar.YEAR) +""+ (calendar.get(Calendar.MONTH)+1) +"" +calendar.get(Calendar.DAY_OF_MONTH)); } } Calendar 平年润年 import java.util.Calendar; public class Demo3_Calendar { public static void main(String[] args) { Calendar calendar = Calendar.getInstance(); int year = 2020; calendar.set(year,2,1); calendar.add(Calendar.DAY_OF_MONTH, -1); System.out.println(calendar.get(Calendar.DAY_OF_MONTH)); if( 29 == calendar.get(Calendar.DAY_OF_MONTH)){ System.out.println( year + "年是润年"); } else { System.out.println( year + "年是平年"); } } }

Java的StringBuffer类 Li.034

String 与 StringBuffer 区别 StringBuffer 是线程安全的可变字符序列 String 是一个不可变的字符序列 StringBuffer 的构造函数 public class Demo1_Sb { public static void main(String[] args) { StringBuffer sb = new StringBuffer(); System.out.println(sb.length()); System.out.println(sb.capacity()); StringBuffer sb2 = new StringBuffer(10); System.out.println(sb2.length()); System.out.println(sb2.capacity()); StringBuffer sb3 = new StringBuffer("hello"); System.out.println(sb3.length()); System.out.println(sb3.capacity()); } } StringBuffer 的append 和 insert 方法 StringBuffer append() 支持int String StringBuffer boolean, 返回本身, StringBuffer insert(int , String) 在指定索引添加int String char等, 返回本身, 有可能会越界 public class Demo2_Sb { public static void main(String[] args) { StringBuffer sb = new StringBuffer(); sb.append(2); System.out.println(sb); sb.append("haihai"); System.out.println(sb); sb.append(true); System.out.println(sb); sb.insert(1,"sss"); System.out.println(sb); } } StringBuffer 的deleteCharAt delete方法 StringBuffer deleteCharAt(int index) 删除指定索引的字符, 返回本身, 可能会越界 StringBuffer delete(int start, int end) 删除从start到end, 返回本身, 可能会越界 public class Demo3_Sb { public static void main(String[] args) { StringBuffer sb = new StringBuffer(); sb.append("Word hen da"); sb.deleteCharAt(10); System.out.println(sb); sb.delete(0,4); System.out.println(sb); } } StringBuffer 的 replace reverse方法 StringBuffer replace(int start, int end, String str) 用户str替换从start到end, 返回本身, 可能越界 StringBuffer reverse() 反转字符串, 返回本身 public class Demo4_StringBuffer { public static void main(String[] args) { StringBuffer sb = new StringBuffer(); sb.append("Hello"); sb.replace(0,1,"Say"); System.out.println(sb); sb.reverse(); System.out.println(sb); } } StringBuffer 的substring方法 String substring(int start) 从索引start开始向后截取, 返回新的String( 不再是StringBuffer本身了 ) String substring(int start, int end) 从start到end(不包含end)截取, 返回新的String( 不再是StringBuffer本身了 ) public class Demo5_StringBuffer { public static void main(String[] args) { StringBuffer sb = new StringBuffer(); sb.append("Hello"); String s = sb.substring(3); System.out.println(sb); System.out.println(s); String s2 = sb.substring(0,2); System.out.println(s2); } } StringBuffer 与 String 转换 String -> StringBuffer 构造方法通过append()方法 StringBuffer -> String 通过构造方法通过toString() 通过substring(0,length); public class Demo6_StringBuffer { public static void main(String[] args) { //String -> StringBuffer StringBuffer sb = new StringBuffer("See You Tomorrow"); StringBuffer sb2 = new StringBuffer(); sb2.append("HaHa"); // StringBuffer -> String StringBuffer sb3 = new StringBuffer("F Word"); String s = new String(sb3); String s2 = sb3.toString(); String s3 = sb3.substring(0,sb3.length()); } } 使用 StringBuffer 指定格式重新拼接成新的字符串,char[] c = {1,2,3}, 转换成[1, 2, 3] 使用 StringBuffer 中的方法 public class Demo7_StringBuffer { public static void main(String[] args) { int [] arr = {1,2,3}; StringBuffer sb = new StringBuffer(); if( arr.length <= 1){ sb = sb.append("[").append(sb).append("]"); }else if ( arr.length > 1 ){ for (int i=0; i<arr.length; i++) { if( 0 == i ){ sb = sb.append("[").append(arr[i]); } else if ( i > 0 && i != (arr.length-1)){ sb = sb.append(", ").append(arr[i]); } else if( i == arr.length-1) { sb = sb.append(", ").append(arr[i]).append("]"); } } } System.out.println(sb); } } StringBuffer 作为参数传递和 String 作为参数传递基本数据类型传递, 不改变其值. 引用数据类型传递改变其值 StringBuffer 作为参数传递 String 作为参数传递 public class Demo8_StringBuffer { public static void main(String[] args) { String s = "Hello"; System.out.println(s); change(s); System.out.println(s); StringBuffer sb = new StringBuffer(s); System.out.println(sb); change(sb); System.out.println(sb); } public static void change(String s){ s = s + "hello"; return ; } public static void change(StringBuffer sb){ sb.append(" Wrold!"); } } 冒泡排序 public class Demo9_StringBuffer { public static void main(String[] args) { int [] arr = {1, 100, 200, 30, 1, 0}; int temp = 0; sortArray(arr); printArray(arr); } public static int[] sortArray(int [] array){ int temp = 0; for (int i=0;i<array.length-1;i++){ for(int j=array.length-1;j > i;j--){ if(array[j] < array[j-1]){ temp = array[j]; array[j] = array[j-1]; array[j - 1] = temp; } } } return array; } public static void printArray(int [] array){ for (int i=0;i<array.length;i++){ System.out.println(array[i]); } } } 选择冒泡排序 public class Demo2_SortArray { public static void main(String[] args) { int [] arr = { 1,22, 30,3, 100}; selectArray(arr); printArray(arr); } public static int[] selectArray(int [] arr){ for (int i=0;i<arr.length;i++){ for (int j=i;j<arr.length;j++){ if(arr[i] > arr[j]) { int temp = arr[i]; arr[i] = arr[j]; arr[j] = temp; } } } return arr; } public static void printArray(int [] array){ for (int i=0;i<array.length;i++){ System.out.println(array[i]); } } } 二分查找 public static int search(int [] arr, int t){ int index = -1; int min = 0; int max = arr.length; int mid = arr.length/2; int count = 0; while ( count <= Math.log(arr.length)/Math.log(2)){ if(arr[mid] == t){ return mid; } else if (arr[mid] < t ){ min = mid + 1; mid = (min + max)/2; } else if (arr[mid] > t){ max = mid - 1; mid = (min + max)/2; } count ++; } return index; } Array 类的排序和二分查找 void sort(int[] a) 排序, 快速排序 int binarySearch(int[] a, int key) 在数组中查找key, 找到返回索引位置, 没找到则返回插入-位置-1 String toString(int[] a) 数组转成String 输出 public class Demo4_SortArray { public static void main(String[] args) { int [] arr = { 15, 100, 1 , 2, 3, 15, 20, 30, 50}; Arrays.sort(arr); System.out.println(Arrays.toString(arr)); int k1 = Arrays.binarySearch(arr, 20); System.out.println(k1); int k2 = Arrays.binarySearch(arr, 200); System.out.println(k2); } } Integer parseInt 方法字符串和数字转换 inter 最大值最小值 2 进制 8进制 16进制转换 public class Demo1_Integer { public static void main(String[] args) { int aInt = Integer.parseInt("190"); System.out.println(Integer.toBinaryString(aInt)); System.out.println(Integer.toHexString(aInt)); System.out.println(Integer.toOctalString(aInt)); Integer bInt = new Integer("180"); System.out.println(bInt); System.out.println(bInt.toString()); System.out.println(Integer.MAX_VALUE); System.out.println(Integer.MIN_VALUE); } } int 和 String 转换 public class Demo2_Integer { public static void main(String[] args) { // int -> String int a = 160; String s1 = a + ""; String s2 = String.valueOf(a); Integer i3 = new Integer(a); String s3 = i3.toString(); String s4 = Integer.toString(a); // String -> int String str = "200"; int a1 = Integer.parseInt(str); Integer ig2 = new Integer(str); int a2 = ig2.intValue(); } } Java 基本数据类型自动装箱和拆箱基本数据类型才可以自动装箱拆箱 Integer Boolean 自动装箱拆箱时需要判断对象不为 byte范围内-128到127内, 不会创建新的对象, 而是从常量池中获取. public class Demo3_Integer { public static void main(String[] args) { Integer i = 200; System.out.println(i); int a = i + 100; System.out.println(a); Integer a2 = 127; Integer a3 = 127; Integer a4 = 128; Integer a5 = 128; System.out.println(a2 == a3); System.out.println(a4 == a5); System.out.println(a2.equals(a3)); System.out.println(a4.equals(a5)); Integer a7 = new Integer(127); Integer a8 = new Integer(127); System.out.println(a7 == a8); // 自动装箱用的Integer.valueOf方法 Integer a9 = Integer.valueOf(127); Integer a10 = Integer.valueOf(127); System.out.println(a9 == a10); // Integer -> int Integer iObject = Integer.valueOf(3); int a11 = iObject.intValue(); System.out.println(a11); } } // 200 300 T F T T F T 3

Java的String类 Li.033

Scanner hasNext() 表示判断下一个输入项是否是字符串, hasNextInt() 表示判断下一个输入项是否是int, 否则返回false next()获取输入项的字符串, nextInt 获取项输入的int import java.util.Scanner; public class Demo1_Scanner { public static void main(String[] args) { Scanner scanner = new Scanner(System.in); if( scanner.hasNextInt()) { int p = scanner.nextInt(); System.out.println(p); } else { System.out.println("输入内容不是int 数字"); } scanner.close(); } } 输入1个整数, 1个数字串, 输出数字和字符串, 错误示例 import java.util.Scanner; public class Demo2_Scanner { public static void main(String[] args) { // 输入1个整数, 1个字符串的错误示例 Scanner sc = new Scanner(System.in); int i = sc.nextInt(); String s = sc.nextLine(); System.out.println(i+" "+s); } } 输入1个整数, 1个数字串, 输出数字和字符串, 正确示例, 都当成一行字符串, 后期String转成int类型 import java.util.Scanner; import java.lang.Integer;; public class Demo2_Scanner { public static void main(String[] args) { Scanner sc = new Scanner(System.in); String s1 = sc.nextLine(); int i = Integer.parseInt(s1); String s2 = sc.nextLine(); System.out.println(s1+ s2); } } String 类 toString()方法, 没重写前显示包和类名@hashcode() 常量池的string, 没有则创建, 有则使用现有的string 使用new String()则会创建常量的副本 public class Demo2_String { public static void main(String[] args) { String s1 = new String("abc"); String s2 = "abc"; System.out.println(s1 == s2); System.out.println(s1.equals(s2)); } } //结果 false ture 使用+号连接的, 是在StringBuffer中连接后, 再把地址指给变量 public class Demo2_String { public static void main(String[] args) { String s4 = "ab"; String s5 = s4 + "c"; System.out.println(s4 == s5); } } // false String 其他方法 startWith endWith equalsIgnoreCase contains public class Demo3_String { public static void main(String[] args) { String sm = "123 sdf z"; String s1 = "123"; String s2 = "z"; String s3 = "123 SDF z"; String s4 = "sdf"; String s5 = "sDf"; System.out.println(sm.startsWith(s1)); System.out.println(sm.endsWith(s2)); System.out.println(sm.equalsIgnoreCase(s3)); System.out.println(sm.contains(s4)); System.out.println(sm.contains(s5)); System.out.println(sm.toLowerCase().contains(s4.toLowerCase())); } } // T T T T F T String练习, 模拟用户登录 import java.util.Scanner; public class Demo4_String { public static void main(String[] args) { Scanner sc = new Scanner(System.in); String name = ""; String passwd = ""; for(int i=0;i<3;i++){ System.out.println("请输入用户名:"); if( sc.hasNextLine() ){ name = sc.nextLine(); } System.out.println("请输入用户密码:"); if( sc.hasNextLine() ){ passwd = sc.nextLine(); } if( "admin".equals(name) && "admin".equals(passwd) ){ System.out.println("欢迎"+ name + "登录!"); break; } else { if( (2-i)==0 ){ System.out.println("请找回密码, 或过1小时后再重试 !"); } else{ System.out.println("用户名或密码错误, 您还有" + (2-i) + "次机会!"); } } name = ""; passwd = ""; } sc.close(); } } String 的方法 char charAt(int index) 获取指定索引位置的字符 int length() 获取字符串长度 int indexOf(int ch) 获取字母在字符串第一次出现的索引 int indexOf(String str) 获取字符串第一次出现的出现的索引位置 int indexOf( int ch, int fromIndex ) 从指定索引位置向后, 字母ch 第一次出现的索引位置 int indexOf( String str, int fromInex ) 从指定索引位置向后, str 第一次出现的索引位置 int lastIndexOf(int ch) 最后一次出现字符的索引, 即从后向前找第1次出现字母的索引 int lastIndexOf(int ch, int fromIndex) 从指定索引向前找, 字素ch 出现的索引 int lastIndexOf(String str) int lastIndexOf(String str, int fromIndex) String substring(int beginIndex) 从索引位置开始到结尾截取字符串 String substring(int beginIndex, int endIndex) 从开始索引到结束索引位置前(结束索引不包含) 截取字符串 public class Demo5_String { public static void main(String[] args) { String sm = "Hello Wrold!"; System.out.println(sm.charAt(0)); //System.out.println(sm.charAt(20)); System.out.println(sm.length()); System.out.println(sm.indexOf('l')); System.out.println(sm.indexOf('z')); System.out.println(sm.indexOf("Wrold")); System.out.println(sm.indexOf("Wrood")); System.out.println(sm.indexOf('o',5)); System.out.println( sm.indexOf( "old", 5 ) ); } } public class Demo6_String { public static void main(String[] args) { String s = "ET drive UFO! So COOL"; System.out.println(s.lastIndexOf('O')); System.out.println(s.lastIndexOf('O',17)); System.out.println(s.lastIndexOf("UFO")); System.out.println(s.lastIndexOf("So", 9)); } } public class Demo7_String { public static void main(String[] args) { String s = "PhpIsBest?"; System.out.println(s.substring(3)); System.out.println(s.substring(0,3)); } } String 练习, 统计大写, 小写, 数字, 其他字符个数 public class Demo9_String { public static void main(String[] args) { String s = "C C++ C# Go Java Javascript Php Swift 2021"; int sumUp = 0; int sumLow = 0; int sumNu = 0; int sumOther = 0; for(int i=0;i<s.length();i++){ char tmpChar = s.charAt(i); if( tmpChar <= 'z' && tmpChar >= 'a'){ sumLow++; } else if( tmpChar <= 'Z' && tmpChar >= 'A' ){ sumUp++; } else if( tmpChar <= '9' && tmpChar >= '0' ){ sumNu++; } else if( ' ' == tmpChar ){ } else { sumOther++; } } System.out.println(sumUp+"," + sumLow + "," + sumNu + "," + sumOther); } } String 的方法 byte[] getBytes() 字符串转换为字节数组 char[] toCharArray() 字符串转换为字符数线 String valueOf(char[] data) 字符数组转化为字符串 String valueOf(int i) 把 int 转换为字符串, (valueOf 可以任意类型的数据转化为字符串) String toLowerCase() 字符串转换成小写 String toUpperCase() 字符串转换大写 concat(String str) 字符串拼接 public class Demo10_String { public static void main(String[] args) { String s = "Hello"; byte [] b = s.getBytes(); for(int i=0;i<b.length;i++){ System.out.print(b[i]+" "); } System.out.println(); char[] c = s.toCharArray(); for(int i=0;i<c.length;i++){ System.out.print(c[i] + " "); } System.out.println(); String s2 = String.valueOf(c); System.out.println(s2); String s3 = String.valueOf(10); System.out.println(s3); System.out.println(s.toLowerCase()); System.out.println(s.toUpperCase()); System.out.println(s.concat(" Wrold!")); } } String 练习, 首字母大写, 其他小写 public class Demo11_String { // 字符串首写字母大写, 其他字母小写 public static void main(String[] args) { String s = "what Is A Youth?"; if(s.length() > 0){ System.out.println(s.substring(0,1).toUpperCase() + s.substring(1).toLowerCase()); } } } String 练习, int [] arr = {1,2,3}; 转换成指定格式 {1, 2, 3}的字符串. public class Demo12_String { // int [] arr = {1,2,3}; 转换成指定格式 {1, 2, 3}的字符串. public static void main(String[] args) { int [] arr = {1,2,3,4,5}; String s = ""; if( 1 == arr.length ){ s = "{"+ arr[0]+"}"; } if( arr.length > 1 ){ for (int i=0; i<arr.length; i++) { if ( 0 == i ){ s = "{" + arr[i] + ", "; } else if( i < arr.length - 1 ){ s = s + arr[i] + ", "; } else if( i == arr.length-1 ){ s = s + arr[i] + "}"; } } } System.out.println(s); } } String 中的方法 String replace(char oldChar, char newChar) 用新字符替换所有旧的字符 String replace(CharSequence target, CharSequence replacement) 使用新字符串替换旧的字符串 int compareTo(String anotherString) 比较2个字符串 int compareToIgnoreCase(String str) 忽略大小写比较2个字符串 public class Demo13_String { public static void main(String[] args) { String s = "Switch Mario"; String s2 = s.replace('i','a'); System.out.println(s2); String s3 = s.replace("Mario", "Zelda"); System.out.println(s3); String s4 = " switch one "; System.out.println("{"+s4.trim()+"}"); String s5 = "sEE"; String s6 = "see"; int sum = s5.compareTo(s6); System.out.println(sum); int sum2 = s5.compareToIgnoreCase(s6); System.out.println(sum2); } } String 练习反转字符串 import java.util.Scanner; public class Demo14_String { //反转字符串 public static void main(String[] args) { Scanner sc = new Scanner(System.in); String s = ""; String s2 = ""; if( sc.hasNextLine() ){ s = sc.nextLine(); } char [] cs = s.toCharArray(); for (int i=0; i<(cs.length/2); i++) { char temp = cs[cs.length-1-i]; cs[cs.length-1-i] = cs[i]; cs[i] = temp; } System.out.println(new String(cs)); char [] cs2 = s.toCharArray(); for (int i=cs2.length-1; i>=0; i--) { s2 = s2 + cs2[i]; } System.out.println(s2); } } String 练习, 查找str在另一个str出现的次数 public class Demo15_String { public static void main(String[] args) { String sMax = "Whatever is worth doing is worth doing well."; String sMin = "worth"; int sum = 0; int j = 0; int index = 0; while(index<sMax.length()){ j = sMax.indexOf(sMin, index); if(-1 == j){ break; } else{ index = j + 1; sum++; } } System.out.println(sum); } }

Java重写equals方法 Li.032

未重写前equals是比较引用的地址, 如比较2个对象, 地址一定是不同的重写后, 可以比较对象的属性相同, 则认为对象相同 public class Demo1_Object { private int age; private String name; Demo1_Object(){} Demo1_Object(int age,String name){ this.age = age; this.name = name; } public static void main(String[] args) { Demo1_Object d = new Demo1_Object(12, "xiaoming"); Demo1_Object d2 = new Demo1_Object(12, "xiaohong"); Demo1_Object d3 = new Demo1_Object(12, "xiaoming"); System.out.println(d.equals(d2)); System.out.println(d.equals(d3)); } public boolean equals(Object obj){ // 重写Object 类中equals方法 if( obj instanceof Demo1_Object ){ Demo1_Object demo = (Demo1_Object)obj; return this.name.equals(demo.name) && this.age == demo.age; } return false; } public boolean equals(Demo1_Object dobj){ //这个不是重写, 是重载 return this.name.equals(dobj.name) && this.age == dobj.age; } public String toString(){ return (age + name); } }

Java面向对象 Li.031

Package 包只能类的首行 com.lizica.java 默认编译是不会创建包的文件夹, 加入参数 -d .才会创建运行类的方式就变了, 必须带上包的路径才的运行类 javac -d . Demo1_Package.java java com.lizicai.java.Demo1_Package package com.lizicai.java; public class Demo1_Package { public static void main(String[] args) { System.out.println("hello"); } } 引用时候3种方式, 推荐导入具体的类,第2种 class Demo2_Package{ public static void main(String[] args) { com.lizicai.java.Demo1_Package d = new com.lizicai.java.Demo1_Package(); d.sprint(); } } import com.lizicai.java.Demo1_Package; class Demo2_Package{ public static void main(String[] args) { Demo1_Package d = new Demo1_Package(); d.sprint(); } } import com.lizicai.java.*; class Demo2_Package{ public static void main(String[] args) { Demo1_Package d = new Demo1_Package(); d.sprint(); } } 不同类修饰符和包访问关系类修饰符本类同一个包下(子类和无关系) 不同包下(子类) 不同包下(无关系) private 默认 protected public Java 修饰符总结修饰符权限修饰符：private，默认的，protected，public 状态修饰符：static，final 抽象修饰符：abstract 类的修饰符权限修饰符：默认修饰符，public ...