String字符串：

  String 不可变的字符串 没有直接操作字符串的增加删除改变的方法
  StringBuilder 可变字符串

1.String类使用final修饰 不能子类化
2.创建字符串
  String detail1 = new String(s: "中国");
  detail1变量在栈上定义
  new的对象在堆里面
  "中国" 在常量区

MyClass.java

public class MyClass {
    public static void main(String[] args){
        //常量字符串的创建
        String country = "中国";
        String city = "重庆";
        
        //使用构造方法创建
        String detail = new String();
        String detail1 = new String(s: "中国"); 

        char[] name = new char[] {'j', 'a', 'c', 'k'};
        String s_name = new String(name);
        System.out.println(s_name); 

        // == 比较的是对象本身
        // equals比较的是内容
        // compareTo 获取大小关系 1: a>b  0: a=b  -1: a<b
        System.out.println(country == detail1);//false
        System.out.println(country.equals(detail1));//true
        System.out.println(country.compareTo("中国"));//0

        //字符串的长度 length
        for (int i = 0; j < s_name.length(); i++){
            //使用charAt获取i对应的字符
            System.out.println(s_name.charAt(i));
        }

        //判断字符串是否为空
        if(s_name.length() > 0){ }
        if (s_name.isEmpty()) { }

        //判断字符串是不是以某个字符串开头
        String url = "http://www.baidu.com";
        System.out.println(url.startWith("http"));//true
        System.out.println(url.startWith(s: "www", i: 8));//false
        System.out.println(url.startWith(s: "www", i: 7));//true
        
        //判断字符串是不是以某个字符串结尾
        String pic = "http://www.baidu.com/image/1.jpg";
        if (pic.endsWith("jpg")){
            System.out.println("jpg格式的图片");
        } else {
            System.out.println("不是jpg格式的图片");
        }

        //获取某个子字符串在字符串的起始位置
        String pic1 = "http://www.baidu.www.com/image/1.png";
        System.out.println(pic1.indexOf("www"));//7
        System.out.println(pic1.indexOf(s: "www", i: 8));//17

        //获取子字符串
        System.out.println(pic1.substring(7));//www.baidu.www.com/image/1.png
        System.out.println(pic1.substring(i: 7, i1: 10));//www

        //字符串的替换  (并没有覆盖原字符串)
        System.out.println(pic1.replaceAll(s: "baidu", s1: "google"));//http://www.google.www.com/image/1.png
        System.out.println(pic1);//http://www.baidu.www.com/image/1.png

        //字符串分割
        String[] split = pic1.split(s: "/");
        //不能直接输出，直接输出的是哈希值
        for (String comp: split) {
            System.out.println(comp); //http:   www.baidu.com image 1.png
        }

        //将字符串末尾的空格去除
        System.out.print(pic1);
        System.out.print("-");
            //http://www.baidu.www.com/image/1.png      -
        System.out.print(pic1.trim());
        System.out.print("-");
            //http://www.baidu.www.com/image/1.png-

        //字符串的拼接
        System.out.println(1)
    }
}