1.什么是字符串

• 使用双引号引住的任意个字符，称做字符串。
  例如

    public static void main(String[] args) {
        String name ="hello";
        String pass ="buzhidao";
    }

2.字符串分类

java创建字符串有两种方式

• 字面量

 1String str ="haha";

• 字符串对象

String str = new String("haha");

3.字面量与字符串对象的区别

字面量

        String str1 = "yaya";
        String str2 = "yaya";
        System.out.println(str1 == str2);
    }

1657850970602.png

字符串对象

    public static void main(String[] args) {
        String str1 = new String("yaya");
        String str2 = new String("yaya");
        System.out.println(str1 == str2);
    }

1657851594591.png

4.String类常用的API

字符串连接 concat

    public static void main(String[] args) {
        String firstName = "李";
        String lastName = "逵";
        String fullName = firstName.concat(lastName);
        System.out.println(fullName);//李逵
    }

字符串相等比较 equals

    public static void main(String[] args) {
        String firstName = "李";
        String lastName = "李";
        //判断firstName和lastName指向同一块内存吗，比较的是值，不是内容
        boolean result1 = firstName.equals(lastName);
        boolean result2 = firstName == lastName;
        System.out.println(result1);//true
        System.out.println(result2);//true
    }

字符串忽略大小写相等比较 equalsIgnoreCase

    public static void main(String[] args) {
        String firstName = "li";
        String lastName = "Li";
        //判断firstName和lastName指向同一块内存吗，比较的是值，不是内容
        boolean result1 = firstName.equalsIgnoreCase(lastName);
        System.out.println(result1);//true
    }

字符串长度length（）

数组的大小是属性：arr.length
字符串的大小是方法：str.length（）

    public static void main(String[] args) {
        String username = "admin";
        int length = username.length();
        System.out.println(length); // 5
    }

字符串全转换大写或者小写toUpperCase，toLowerCase

    public static void main(String[] args) {
        String word = "aoHaYgaoZayiMskongbaWAATAXIWA";
        System.out.println(word.toUpperCase());
        System.out.println(word);
    }

运行结果：AOHAYGAOZAYIMSKONGBAWAATAXIWA

  public static void main(String[] args) {
        String word = "aoHaYgaoZayiMskongbaWAATAXIWA";
        System.out.println(word.toLowerCase());
        System.out.println(word);
    }

运行结果：aohaygaozayimskongbawaataxiwa

切割字符串

判断”admin@itlaobing.com“ 这个邮箱中是否只包含一个@符合

方法1：

public static void main(String[] args) {
        String mail = "admin@itlaobing@.com";
        int index1 = mail.indexOf("@");
        int index2 = mail.lastIndexOf("@");
        System.out.println(index1);//5
        System.out.println(index2);//15
        if((index1 == index2)&& index1!=-1 && index2!=-1){
            System.out.println("合法");
        }else{
            System.out.println("不合法");
        }
    }

方法2：

    public static void main(String[] args) {
        String mail = "admin@itlaobing.com";
        String[] split = mail.split("@");
        if(split.length==2){
            System.out.println("合法");
        }else{
            System.out.println("不合法");
        }
    }

判断是否以什么开头，什么结尾（startsWith ，endsWith）

    public static void main(String[] args) {
        String website = "www.itlaobing.cn";
        System.out.println(website.startsWith("www"));
    }

    public static void main(String[] args) {
        String website = "www.itlaobing.cn";
        System.out.println(website.endsWith("com"));
    }

替换字符串内容 replace

    public static void main(String[] args) {
        String str = "今天我吃了一个坏蛋黄派";
        str = str.replace("坏蛋","**");
        System.out.println(str);
    }

结果：今天我吃了一个**黄派

截取子串 substring

    public static void main(String[] args) {
        String file = "d:\\2002-07-09\\javaapi\\jdk_api_1.6_zh_cn.chm";
        int startIndex  = file.lastIndexOf("\\")+1;
        int dotIndex = file.lastIndexOf(".");
        String filename = file.substring(startIndex, dotIndex);
        System.out.println(filename);
    }

截取字符 charAt

    public static void main(String[] args) {
        String str = "admin@123456";
        char ch = str.charAt(5);
        int index = str.indexOf("@");
        System.out.println(ch);
        System.out.println(index);
    }