String和StringBuilder

3.1 StringBuilder概述

StringBuilder是一个可变的字符串类，我们可以把它看成是一个容器这里的可变指的是StringBuilder对象中的内容是可变的

String和StringBuilder的区别:

String:内容是不可变的
StringBuilder:内容是可变的

    public static void main(String[] args) {
        StringBuilder s = new StringBuilder();
        System.out.println("s:"+s);
        System.out.println("s.length"+s.length());

        StringBuilder sd = new StringBuilder("hello");
        System.out.println("sd:"+sd);
        System.out.println("sd.length:"+sd.length());
    }

StringBuilder中的方法：

练习：字符串的拼接，反转

import java.sql.SQLOutput;

public class StringBuilderDemo01 {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder();

//        StringBuilder s1 = sb.append("hello");
//        System.out.println("s1:"+s1);
//        System.out.println("sb:"+sb);
//        System.out.println(sb==s1); //存储地址相同

//
//        sb.append("I ");
//        sb.append("live ");
//        sb.append("java");
//        System.out.println(sb);
        //链式编程
        sb.append("I ").append("love ").append("java").append(100);
        System.out.println("sb:"+sb);

        //reverse():返回相反的字符序列
        sb.reverse();
        System.out.println("反转后的sb："+sb);
    }
}

public class StringBuilderDemo02 {
    public static void main(String[] args) {

        /*
        //StringBuilder转换为String
        StringBuilder sb =new StringBuilder();
        sb.append("hello");
        String s=sb.toString(); //通过toString()可以实现把StringBuilder转化为String
        System.out.println(s);
        */
        String s = "hello";
        //通过构造的方法就可以实现把String转化为StringBuilder
        StringBuilder sb = new StringBuilder(s);
        System.out.println(sb);


    }
}

StringBuilder的应用

字符串的拼接（升级版）

public class StringBuilderTest01 {
    public static void main(String[] args) {
        int[] a = {1, 2, 3};

        String s = arrayToSting(a);
        System.out.println("s:"+s);
    }

    public static String arrayToSting(int[] arr) {
        StringBuilder sb = new StringBuilder();
        sb.append("[");
        for (int i = 0; i < arr.length; i++){
            if(i<arr.length-1){
                sb.append(arr[i]).append(", ");
            }
            else{
                sb.append(arr[i]);
            }
        }
        sb.append("]");
        String s = sb.toString();  //把结果转化为String
        return s;
    }
}

字符串的反转（升级版）（StringBuilder）

从接盘上获取一个字符串，用StringBuilder中的Reverse（）的方法把字符串反转
思路：
先把String转化为StringBuilder
然后用StringBuilder中的反转方法Reverse（）
最后把StringBuilder转化为String

import java.util.Scanner;

public class StringBuilderDemo03 {
    public static void main(String[] args) {
        System.out.println("请输入一个字符串");
        Scanner scanner = new Scanner(System.in);
        String s = scanner.nextLine();

        String reverse = Reverse(s);
        System.out.println(reverse);


    }

    public static String Reverse(String s) {
//        StringBuilder sb = new StringBuilder(s);  //String转化为StringBuilder
//        StringBuilder reverse = sb.reverse();   //反转
//        String s2 = reverse.toString();  //StringBuilder转化为String
//        return s2;

        return new StringBuilder(s).reverse().toString();
    }
}