String Builder

1.定义：一种比String更灵活，长度可变的字符串类

2.构造方法

package com.ljh.StringBuilderDemo;

/**
 * @Author: ljh
 * @Date: 2022/1/8 21:24
 * @Description:
 */
public class StringBuilderDemo {
    public static void main(String[] args) {
        //构造方法,无参构造方法，长度为0
        StringBuilder stringBuilder = new StringBuilder();
        System.out.println(stringBuilder.length());//0
        //有参构造方法
        StringBuilder stringBuilder1 = new StringBuilder("hello !");
        System.out.println(stringBuilder1.length());//7(空格算字符)
    }
}

3.常用方法

package com.ljh.StringBuilderDemo;

/**
 * @Author: ljh
 * @Date: 2022/1/8 21:24
 * @Description:
 */
public class StringBuilderDemo {
    public static void main(String[] args) {

        StringBuilder stringBuilder = new StringBuilder();
        //添加方法
        stringBuilder.append("hello");
        stringBuilder.append("world");
        System.out.println(stringBuilder);//helloworld
        //该方法使用时返回其自身，因此可以链式编程
        stringBuilder.append("for").append("world");
        System.out.println(stringBuilder);//helloworldforworld
        //倒序字符串
        stringBuilder.reverse();
        System.out.println(stringBuilder);//dlrowrofdlrowolleh
        }

}

4.String和StringBuilder的转换

package com.ljh.StringBuilderDemo;

/**
 * @Author: ljh
 * @Date: 2022/1/8 21:24
 * @Description:
 */
public class StringBuilderDemo {
    public static void main(String[] args) {
        //1.把StringBuilder转为String用toString（）方法
        StringBuilder stringBuilder = new StringBuilder();
        //添加方法
        stringBuilder.append("hello");
        String s = stringBuilder.toString();
        System.out.println(s);//hello
        //2.把String转为StringBuilder用构造方法
        String s1 ="233";
        StringBuilder stringBuilder1 = new StringBuilder(s1);
        System.out.println(stringBuilder1);//233

    }

}

5.练习

需求:定义一个方法,把int数组中的数据按照指定的格式拼接成一个字符串返回,调用该方法,并在控制台
输出结果。例如，数组为int[ arr= {1,2,3};, 执行方法后的输出结果为: [1,2, 3]

package com.ljh.StringBuilderDemo;

/**
 * @Author: ljh
 * @Date: 2022/1/9 16:00
 * @Description: 练习1
 */
public class StringBuilderDemo02 {
    public static void main(String[] args) {
        int[] arr = {1,2,3};
        System.out.println(show(arr));

    }

    public static String show(int[] ints){
        StringBuilder stringBuilder = new StringBuilder("[");
        for (int i=0;i<ints.length;i++){
            if (i==ints.length-1){
                stringBuilder.append(ints[i]);
            }
            else{
                stringBuilder.append(ints[i]).append(",");
            }
        }
        stringBuilder.append("]");
        return stringBuilder.toString();

    }
}

需求:定义一个方法，实现字符串反转。键盘录入一个字符串，调用该方法后，在控制台输出结果
例如，键盘录入abc,输出结果cba

package com.ljh.StringBuilderDemo;

import java.util.Scanner;

/**
 * @Author: ljh
 * @Date: 2022/1/9 16:11
 * @Description:练习2
 */
public class StringBuilderDemo03 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String s = scanner.next();
        System.out.println(getString(s));
        scanner.close();

    }
    public static String getString(String s){
        StringBuilder stringBuilder = new StringBuilder(s);
        stringBuilder.reverse();
        return stringBuilder.toString();
    }
}