StringBuffer和StringBuilder

import org.junit.Test;

/*
String是不可变的字符串序列，涉及到字符串的改变，会创建大量的对象

StringBuffer:是线程安全的，效率低,底层是采用char类型的数组存储数据，char数组默认长度是16

StringBuilder:是线程不安全的，效率高，底层是采用char类型的数组存储数据，char数组默认长度是16

共同点:他们的父亲都是AbstractStringBuilder抽象类

* */
public class Demo8 {
    @Test
    public void test1(){
        String s ="";
        //StringBuffer sbf = new StringBuffer("从前有座山");
        //StringBuffer sbf2 = sbf.append("1").append("2").append(3);

        StringBuilder sbf = new StringBuilder("从前有座山");
        StringBuilder sbf2 = sbf.append("1").append("2").append(3);
        System.out.println("sbf2 = " + sbf2);

        int length=sbf2.length();//sbf2 = 从前有座山123
        System.out.println("length = " + length);//length = 8

        sbf2.setLength(10);
        System.out.println(sbf2);//从前有座山123  ;这有两个空格
        int length2=sbf2.length();
        System.out.println("length2 = " + length2);//length2 = 10

        StringBuffer sbf3 = new StringBuffer("从前有座山山里有座庙");
        //返回元素第一次出现的下标
        int idx1=sbf3.indexOf("山");
        System.out.println("idx1 = " + idx1);//idx1 = 4
        //返回元素最后出现的下标
        int idx2=sbf3.lastIndexOf("山");
        System.out.println("idx2 = " + idx2);//idx2 = 5

        //将指定元素添加到指定位置
        sbf3.insert(6,"那边");
        System.out.println(sbf3);//从前有座山山那边里有座庙

        //删除指定元素[开始下标,结束下标)
        sbf3.delete(1,3);
        System.out.println(sbf3);//从座山山那边里有座庙

        //删除指定位置的字符
        sbf3.deleteCharAt(0);
        System.out.println(sbf3);//座山山那边里有座庙

        //设置指定位置的字符
        sbf3.setCharAt(0,'一');
        System.out.println(sbf3);//一山山那边里有座庙
    }
}

代码2

package com.atguigu.day15;

public class Demo9 {
    public static void main(String[] args) {
        testString();
        testStringBuffer();
        testStringBuilder();
    }

    private static void testStringBuilder() {
        long start=System.currentTimeMillis();
        StringBuilder sbd=new StringBuilder("");
        for (int i = 0; i <10000 ; i++) {
            sbd.append(i);
        }
        long end =System.currentTimeMillis();
        System.out.println("sbd用时:"+(end-start));
    }

    private static void testStringBuffer() {
        long start=System.currentTimeMillis();
        StringBuffer sbf=new StringBuffer("");
        for (int i = 0; i <10000 ; i++) {
            sbf.append(i);
        }
        long end =System.currentTimeMillis();
        System.out.println("sbf用时:"+(end-start));
    }

    private static void testString() {
        long start=System.currentTimeMillis();
        String s1="";
        for (int i = 0; i <10000 ; i++) {
            s1+=i;
        }
        long end =System.currentTimeMillis();
        System.out.println("s1用时:"+(end-start));
    }
}

输出

s1用时:774
sbf用时:2
sbd用时:1