P465 StringBuffer与StringBuilder

    @Test
    public void test1(){
        StringBuffer sb1 = new StringBuffer("abc");
        sb1.setCharAt(0,'m');//StringBuffer没有返回，所以可变
        System.out.println(sb1);

        StringBuffer sb2 = new StringBuffer();
        System.out.println(sb2.length());//0，返回的是里面到底有几个，不是value.length
    }
}

    @Test
    public void test2(){
        StringBuffer s1=new StringBuffer("abc");
        s1.append(1);
        s1.append('1');
        System.out.println(s1);
 //       s1.delete(2,4);//左闭右开，删掉的是c1
 //       s1.replace(2,4,"hello");
 //       s1.insert(2,false);
 //       s1.reverse();
        String s2 = s1.substring(1, 3);//要重新接收
        System.out.println(s1);
        System.out.println(s1.length());//把false看成5个字符
        System.out.println(s2);

    }

 
    @Test
    public void test3(){
        //初始设置
        long startTime = 0L;
        long endTime = 0L;
        String text = "";
        StringBuffer buffer = new StringBuffer("");
        StringBuilder builder = new StringBuilder("");
        //开始对比
        startTime = System.currentTimeMillis();
        for (int i = 0; i < 20000; i++) {
            buffer.append(String.valueOf(i));
        }
        endTime = System.currentTimeMillis();
        System.out.println("StringBuffer的执行时间：" + (endTime - startTime));
        startTime = System.currentTimeMillis();
        for (int i = 0; i < 20000; i++) {
            builder.append(String.valueOf(i));
        }
        endTime = System.currentTimeMillis();
        System.out.println("StringBuilder的执行时间：" + (endTime - startTime));
        startTime = System.currentTimeMillis();
        for (int i = 0; i < 20000; i++) {
            text = text + i;
        }
        endTime = System.currentTimeMillis();
        System.out.println("String的执行时间：" + (endTime - startTime));
    }