StringBuilder几种清空方法的效率比较

分类：Java, Java语言阅读 (2,924) Add comments

6月 202016

转载请注明原文地址：http://bcoder.com/java/speed-compare-of-several-clear-method-of-stringbuilder

一共有三种方法可以实现StringBuilder的清空

New一个新的StringBuilder
使用StringBuilder的delete方法
使用StringBuilder的setLength方法

下面通过代码验证三种方法的效率

public class StringBuilderClearTest {
    public static void main(String[] args){
        long time = System.currentTimeMillis();
        System.out.println("begin: " + time);
        StringBuilder sbMain1 = new StringBuilder();
        StringBuilder sbMain2 = new StringBuilder();
        StringBuilder sbMain3 = new StringBuilder();

        StringBuilder sb1 = null;
        for(int i=0; i<1000000; i++){
            sb1 = new StringBuilder("a1");
            sb1.append("a2");
            sb1.append("a3");
            sbMain1.append(sb1);
        }

        long time1 = System.currentTimeMillis();
        StringBuilder sb2 = new StringBuilder();
        for(int i=0; i<1000000; i++){
            sb2.delete(0, sb2.length());
            sb2.append("b1");
            sb2.append("b2");
            sb2.append("b3");
            sbMain2.append(sb2);
        }

        long time2 = System.currentTimeMillis();
        StringBuilder sb3 = new StringBuilder();
        for(int i=0; i< 1000000; i++){
            sb3.setLength(0);
            sb3.append("c1");
            sb3.append("c2");
            sb3.append("c3");
            sbMain3.append(sb3);
        }

        long time3 = System.currentTimeMillis();


        System.out.println(sbMain1.length());
        System.out.println(sbMain2.length());
        System.out.println(sbMain3.length());

        System.out.println("New耗时： " + (time1 - time));
        System.out.println("delete耗时： " + (time2 - time1));
        System.out.println("setLength： " + (time3 - time2));
    }
}

public class StringBuilderClearTest {

public static void main(String[] args){

long time = System.currentTimeMillis();

System.out.println("begin: " + time);

StringBuilder sbMain1 = new StringBuilder();

StringBuilder sbMain2 = new StringBuilder();

StringBuilder sbMain3 = new StringBuilder();

StringBuilder sb1 = null;

for(int i=0; i<1000000; i++){

sb1 = new StringBuilder("a1");

sb1.append("a2");

sb1.append("a3");

sbMain1.append(sb1);

}

long time1 = System.currentTimeMillis();

StringBuilder sb2 = new StringBuilder();

for(int i=0; i<1000000; i++){

sb2.delete(0, sb2.length());

sb2.append("b1");

sb2.append("b2");

sb2.append("b3");

sbMain2.append(sb2);

}

long time2 = System.currentTimeMillis();

StringBuilder sb3 = new StringBuilder();

for(int i=0; i< 1000000; i++){

sb3.setLength(0);

sb3.append("c1");

sb3.append("c2");

sb3.append("c3");

sbMain3.append(sb3);

}

long time3 = System.currentTimeMillis();

System.out.println(sbMain1.length());

System.out.println(sbMain2.length());

System.out.println(sbMain3.length());

System.out.println("New耗时： " + (time1 - time));

System.out.println("delete耗时： " + (time2 - time1));

System.out.println("setLength： " + (time3 - time2));

}

测试了十几次，选了一次比较有代表性的结果：

New耗时 90
delete耗时： 73
setLength： 63

New耗时 90

delete耗时： 73

setLength： 63

由结果可见setLength是最快的，delete不相上下，new是最慢的

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