String+,StringBuilder,String.format运行效率比较

时间:2021-07-18 21:26:27

实现String字符串相加的方法有很多,常见的有直接相加,StringBuilder.append和String.format,这三者的运行效率是有差异的,String是final类型的,每次相加都会new一个

新的String对象,如果这种操作很多的话,很占用很大的内存。而StringBuilder.append方法是在原对象上进行操作,如果长度不够就自行扩展。

测试代码1:

<span style="white-space:pre">	</span>String success_code = "0";

	byte splite = 0x01;

	private void method1(){
String resultMsg = "";
long time1 = System.nanoTime();
String.format("ErrorCode=%s%cErrorMsg=心跳包接收成功%c", success_code, splite, splite);
long time2 = System.nanoTime();
System.out.println("StringFormat:"+(time2-time1)+"ns"); long time3 = System.nanoTime();
resultMsg = "ErrorCode="+success_code+splite+"ErrorMsg=心跳包接收成功"+splite;
long time4 = System.nanoTime();
System.out.println("String add:"+(time4-time3)+"ns"); long time5 = System.nanoTime();
sb.append("ErrorCode=").append(success_code).append(splite).append("ErrorMsg=心跳包接收成功").append(splite);
long time6 = System.nanoTime();
System.out.println("StringBuilder add:"+(time6-time5)+"ns");
System.out.println("-------------------------------------------------");
} @Test
public void test1(){
for(int i=0; i<1000; i++){
method1();
}
}

运行结果:

StringFormat:58025ns
String add:3158ns
StringBuilder add:1579ns
-------------------------------------------------
StringFormat:43026ns
String add:3948ns
StringBuilder add:1974ns
-------------------------------------------------
.....
StringFormat:46973ns
String add:1579ns
StringBuilder add:790ns
-------------------------------------------------
StringFormat:52499ns
String add:1578ns
StringBuilder add:790ns
-------------------------------------------------
StringFormat:43026ns
String add:1579ns
StringBuilder add:790ns
-------------------------------------------------

从上述结果可知,StringBuilder与String直接相加的执行效率都比String.format高, 而StringBuilder的执行效率要比String直接相加要高点。下面针对String,StringBuilder再

做一组测试。

测试代码2:

<span style="white-space:pre">	</span>public void method2(int num){
String text = "";
long beginTime = System.nanoTime();
for(int i = 0; i < num; i++){
text += i;
}
long endTime = System.nanoTime();
System.out.println("String直接相加"+num+"次耗费时间:" + (endTime - beginTime)+"ns");
StringBuilder builder = new StringBuilder("");
beginTime = System.nanoTime();
for(int i = 0; i < num; i++){
builder.append(i);
}
endTime = System.nanoTime();
System.out.println("StringBuilder相加"+num+"次耗费时间:" + (endTime - beginTime)+"ns");
System.out.println("---------------------------------------------------");
} @Test
public void test2(){
method2(10);
method2(100);
method2(10000);
method2(100000);
}

运行结果:

String直接相加10次耗费时间:19737ns
StringBuilder相加10次耗费时间:3553ns
---------------------------------------------------
String直接相加100次耗费时间:56447ns
StringBuilder相加100次耗费时间:47762ns
---------------------------------------------------
String直接相加10000次耗费时间:266082677ns
StringBuilder相加10000次耗费时间:999061ns
---------------------------------------------------
String直接相加100000次耗费时间:45212528095ns
StringBuilder相加100000次耗费时间:3040604ns
---------------------------------------------------

从测试结果可分析出,StringBuilder的效率是比String高。

再来看一个测试,代码和上面的一样,只是SringBuilder加上个toString

测试代码:

String success_code = "0";

	byte splite = 0x01;

	private void method1(){
String resultMsg = "";
long time1 = System.nanoTime();
resultMsg = String.format("ErrorCode=%s%cErrorMsg=心跳包接收成功%c", success_code, splite, splite);
long time2 = System.nanoTime();
System.out.println("StringFormat:"+(time2-time1)+"ns"); long time3 = System.nanoTime();
resultMsg = "ErrorCode="+success_code+splite+"ErrorMsg=心跳包接收成功"+splite;
long time4 = System.nanoTime();
System.out.println("String add:"+(time4-time3)+"ns"); long time5 = System.nanoTime();
resultMsg = sb.append("ErrorCode=").append(success_code).append(splite).append("ErrorMsg=心跳包接收成功").append(splite).toString();
long time6 = System.nanoTime();
System.out.println("StringBuilder add:"+(time6-time5)+"ns");
System.out.println("-------------------------------------------------");
}
<span style="white-space:pre">	</span>@Test
public void test1(){
for(int i=0; i<10; i++){
method1();
}
}

运行结果:

StringFormat:564859ns
String add:55657ns
StringBuilder add:3158ns
-------------------------------------------------
StringFormat:98683ns
String add:2368ns
StringBuilder add:1974ns
-------------------------------------------------
StringFormat:69867ns
String add:2369ns
StringBuilder add:1974ns
-------------------------------------------------
StringFormat:77762ns
String add:3552ns
StringBuilder add:2369ns
-------------------------------------------------
StringFormat:105788ns
String add:3948ns
StringBuilder add:2368ns
-------------------------------------------------
StringFormat:78552ns
String add:2763ns
StringBuilder add:1974ns
-------------------------------------------------
StringFormat:68683ns
String add:2368ns
StringBuilder add:1974ns
-------------------------------------------------
StringFormat:67894ns
String add:2369ns
StringBuilder add:1973ns
-------------------------------------------------
StringFormat:67499ns
String add:2369ns
StringBuilder add:1974ns
-------------------------------------------------
StringFormat:116840ns
String add:3948ns
StringBuilder add:3552ns
-------------------------------------------------

当运行10次时,均显示StringBuilder.append.toString的效率比String的直接相加高。

测试执行10000次,结果如下:

StringFormat:9079ns
String add:789ns
StringBuilder add:153550ns
-------------------------------------------------
StringFormat:18552ns
String add:789ns
StringBuilder add:141708ns
-------------------------------------------------
StringFormat:9078ns
String add:395ns
StringBuilder add:122761ns
-------------------------------------------------
。。。。

发现当执行10000次时,出现StringBuilder的执行效率比String低了很多,原因暂未发现。