StringBuffer/StringBuidler/String_string buffer buidler区别-优快云博客

本文链接：https://blog.youkuaiyun.com/Torkit/article/details/10367033

本文通过实验对比了Java中StringBuffer和StringBuilder的性能差异，重点分析了内存分配、同步机制以及创建新对象的成本对运行时间的影响。

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1.StringBuffer is synchronized, but StringBuilder not. So StringBuilder is faster than StringBuffer.

1) code: append empty string

public static void main(String[] args) {

int N = 5000000;

long t;

StringBuffer sBuffer = new StringBuffer();

t = System.currentTimeMillis();

int i = 0;

while (i++ < N)

sBuffer.append("");

System.out.println("sBuffer time: " + (System.currentTimeMillis() - t));

StringBuilder sBuilder = new StringBuilder();

t = System.currentTimeMillis();

i = 0;

while (i++ < N)

sBuilder.append("");

System.out.println("sBuilder time: " + (System.currentTimeMillis() - t));

}

Output:

sBuffer time: 227

sBuilder time: 19

2) append a space char: sBuffer.append(" ")

sBuffer time: 403

sBuilder time: 144

3)initialize Buffer size first, append space char:

StringBuffer sBuffer = new StringBuffer(N);

StringBuilder sBuilder = new StringBuilder(N);

Output:

sBuffer time: 225

sBuilder time: 19

In conclusion, StringBuilder is faster than StringBuffer many times. When buffer memory is been allocated dynamic in running time, Array.copy() time will larger than synchronized time, it lead to difference of time be smaller.

2.create a new StringBuilder object is expensive, we should always reuse the object.

code:

long
t = System.currentTimeMillis();

int
i = 0, N = 5000000;

while
(i++ < N) {

StringBuilder sb = new StringBuilder();

sb.append(
"test for something");

sb.append(
"test for something2" + i);

sb.append(
" test for something3" + i);

sb.append(
"test for something4" + i);

a = sb.toString();

}

System.out.println( "in the loop time: " + (System.currentTimeMillis() - t));

t = System.currentTimeMillis();

StringBuilder sb = new StringBuilder();

i = 0;

while
(i++ < N) {

sb.setLength(0);

sb.append(
"test for something");

sb.append(
"test for something2" + i);

sb.append(
" test for something3" + i);

sb.append(
"test for something4" + i);

}

System.out.println( " out the loop time: " + (System.currentTimeMillis() - t)

Output:

in the loop time: 4725

out the loop time: 2816