java set clear_Java StringBuilder 清空Clear方法

StringBuilder,StringBuffer中 没有提供clear或empty方法。 清空有3种方法： 1)新生成一个，旧的由系统自动回收 2)重用StringBuilder对象，使用delete(start,end) 3)重用StringBuilder对象，使用setLength(newLength)

将三种方法循环1000万次，代码：

public class StringBuilderClear {

static String a;

static long time;

public static void main(String[] args) throws Exception {

StringBuilder sb = new StringBuilder();

StringBuilder sb3 = new StringBuilder();

time = System.currentTimeMillis();

for (int i = 0; i < 10000000; i++) {

StringBuilder sb2 = new StringBuilder();

sb2.append("someStr6ing");

sb2.append("someS5tring2");

sb2.append("some3Strin4g");

sb2.append("so3meStr5ing");

sb2.append("so2meSt7ring");

}

System.out.println("new StringBuilder方式：" + (System.currentTimeMillis() - time)+"ms ");

time = System.currentTimeMillis();

for (int i = 0; i < 10000000; i++) {

sb.append("someString");

sb.append("someString2");

sb.append("someStrin4g");

sb.append("someStr5ing");

sb.append("someSt7ring");

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

}

System.out.println("StringBuilder。delete()方式：" + (System.currentTimeMillis() - time)+"ms -> "+sb.capacity());

time = System.currentTimeMillis();

for (int i = 0; i < 10000000; i++) {

sb3.append("someStr55ing");

sb3.append("some44String2");

sb3.append("som55eStrin4g");

sb3.append("some66Str5ing");

sb3.append("so33meSt7ring");

sb3.setLength(0);

}

System.out.println("StringBuilder.setLength方式" + (System.currentTimeMillis() - time)+"ms -> "+sb3.capacity());

}

}

注意append的字符串要都不相同，否则会因为java 的String  pool对结果造成影响(即3好于2)

结果：

new StringBuilder方式：861

StringBuilder.delete()方式：305 ms -> 70

StringBuilder.setLength方式337 ms -> 70

可以将各方法多重复几次，颠倒顺序，等 总体来看：方法2略好于方法3好于方法1

delete方法和setLength方法都是将count置为0；setLength方法只是多了一次ensureCapacityInternal(newLength)的函数调用

public AbstractStringBuilder delete(int start, int end) {

if (start < 0)

throw new StringIndexOutOfBoundsException(start);

if (end > count)

end = count;

if (start > end)

throw new StringIndexOutOfBoundsException();

int len = end - start;

if (len > 0) {

System.arraycopy(value, start+len, value, start, count-end);

count -= len;

}

return this;

}

其中

public static native void arraycopy(Object src, int srcPos,

Object dest, int destPos,

int length);

是一个本地方法。

public void setLength(int newLength) {

if (newLength < 0)

throw new StringIndexOutOfBoundsException(newLength);

ensureCapacityInternal(newLength){

// overflow-conscious code

if (minimumCapacity - value.length > 0)

expandCapacity(minimumCapacity);

}

if (count < newLength) {

Arrays.fill(value, count, newLength, '');

}

count = newLength;

}