Collection的<T> T[] toArray(T[] a) 性能优化

from：http://landerrooter.iteye.com/blog/725652

最近在使用PMD进行代码缺陷扫描时,有一类问题PMD称为"call to Collection.toArray() may be optimizable"
下面一行为问题代码:

result = (IResearch[]) list.toArray(new IResearch[0]);

PMD对这类问题给出的解决方案如下:

Java代码

class Foo {

void bar(Collection x) {

// A bit inefficient

x.toArray(new Foo[0]);

// Much better; this one sizes the destination array, avoiding

// a reflection call in some Collection implementations

x.toArray(new Foo[x.size()]);

}

}

再看下ArrayList的<T> T[] toArray(T[] a) 源码:
Java代码

public <T> T[] toArray(T[] a) {

if (a.length < size)

a = (T[])java.lang.reflect.Array.

newInstance(a.getClass().getComponentType(), size);

System.arraycopy(elementData, 0, a, 0, size);

if (a.length > size)

a[size] = null;

return a;

}

你就发现如果采用大家常用的把a的length设为0,就需要反射API来创建一个大小为size的数组,而这对性能有一定的影响.

所以最好的方式就是直接把a的length设为Collection的size从而避免调用反射API来达到一定的性能优化.

Java代码

import java.util.ArrayList;

publicclass ToArrayTest {

publicstaticvoid main(String[] args) {

ArrayList al = new ArrayList();

for (int i = 0; i < 10; i++) {

al.add(String.valueOf(i));

}

String[] s1 = (String[]) al.toArray(new String[15]);

for (int i = 0; i < s1.length; i++) {

System.out.println(i+","+s1[i]);

}

String[] s2 = (String[]) al.toArray(new String[5]);

for (int i = 0; i < s2.length; i++) {

System.out.println(i+","+s2[i]);

}

System.out.println("OptimizableToArrayCall");

String[] s3 = (String[]) al.toArray(new String[al.size()]);

for (int i = 0; i < s3.length; i++) {

System.out.println(i+","+s3[i]);

}

}

}