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:
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:
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.
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]);
}
}
}