Question

可能重复：
  Which loop has better performance? Why?
  Which is optimal ?
  Efficiency of Java code with primitive types

循环时

，例如：

for ( int j = 0; j < 1000; j++) {};我需要实例化1000个对象，当我声明循环内的对象从循环外声明它时，它有什么不同？

 for ( int j = 0; j < 1000; j++) {Object obj; obj =}

vs

Object obj; 
 for ( int j = 0; j < 1000; j++) {obj =}

很明显，只能从循环范围或从它周围的范围访问该对象。但我不了解性能问题，垃圾收集等。

最佳做法是什么？谢谢

Answer 1

第一种形式更好。限制变量的范围使读者更容易理解变量的使用位置和方式。

在性能方面，限制范围也有一些小优势，您可以在another answer.中阅读这些优点但这些问题仅次于代码理解。

Answer 2

没有区别。编译器会将它们优化到同一个地方。

Answer 3

我在我的机器上测试了这个问题，差异大约是2-4个超过10000个实例，我测试了所有类型的东西，比如你实例化并赋值：

int i=0;

与以下相比：

int i;
i=0;

这是我用于测试的代码，当然我更改了它进行测试，并且在机器达到优化之前有一个初始的平衡效果，一旦你测试就可以看清楚：

package initializer;
public final class EfficiencyTests {
private static class Stoper {
    private long initTime;
    private long executionDuration;

    public Stoper() {
    // TODO Auto-generated constructor stub
    }
    private void start() {
        initTime = System.nanoTime();
    }
    private void stop() {
        executionDuration = System.nanoTime() - initTime;
    }
    @Override
    public String toString() {
        return executionDuration + " nanos";
    }
}

private static Stoper stoper = new Stoper();

public static void main(String[] args) {
    for (int i = 0; i < 100; i++) {
        theCycleOfAForLoop(100000);
        theCycleOfAForLoopWithACallToSize(100000);
        howLongDoesItTakeToSetValueToAVariable(100000);
        howLongDoesItTakeToDefineAVariable(100000);
        System.out.println("\n");
    }
}
private static void theCycleOfAForLoop(int loops) {
    stoper.start();
    for (int i = 0; i < loops; i++);
    stoper.stop();
    System.out.println("The average duration of 10 cycles of an empty 'for' loop over " + loops + " iterations is: " + stoper.executionDuration * 10 / loops);
}
private static void theCycleOfAForLoopWithACallToSize(int loops) {
    ArrayList<Object> objects=new ArrayList<Object>();
    for (int i = 0; i < loops; i++)
        objects.add(new Object());
    stoper.start();
    for (int i = 0; i < objects.size(); i++);
    stoper.stop();
    System.out.println("The average duration of 10 cycles of an empty 'for' loop with call to size over " + loops + " iterations is: " + stoper.executionDuration * 10 / loops);

}
private static void howLongDoesItTakeToSetValueToAVariable(int loops) {
    int value = 0;
    stoper.start();
    for (int i = 0; i < loops; i++) {
        value = 2;
    }
    stoper.stop();
    System.out.println("The average duration of 10 cycles of setting a variable to a constant over " + loops + " iterations is: " + stoper.executionDuration * 10 / loops);
}
private static void howLongDoesItTakeToDefineAVariable(int loops) {
    stoper.start();
    for (int i = 0; i < loops; i++) {
        int value = 0;
    }
    stoper.stop();
    System.out.println("The average duration of 10 cycles of initializing and setting a variable to a constant over " + loops + " iterations is: " + stoper.executionDuration * 10 / loops);

}

private static void runAForLoopOnAnArrayOfObjects() {
// TODO Auto-generated method stub

}}

如果你减少另一个人的时间，你可以得出你需要多长时间......（如果你理解我的意思）

希望这可以节省你一些时间。

你需要了解的是，我测试了这些东西以优化我的平台的绘制更新循环并且它有所帮助。亚当。

java循环 - 在循环外部/内部声明类

3 个答案: