Java速度访问数组索引与临时变量 [英] Java speed access array index versus temp variable

问题描述

Java的速度更快。直接多次访问数组索引，或者将数组索引的值保存到新变量并使用它来进行后续计算？

What is faster in Java. Accessing an array index directly multiple times, or saving the value of the array index to a new variable and use this for following compution?

访问索引

if ((shape.vertices[0].x >= fromX && shape.vertices[0].x <= toX) || // left side of shape in screen
    (shape.vertices[0].x <= fromX && shape.vertices[0].x + shape.width >= fromX) || // right side of shape in screen
    (shape.vertices[0].x >= fromX && shape.vertices[0].x + shape.width <= toX)) { // shape fully in screen

    // ...
}

临时变量

float x = shape.vertices[0].x;
float y = shape.vertices[0].y;
if ((x >= fromX && x <= toX) || // left side of shape in screen
    (x <= fromX && x + shape.width >= fromX) || // right side of shape in screen
    (x >= fromX && x + shape.width <= toX)) { // shape fully in screen

        // ...
    }

推荐答案

第二种方法肯定更快。但是你可以使用 final 关键字提供更多帮助：

The second approach is definitely faster. But you can help even more with the final keyword:

final float x = shape.vertices[0].x;
final float y = shape.vertices[0].y;
final int rightEdge = x + shape.width;
if ((x >= fromX && x <= toX) || // left side of shape in screen
(x <= fromX && rightEdge >= fromX) || // right side of shape in screen
(x >= fromX && rightEdge <= toX)) { // shape fully in screen

    // ...
}

当然不是一个显着的改进（但仍然是一个改进，也是使意图明确）。您可以阅读以下讨论： http：/ /old.nabble.com/Making-copy-of-a-reference-to-ReentrantLock-tt30730392.html#a30733348

Not a significant improvement of course (but still an improvement and also makes the intent clear). You can read this discussion: http://old.nabble.com/Making-copy-of-a-reference-to-ReentrantLock-tt30730392.html#a30733348

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