java - 哪一个是迭代特定集合元素的最佳方式？

Question

在遍历特定集合 的元素时，我总是使用for each 循环。

只是为了检查每个循环过程消耗了多少时间，我这样编码，

public class LoopingDemo {
    static ArrayList<String> arrayList = new ArrayList<String>();
    static double start;
    static double end;

    public static void iterator() {
        simpleForLoop();
        System.out.println();
        forEachLoop();
        System.out.println();
        useWhileLoop(arrayList);
        System.out.println();
        useForLoop(arrayList);
        System.out.println();
        enumerator();
    }
    public static void simpleForLoop(){
        start = System.nanoTime();
        for(int i=0;i<arrayList.size();i++){
            String str = arrayList.get(i);
            System.out.println(": "+str);
        }
        end = System.nanoTime();
        System.out.println("Taken taken in simpleForLoop process: "
            + (end - start));
    }

    public static void forEachLoop() {
        start = System.nanoTime();
        for (String str : arrayList) {
        System.out.println(str);
    }
        end = System.nanoTime();
        System.out.println("Taken taken in forEachLoop process: "
            + (end - start));
    }

    public static void enumerator() {
        start = System.nanoTime();

        // get the Enumeration object
        Enumeration<String> en = Collections.enumeration(arrayList);

        // enumerate through the ArrayList elements
        System.out.println("Enumerating through Java ArrayList");

        while (en.hasMoreElements()) {
            System.out.println(en.nextElement());
            /*
             * String name = (String) en.nextElement();
             * System.out.println(name);
             */
        } 
        end = System.nanoTime();
        System.out.println("Taken taken in enumeration process: "
            + (end - start));
    }

    private static void useWhileLoop(Collection<String> myList) {
        start = System.nanoTime();
        Iterator<String> itr = myList.iterator();
        while (itr.hasNext()) {
            String str = itr.next(); // Returns the next element in the
                                    // iteration.
            System.out.println(str);
            // System.out.println(itr.next()); // in one line
        }
        end = System.nanoTime();
        System.out.println("Taken taken in useWhileLoop process: "
            + (end - start));
    }

    /**
     * Note that this for-loop does not use an integer index.
     */
    private static void useForLoop(Collection<String> myList) {
        start = System.nanoTime();
        for (Iterator<String> itr = myList.iterator(); itr.hasNext();) {
            System.out.println(itr.next());
        }
        end = System.nanoTime();
        System.out.println("Taken taken in useForLoopWithIterator process: "
            + (end - start));
    }

    public static void addElements() {

        // Add elements to the array list.
        arrayList.add("C");
        arrayList.add("A");
        arrayList.add("E");
        arrayList.add("B");
        arrayList.add("D");
        arrayList.add("F");
        arrayList.add(1, "A2");

    }

    public static void main(String[] args) {
        addElements();
        iterator();

    }

}

令人惊讶的是，通过for each 循环完成的循环过程仅落后于简单的for 循环。(结果可能因具有不同配置的不同机器而异。)

控制台输出:

Taken taken in simpleForLoop process:              853200.0
Taken taken in forEachLoop process:                788993.0
Taken taken in useWhileLoop process:               452014.0
Taken taken in useForLoopWithIterator process:     299775.0
Taken taken in enumeration process:                766756.0

那么为什么人们更喜欢通过 for each 循环来完成它？是否有任何基于性能的原因？

Answer 1

IIRC，for-each 循环只是一个语法糖，在遍历集合时将被编译为带有迭代器的 for 循环。在运行时应该没有性能差异。

您看到的差异是基准测试逻辑编写不当的典型结果。如果你把“for loop with iterator”作为第一个调用的方法，你会发现它运行缓慢。

且不说每次测试的持续时间太短，意义不大，在我添加了一些热身后(先运行所有东西5次，然后看第6次的结果)，结果变得正常:

Taken taken in useForLoopWithIterator process: 105110.0
Taken taken in simpleForLoop process: 122181.0
Taken taken in useWhileLoop process: 104774.0
Taken taken in enumeration process: 123520.0
Taken taken in forEachLoop process: 106782.0

forEachloop、useForLoopWithIterator 和 useWhileLoop 的结果几乎相同(它们应该是相同的)。遍历集合时基于索引的访问几乎是最不受欢迎的。虽然这里的差异并不显着，但如果您使用非基于数组的集合，您会看到更大的差异，例如链表。