【java容器的刻意练习】【十五】ArrayDeque的性能分析

这篇我们来看看ArrayDeque的性能分析。

之前我们在《【八】ArrayList与LinkedList的遍历》中提到，ArrayList作为数组，用for + get(i)的遍历性能是LinkedList的两倍。

那么今天我们看看ArrayList与ArrayDeque的遍历性能。

        start = System.currentTimeMillis();
        ArrayList arrayList = new ArrayList(2000000);
        for (int i=0; i < 2000000; i++){
            arrayList.add(i);
        }
        end = System.currentTimeMillis();
        System.out.println("ArrayList init 2000000 cost time = " + (end-start) + "ms");


        start = System.currentTimeMillis();
        for (int i=0; i < arrayList.size(); i++) {
            arrayList.get(i);
        }
        end = System.currentTimeMillis();
        System.out.println("for get 2000000 cost time = " + (end-start) + "ms");

ArrayList init 2000000 cost time = 75ms
for get 2000000 cost time = 10ms

        long start, end;
        int size = 2000000;

        start = System.currentTimeMillis();
        ArrayDeque arrayDeque = new ArrayDeque(size);

        for (int i=0; i < size; i++){
            arrayDeque.add(i);
        }
        end = System.currentTimeMillis();
        System.out.println("ArrayDeque init 2000000 cost time = " + (end-start) + "ms");

        start = System.currentTimeMillis();
        for (Object item : arrayDeque) {
            //System.out.println(item);
        }
        end = System.currentTimeMillis();

        System.out.println("for 1 cost time = " + (end-start) + "ms");

        start = System.currentTimeMillis();
        Iterator it = arrayDeque.iterator();
        while (it.hasNext()) {
            it.next();
        }
        end = System.currentTimeMillis();
        System.out.println("Iterator cost time = " + (end-start) + "ms");

        start = System.currentTimeMillis();
        while (arrayDeque.size() != 0) {
            arrayDeque.removeFirst();
        }
        end = System.currentTimeMillis();
        System.out.println("removeFirst cost time = " + (end-start) + "ms");

ArrayDeque init 2000000 cost time = 70ms
for 2000000 cost time = 10ms
Iterator cost time = 10ms
removeFirst cost time = 10ms

看来作为数组，两者性能都是差不多的。

那作为队列呢，ArrayDeque就要跟同样实现了Deque接口的LinkedList对比一番啦。

先来看看FIFO先进先出列队性能对比。

        start = System.currentTimeMillis();
        LinkedList linkedList = new LinkedList();
        for (int i=0; i<2000000; i++) {
            linkedList.addFirst(i);
        }
        end = System.currentTimeMillis();
        System.out.println("LinkedList addFirst 2000000 cost time = " + (end-start) + "ms");

LinkedList addFirst 2000000 cost time = 262ms

        ArrayDeque arrayDeque = new ArrayDeque();
        start = System.currentTimeMillis();
        for (int i=0; i < 2000000; i++){
            arrayDeque.addFirst(i);
        }
        end = System.currentTimeMillis();
        System.out.println("ArrayDeque addFirst 2000000 cost time = " + (end-start) + "ms");

ArrayDeque addFirst 2000000 cost time = 120ms

即使是不用预先分配空间的构造函数，而是用了ArrayDeque的自动扩容能力，还是比LinkedList快了将近一倍。在入队能力上，LinkedList还是个弟弟啊。

继续看出队能力。

LinkedList的出队：

        start = System.currentTimeMillis();
        while (linkedList.size() != 0) {
            linkedList.removeFirst();
        }
        end = System.currentTimeMillis();
        System.out.println("LinkedList removeFirst cost time = " + (end-start) + "ms");

LinkedList removeFirst cost time = 40ms

        start = System.currentTimeMillis();
        while (arrayDeque.size() != 0) {
            arrayDeque.removeFirst();
        }
        end = System.currentTimeMillis();
        System.out.println("ArrayDeque removeFirst cost time = " + (end-start) + "ms");

ArrayDeque removeFirst cost time = 10ms

ArrayDeque在出队性能上碾压LinkedList呀，都快了3倍了。也难怪，ArrayDeque就是通过Head的偏移地址直接计算到元素的内存地址，读完了就顺手置空而已。而LinkedList虽然有First引用快速找到头部，但是元素删除方面还得一通操作。

那可能有人问，FILO先进后出呢？性能也是一样的。为什么？本质上，双向链表出队都要操作节点，就导致了它比数组慢。因为ArrayDeque的出队不改变任何内存结构。

就好像哥哥写错字就画条斜线，而弟弟写错字还要用涂改液修改一番。

好了，最后我们来总结下今天的结论（在我的机器上）：

• ArrayDeque与ArrayList作为数组，性能都差不多
• ArrayDeque与LinkedList作为队列，入队快1倍，出队快3倍