分析輪子（一）-ArrayList.java

時間 2019-11-06

標籤分析輪子 arraylist.java arraylist java 欄目 Java 简体版

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前言：以前也看過一些JDK源碼，不過沒有留下痕跡，經久年月就淡忘了，如今的時機也差很少了，想再看一次，而且記錄下來本身的感想，因而從本身使用最多最熟悉的地方開始！而且看的過程當中，我但願本身思考一下以下的問題：數組

1：若是讓我設計，我怎麼玩？數據結構

2：原做者爲何這樣設計？app

3：它的底層實現是什麼玩意？有什麼特色？dom

4：平時使用的時候，須要注意些什麼東西？函數

5：原設計有什麼特別好玩的點？性能

注：玩的是JDK1.7版本ui

一：先上類的繼承結構圖this

細細看上圖，其實就能透露出一些關鍵的信息，好比：spa

1：實現Serializable接口，表示ArrayList是可序列化的設計

2：實現RandomAccess接口，表示ArrayList是可快速隨機訪問的

3：實現Cloneable接口，表示ArrayList是能夠調用Object.clone方法的

4：實現Iterable接口，表示容許ArrayList使用迭代遍歷的方式循環

二：再看一下他的底層實現數據結構

三：而後從源碼中找點好玩的東西

從源碼中能夠看出ArrayList的底層數據結構是一個數組，因此，它全部的操做都是圍繞着數組來玩的。

1）觀察以下代碼，咱們能夠了解到以下信息

private 表示不能被類外操做

transient 表示經過實現Serializable接口的方式來序列化時，被其標識的類變量能夠不被序列化

Object[] 表示ArrayList的通用性

    /**
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer. Any
     * empty ArrayList with elementData == EMPTY_ELEMENTDATA will be expanded to
     * DEFAULT_CAPACITY when the first element is added.
     */
    private transient Object[] elementData;

2）size表示數組中元素的個數，對於數組數據結構而言，數據長度至關重要

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    private int size;

3)觀察以下源碼，咱們能夠看到以下信息

ArrayList的無參構造函數，初始化的數組對象是一個空數組

ArrayList的有參構造函數，初始化的對象是根據指定的空間大小來分配數組的空間的

ArrayList是一個動態擴展的數據集合從 grow 方法中可見一二，而且當它的空間不夠的時候會擴展爲（大概是這樣他還有一些最大最小長度的判斷邏輯）：原總長度+原總長度的一半

固然ArrayList的元素的值也是有限的最大是 Integer.MAX_VALUE

    /**
     * Shared empty array instance used for empty instances.
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        super();
        this.elementData = EMPTY_ELEMENTDATA;
    }

    /**
     * Constructs an empty list with the specified initial capacity.
     *
     * @param  initialCapacity  the initial capacity of the list
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative
     */
    public ArrayList(int initialCapacity) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
    }

    /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }

    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

    /**
     * Increases the capacity to ensure that it can hold at least the
     * number of elements specified by the minimum capacity argument.
     *
     * @param minCapacity the desired minimum capacity
     */
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }

    /**
     * The maximum size of array to allocate.
     * Some VMs reserve some header words in an array.
     * Attempts to allocate larger arrays may result in
     * OutOfMemoryError: Requested array size exceeds VM limit
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

4）觀察以下源代碼，能夠看到ArrayList是能夠放null的，而且刪除元素涉及到數據的移位複製操做是比較耗費性能的

    /**
     * Removes the first occurrence of the specified element from this list,
     * if it is present.  If the list does not contain the element, it is
     * unchanged.  More formally, removes the element with the lowest index
     * <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
     * (if such an element exists).  Returns <tt>true</tt> if this list
     * contained the specified element (or equivalently, if this list
     * changed as a result of the call).
     *
     * @param o element to be removed from this list, if present
     * @return <tt>true</tt> if this list contained the specified element
     */
    public boolean remove(Object o) {
        if (o == null) {
            for (int index = 0; index < size; index++)
                if (elementData[index] == null) {
                    fastRemove(index);
                    return true;
                }
        } else {
            for (int index = 0; index < size; index++)
                if (o.equals(elementData[index])) {
                    fastRemove(index);
                    return true;
                }
        }
        return false;
    }

    /*
     * Private remove method that skips bounds checking and does not
     * return the value removed.
     */
    private void fastRemove(int index) {
        modCount++;
        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work
    }