源碼的解讀邏輯按照程序運行的軌跡展開

1. Arraylist的繼承&實現關系

   
   
   
   

   打開ArrayList源碼，會看到有如下的屬性定義，

2. ArrayList中定義的屬性

    /**
     * Default initial capacity. 
     * 初始容量
     */
    private static final int DEFAULT_CAPACITY = 10;

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

    /**
     * Shared empty array instance used for default sized empty instances. We
     * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
     * first element is added.
     * 默認容量的空數組
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    /**
     * 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 == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     * will be expanded to DEFAULT_CAPACITY when the first element is added.
     * 真正存放對象的數組
     */
    transient Object[] elementData; 

     /**
     * The size of the ArrayList (the number of elements it contains).
     * 實際數據的數量
     * @serial
     */
    private int 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
     * Integer 最大值
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

當運行 ArrayList<Integer> list = new ArrayList<>() ; ，因為它沒有指定初始容量，所以它調用的是它的無參構造

//無參構造，
public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
// 指定初始容量
public ArrayList(int initialCapacity) {
    if (initialCapacity > 0) {
        this.elementData = new Object[initialCapacity];//創建數組
    } else if (initialCapacity == 0) {
        this.elementData = EMPTY_ELEMENTDATA;
    } else {
        throw new IllegalArgumentException("Illegal Capacity: "+                                     initialCapacity);
    }
}

當我們僅僅new出一個ArrayList時，它僅僅只會創建一個空數組，由此我們可以得知它的初始化操作被延遲到了第一次add()

    //添加一個元素
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }
    
    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }
    public static int max(int a, int b) {
        return (a >= b) ? a : b;
    }
//判斷是否要擴容,minCapacity的值大于add數據之前的大小，就調用grow方法，進行擴容，否則什么也不做
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;
        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);//擴充capacity,將其向右一位再加上原來的數，實際上是擴充了1.5倍
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)//確保數組的容量不大于Integer的最大值
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);//復制
    }

對源碼閱讀有問題的可以把以下代碼復制自行運行，這是一個簡版的ArrayList,是我從JDK源碼中抽取出來的，理解下面的代碼再去看JDK的源碼相信就很簡單了

package com.tanoak.list.arraylist;

import java.io.Serializable;
import java.util.Arrays;
import java.util.Collection;

/**
 * @Desc  自定義ArrayList集合類， 基于數組實現
 */
public class TkArrayList<E> implements Serializable {


    /**
     *
     * 初始容量
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * 空數組
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * 默認容量的空數組
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    /**
     * 真正存放數據的數組
     */
     transient Object[] elementData;

    /**
     * 實際數據的數量
     */
    private int size;

    /**
     * 記錄了ArrayList結構性變化的次數
     */
    protected transient int modCount = 0;


    /**
     * Integer 最大值
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;


    public TkArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }


    /**
     * 指定數組大小
     * @param initialCapacity
     */
    public TkArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                    initialCapacity);
        }
    }

    /**
     * 構造一個包含指定元素的list，這些元素的是按照Collection的迭代器返回的順序排列的
     * @param c
     */
    public TkArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            if (elementData.getClass() != Object[].class){
                elementData = Arrays.copyOf(elementData, size, Object[].class);
            }
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }
    //增
    /**
     *  新增元素
     * @param e
     * @return
     */
    public boolean add(E e) {
        // Increments modCount!!
        ensureCapacityInternal(size + 1);
        elementData[size++] = e;
        return true;
    }

    /**
     *
     * @param minCapacity
     */
    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }

    /**
     * 判斷是否擴容
     * @param minCapacity
     */
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

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

    }
   //進行擴容
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;

        //擴充capacity,將其向右一位再加上原來的數，實際上是擴充了1.5倍
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0){
            newCapacity = minCapacity;
        }
        //確保數組的容量不大于Integer類型最大值
        if (newCapacity - MAX_ARRAY_SIZE > 0){
            newCapacity = hugeCapacity(minCapacity);
        }
        // //復制數據
        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;
    }
    //查
    /**
     * 根據索引 調用 elementData 返回值
     * @param index
     * @return
     */
    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }

    /**
     * 根據索引取出值
     * @param index
     * @return
     */
    E elementData(int index) {
        return (E) elementData[index];
    }

    private void rangeCheck(int index) {
        if (index >= size){
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
        }
    }

    /**
     * 越界信息
     * @param index
     * @return
     */
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

    //刪
    /**
     *
     * @param index
     * @return
     */
    public E remove(int index) {
        rangeCheck(index);
        modCount++;
        E oldValue = elementData(index);

        int numMoved = size - index - 1;
        if (numMoved > 0){
            System.arraycopy(elementData, index+1, elementData, index, numMoved);
        }
        // clear to let GC do its work
        elementData[--size] = null;

        return oldValue;
    }
    
    //改
    public E set(int index, E element) {
        rangeCheck(index);

        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }
}

ArrayList比較難理解的就是擴容，思路首先理清楚，但是只要理清楚幾個屬性在方法中所做的判斷，然后運行上面簡版的源碼，多熟悉幾次就不成問題了

• 如理解有誤，請指正