ArrayList源碼解析

1、ArrayList 屬性

// 默認容量
private static final int DEFAULT_CAPACITY = 10;
// 一個空對象
private static final Object[] EMPTY_ELEMENTDATA = {};
// 一個空對象，無參默認建立
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
// 數據對象存放的地方，不參與序列化
transient Object[] elementData;// 1.transient 不參與序列化，思考：這個變量爲何不須要序列化？是能夠經過計算獲得？顯然不是，那爲何這麼設計呢？
// 實際的長度
private int size;
// 數組最大長度
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

2、構造函數

1.無參構造函數

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

也就是說，此時的ArrayList 爲空對象，建立出來的size爲0

2.帶int類型的構造參數

public ArrayList(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);
        }
    }

3.帶Collection類型的構造參數

public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();// 淺拷貝
        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);// 深拷貝
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

由此看出，ArrayList底層就是Array數組

3、方法

1.add()方法

1.1 add(E e)

public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!  // 1.1.1 ensureCapacityInternal 確認size+1以後不超過容量
        elementData[size++] = e;// 將elementData[size] = e; size自增
        return true;
    }

1.1.1 ensureCapacityInternal

private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }

private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {//若是空數組，則返回（默認容量和數組+1以後的容量）的最大值，也就是這裏會返回默認容量10
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
        }

private void ensureExplicitCapacity(int minCapacity) {
        modCount++; // abstractList protected transient int modCount = 0; 記錄變動次數

        // overflow-conscious code
        if (minCapacity - elementData.length > 0) //容量>數組實際長度
            grow(minCapacity);
    }

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;
    }

第一次add時，elementData 長度變爲10，而後一直能夠添加，若添加到11個的時候，執行grow()方法， 將當前數組的長度變爲原來容量的1.5倍，繼續循環添加，

1.2 add(int index, E element)

public void add(int index, E element) {
        rangeCheckForAdd(index); //參數判斷，若超出數組長度或者小於0則拋出IndexOutOfBoundsException

        ensureCapacityInternal(size + 1);  // Increments modCount!!
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);// index 以後的元素都日後移動1，包括index的元素
        elementData[index] = element;// 而後將index的元素賦值爲插入的元素
        size++;
    }

private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

2.remove

2.1 remove(int index)

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);
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }

此方法不縮減數組容量

將最後一個元素置爲null，以便垃圾回收

返回被移除的元素

2.2 remove(Object o)

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 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
}

若移除，返回true；若沒有找到，返回false

 

 

 

參考文檔：

https://blog.csdn.net/fighterandknight/article/details/61240861