ArrayList 源码笔记

ArrayList 源码

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相关常量

• DEFAULT_CAPACITY ：10
• MAX_ARRAY_SIZE : Integer.MAX_VALUE - 8
• Object[] EMPTY_ELEMENTDATA
• Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA
• Object[] elementData：存储数据的数组

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构造方法

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);
    }
}

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

public ArrayList(Collection 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;
        }
    }

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扩容方法

private void grow(int minCapacity) {
    // overflow-conscious code
    int oldCapacity = elementData.length;
    //新容量为旧容量加上旧容量除2，即新容量是旧容量的1.5倍
    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:
    //这里底层调用的是 Systems.arraycopy() 这是个native方法
    elementData = Arrays.copyOf(elementData, newCapacity);
}

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常用方法

public E get(int index) {
    //检查索引是否在范围内，其实就是简单的判断index是不是大于size，是的话抛出越界异常。
    rangeCheck(index);
    //返回elementData数组在该索引的值
    return elementData(index);
}

public E set(int index, E element) {
    rangeCheck(index);
    E oldValue = elementData(index);
    elementData[index] = element;
    return oldValue;
}

public boolean add(E e) {
    //检查是否超出elementData数组的长度，是的话进行扩容
    ensureCapacityInternal(size + 1);  // Increments modCount!!
    elementData[size++] = e;
    return true;
}

public void add(int index, E element) {    
    rangeCheckForAdd(index);
    ensureCapacityInternal(size + 1);  // Increments modCount!!
    //将index及之后的数据整体向后移动一位
    System.arraycopy(elementData, index, elementData, index + 1,size - index);  
    elementData[index] = element;    
    size++;    
}
//按照索引删除
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);
    //设置null是方便垃圾收集器可以回收内存
    elementData[--size] = null; // clear to let GC do its work
    return oldValue;    
}
//是上面方法的快速版本，因为不用检查边界，这意味着越界也不会报异常
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    
}

//删除第一个匹配的对象
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;
}
//删除 [fromIndex,toIndex)内的元素
protected void removeRange(int fromIndex, int toIndex) {        
    modCount++;    
    int numMoved = size - toIndex;    
    System.arraycopy(elementData, toIndex, elementData, fromIndex,numMoved);        
    // clear to let GC do its work      
    //元素指向null后方便垃圾收集器清理
    int newSize = size - (toIndex-fromIndex);    
    for (int i = newSize; i < size; i++) {    
        elementData[i] = null;        
    }        
    size = newSize;    
}