ArrayList源码详解

r囧r小猫 2022-05-18 08:23 192阅读 0赞

##### 成员变量的声明 #####

// 成员变量的声明
    private static final Object[] EMPTY_ELEMENTDATA = {};
    transient Object[] elementData;//存储元素的数组
    private static final int DEFAULT_CAPACITY = 10;

##### 构造方法初始化 #####

//初始数组的大小为传入的initialCapacity
    public ArrayList(int initialCapacity) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
    }
    
     //默认的构造方法
    public ArrayList() {
        super();
        //将空的Object对象数组赋予elementData,完成初始化
        this.elementData = EMPTY_ELEMENTDATA;
    }

##### add方法 #####

//添加元素
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;//赋值并且size+1
        return true;
    }
    private void ensureCapacityInternal(int minCapacity) {
        //首次插入数据，数组的大小为10，如果传入的值超过10，则最小容量值设置为传入值
        if (elementData == EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }
    
        ensureExplicitCapacity(minCapacity);
    }
    
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;
    
        // 如果传入的值比之前数组的长度大，需要扩容
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        //新的容量值变为原来的1.5倍
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        //如果新的容量值还不够，将传入的值赋值给新容量值
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // 不得不说，Arrays.copyOf的实现方法才是真正扩容的核心啊
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

##### get方法 #####

//按下标访问数组的值
    public E get(int index) {
        rangeCheck(index);
    
        return elementData(index);
    }
    E elementData(int index) {
        return (E) elementData[index];
    }

##### set方法 #####

//对下标为index的元素做替换操作
    public E set(int index, E element) {
        rangeCheck(index);
    
        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

##### remove方法 #####

public E remove(int index) {
        rangeCheck(index);
    
        modCount++;
        E oldValue = elementData(index);
    
        int numMoved = size - index - 1;
        if (numMoved > 0)
            // 把后面的往前移
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        //最后的置为空，等待GC
        elementData[--size] = null; // clear to let GC do its work
        //返回值为原来位置上的元素
        return oldValue;
    }