ArrayList简单理解

刺骨的言语ヽ痛彻心扉 2023-07-09 10:25 2阅读 0赞

**ArrayList的底层实现是一个Object类型数组，初始容量为10，扩容时复制当前数组并将数组容量扩展到原来的1.5倍  
特点：相对有序，可以通过下角标访问元素，不排重，可以存放null值**

transient Object[] elementData; //存放元素的Object 数组
    //默认容量空元素数据
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = { };
    private static final int DEFAULT_CAPACITY = 10;//初始容量值为10

**ArrayList构造方法之一**

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

创建ArrayList无参数对象时，调用构造方法得到elementData空数组  
**ArrayList中的add方法**

public boolean add(E e) { 
            ensureCapacityInternal(size + 1); 
            //数组中element[0]=e，即刚添加的元素，并将size的值+1
            elementData[size++] = e;
            return true;
    }
    private void ensureCapacityInternal(int minCapacity) { 
    //初次执行时将空数组和值为1的minCapacity参数传入
            ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }
    private static int calculateCapacity(Object[] elementData, int minCapacity) { 
            if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { 
                return Math.max(DEFAULT_CAPACITY, minCapacity);//返回较大的值
            }
            return minCapacity;
    }
    private void ensureExplicitCapacity(int minCapacity) { 
            modCount++;
            if (minCapacity - elementData.length > 0)
                grow(minCapacity);
    }
    private void grow(int minCapacity) { 
            int oldCapacity = elementData.length;
            int newCapacity = oldCapacity + (oldCapacity >> 1);//扩容为原来的1.5倍
            if (newCapacity - minCapacity < 0)
                newCapacity = minCapacity;
            if (newCapacity - MAX_ARRAY_SIZE > 0)
                newCapacity = hugeCapacity(minCapacity);
                //复制原来的空数组并赋予初始容量
            elementData = Arrays.copyOf(elementData, newCapacity);
    }

集合调用add方法添加元素时，逐步调用上图方法，size为当前集合中元素个数