ArrayList源码解读

1.创建ArrayList

List<String> list = new ArrayList<>();
list.add("wang");

2.构造方法：

elementData的长度就是ArrayList的容量，在第一次使用时，elementData的长度会扩展到10

/**
 * 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.
 */
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.
 */
transient Object[] elementData; // non-private to simplify nested class access

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

/** * Constructs an empty list with an initial capacity of ten. */ public ArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; }

3.add方法/**     * Appends the specified element to the end of this list.

     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
　　　　　　扩容
        ensureCapacityInternal(size + 1);  // Increments modCount!!
　　　　　　插入元素
        elementData[size++] = e;
        return true;
    }

  扩容
private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }

private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

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

    /**
     * Increases the capacity to ensure that it can hold at least the
     * number of elements specified by the minimum capacity argument.
     *
     * @param minCapacity the desired minimum capacity
     */
    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);
    }

Arrays的copyOf()方法传回的数组是新的数组对象，改变传回数组中的元素值，不会影响原来的数组。

copyOf()的第二个自变量指定要建立的新数组长度，如果新数组的长度超过原数组的长度，则保留数组默认值，例如：

import java.util.Arrays;

public class ArrayDemo {
    public static void main(String[] args) {
        int[] arr1 = {1, 2, 3, 4, 5}; 
        int[] arr2 = Arrays.copyOf(arr1, 5);
        int[] arr3 = Arrays.copyOf(arr1, 10);
        for(int i = 0; i < arr2.length; i++) 
            System.out.print(arr2[i] + " "); 
            System.out.println();
        for(int i = 0; i < arr3.length; i++) 
            System.out.print(arr3[i] + " ");
    }
} 

运行结果：

1 2 3 4 5 
1 2 3 4 5 0 0 0 0 0