用Java实现一个动态数组

package com.shixianjun;

@SuppressWarnings("unchecked")
public class ArrayList {
    
    private static final int DEFAULT_CAPACITY = 10;
    private static final int ELEMENT_NOT_FOUND = -1;
    
    private int size;
    private E[] elements;
    
    public ArrayList(){
        this(DEFAULT_CAPACITY);
    }
    
    public ArrayList(int capacity){
        capacity = capacity < DEFAULT_CAPACITY ? DEFAULT_CAPACITY : capacity;
        elements = (E[]) new Object[capacity];
    }
    
    
    /// 数组大小
    public int size() {
        return size;
    }
    
    /// 是否为空
    public boolean isEmpty() {
        return size == 0;
    }
    
    /// 是否包含某元素
    boolean contains(E element) {
        return indexOf(element) != ELEMENT_NOT_FOUND;
    }
    
    /// 添加元素
    void add(E element) {
        this.add(size, element);
    }
    
    /// 返回index位置对应的元素
    E get(int index) {
        this.rangeCheck(index);
        return elements[index];
    }
    
    /// 替换index位置的元素
    E replace(int index, E element) {
        this.rangeCheck(index);
        E oldE = elements[index];
        elements[index] = element;
        return oldE;
    }
    
    // 向index位置添加元素
    void add(int index, E element) {
        this.rangeCheckForAdd(index);
        this.ensureCapacity(size + 1);
        
        for (int i = size; i > index; i--) elements[i] = elements[i - 1];
        
        elements[index] = element;
        size++;
    }
    
    /// 删除index位置的元素
    E remove(int index) {
        this.rangeCheck(index);
        E oldE = elements[index];
        for (int i = index + 1; i < size; i++) {
            elements[i-1] = elements[i];
        }
        elements[--size] = null;
        return oldE;
    }
    
    /// 查看元素的位置
    int indexOf(E element) {
        if (element == null) {
            for (int i = 0; i < size; i++) {
                if (elements[i] == null) return i;
            }
        }else {
            for (int i = 0; i < size; i++) {
                if (element.equals(elements[i])) return i;
            }
        }
        return ELEMENT_NOT_FOUND;
    }
    
    /// 清除所有元素
    void clear() {
        for (int i = 0; i < size; i++) elements[i] = null;
        size = 0;
    }
    
    private void rangeCheck(int index) {
        if (index < 0 || index >= size) outOfBounds(index);
    }
    
    private void rangeCheckForAdd(int index) {
        if (index < 0 || index > size) outOfBounds(index);
    }
    
    private void outOfBounds(int index) {
        throw new IndexOutOfBoundsException("index:" + index + ", size:" + size);
    }
    
    private void ensureCapacity(int capacity) {
        int oldCapaticy = elements.length;
        if (oldCapaticy >= capacity) return;
        
        int newCapacity = oldCapaticy + oldCapaticy << 2;
        E[] newElements = (E[]) new Object[newCapacity];
        for (int i = 0; i < size; i++) {
            newElements[i] = elements[i];
        }
        
        elements = newElements;
        
        System.out.println(oldCapaticy + "扩容为" + newCapacity);
    }
}