java实现动态数组

public class Array<E> {
    private E[] data;
    private Integer size;
 
    
    public Array(Integer capacity) {
        // 在java里不能直接创建一个泛型数组，只能这样转换
        data = (E[]) new Object[capacity];
        size = 0;
    }
 
    public Array() {
        this(20);// 默认容量为20
    }
 
    
    public E get(Integer key) throws Exception {
        if (key >= data.length || key < 0 || key >= size)
            throw new Exception("该索引非法");
        return data[key];
    }
    
    
    public void insert(Integer key, E value) throws Exception {
        this.resize(true);// 要在key检查之前执行
        this.checkKey(key, false);
        // 后移一位
        for (Integer i = size - 1; i >= key; i--)
            data[i + 1] = data[i];
        data[key] = value;
        size++;
    }
 
    
    public void delete(Integer key) throws Exception {
        checkKey(key, true);
        for (Integer i = key; i < size - 1; i++)
            data[i] = data[i + 1];
        // 实际上是通过覆盖的方式删除掉key索引的值，但是最后还需要删除掉size-1索引的值
        data[size - 1] = null;
        size--;
        this.resize(false);
    }
 
    
    private void checkKey(Integer key, Boolean equal) throws Exception {
        if (equal) {
            if (key >= data.length || key < 0 || key >= size)
                throw new Exception("该索引非法");
        } else {
            if (key >= data.length || key < 0 || key > size)
                throw new Exception("该索引非法");
        }
    }
 
    
    private void resize(Boolean isExpand) {
        if (size == data.length && isExpand) {
            //如果数组已经满了，那么就扩容为当前容量的1.5倍
            E[] buffer = (E[]) new Object[size + (size / 2)];
            for (Integer i = 0; i < size; i++)
                buffer[i] = data[i];
            data = buffer;
        } else if (size < data.length / 4 && (!isExpand) && data.length / 2 != 0) {
            //如果数组的size小于容量的四分之一，则容量降低一半
            E[] buffer = (E[]) new Object[data.length / 2];
            for (Integer i = 0; i < size; i++)
                buffer[i] = data[i];
            data = buffer;
        }
    }
}