unchecked cast from object to E

[sx2woshinidie]

package DateStruct;


public class stack<E> {
	private Object[] a;
	private int top = -1;
	public stack(int size){
		a = new Object[size];
		//ArrayList a = new ArrayList();
	}
	public void push(E i) throws Exception{
		if(isFull() == false){
			top++;
			a[top] = i;
		}else{
			throw new Exception("stack is full");
		}
	}
	
	public E pop() throws Exception{
		if(isEmpty() == false){
			top--;
			return (E) a[top+1]; //就是出现警告的地方。
		}else{
			throw new Exception("stack is empty");
		}
	}
	private boolean isEmpty(){
		if(top < 0)return true;
		else return false;
	}
	private boolean isFull(){
		if(top+1 > a.length)return true;
		else return false;
	}
	public int getNumbers(){
		return top+1;
	}
}

小弟菜鸟。
想问问各位大大。为什么这里会出现这个。而在jdk自己实现的arraylist里面也是强转成E却没有出现