java - Ljava.lang.Object;不能转换为 [Ljava.lang.Integer

Question

我写了一个泛型类，下面是类的构造函数。我想按照行写的那样做这样的事情

elements = (E[])new Object[size] 

因为我在运行时不知道泛型类型所以它会抛出异常。

public class Stack<E> implements IStack<E> {
protected E[] elements = null;
protected int top = -1;
protected int size= 0;
private static final int DEFAULT_CAPACITY = 10;

public Stack(){
    this(DEFAULT_CAPACITY);
}

public Stack(int size){
    if(size <0){
        throw new IllegalArgumentException("Initial capacity cannot be negative or zero");
    }
    ArrayList<Integer> ar = new ArrayList<Integer>();
    elements = (E[])new Object[size];
}
}

有什么办法可以解决这样的问题吗？ E的声明是

protected E[] elements = null;    

这就是我想要调用的方式

Random ran = new Random();
Stack<Integer> st = new Stack<Integer>();
st.push(ran.nextInt(100));

更新伙计们，谢谢你的帮助。我在搞乱泛型，所以问题就产生了。这是造成问题的所有代码 -

public class StackMain {
    public static void main(String[] args) {
        MinMaxStack minMaxStack = new MinMaxStack();
        Random ran = new Random();
        for (int k = 0; k < 10; k++) {
            minMaxStack.push(ran.nextInt(100));
        }
        System.out.println(minMaxStack);
    }
    }

public class MinMaxStack extends Stack<Integer> implements IMinMaxStack<Integer>{

private int min;
private int max;
/*
 * Approach 1:
 * For push method we can push and update the minimum/maximum value 
 * For pop method we will be traversing whole stack to find out the new minimum/maximum
 *
 */
@Override
public void push(Integer element){
    if(isEmpty()){
        this.min = element;
        this.max = element;
        elements[top+1] = element;
        size++;
    }else{
        if(element < min){
            min = element;
        }
        if(element > max){
            max = element;
        }
        elements[top+1] = element;
        size++;
    }
}
}

public  class Stack<E> implements IStack<E> {
protected E[] elements = null;
protected int top = -1;
protected int size= 0;
private static final int DEFAULT_CAPACITY = 10;

public Stack(){
    this(DEFAULT_CAPACITY);
}

public Stack(int size){
    if(size <0){
        throw new IllegalArgumentException("Initial capacity cannot be negative or zero");
    }
    elements = (E[])new Object[size];
}

public void push(E element) {
    ensureCapacity();
    elements[top+1] = element;
    size++;
}
}

public interface IStack<E> {    
public void push(E element );
}


public interface IMinMaxStack<E> extends IStack<E> {    
public int min();   
public int max();   
}

更新 2:似乎，除了下面的答案中提到的传递类类型之外，我们无法做到这一点。

Answer 1

这是重现异常所需的最少代码。

class Stack<E> {
    protected E[] elements = (E[])new Object[1];
}

class IntStack extends Stack<Integer> {
    void push(Integer i) {
        // subtly accessing elements as Integer[] which it's not
        elements[0] = i;
    }
}

Java 泛型是用 type erasure 实现的所以在编译之后，这段代码会翻译成这样:

class Stack {
    protected Object[] elements = new Object[1];
}

class IntStack extends Stack {
    void push(Integer i) {
        // throws ClassCastException
        ((Integer[])elements)[0] = i;
    }
}

显然 new Object[] 不是 Integer[]。请注意类型转换是如何移动到您没有明确放置的地方的。这就是为什么 (E[])new Object[size] 是未经检查的转换并显示警告的原因。

相反，您应该使用 Object[] 并仅在需要向外界返回元素时才执行未经检查的转换。

class Stack<E> {
    private Object[] elements;
    private int size;

    Stack(int len) {
        elements = new Object[len];
    }

    void push(E e) {
        elements[size] = e;
        size++;
    }

    E pop() {
       @SuppressWarnings("unchecked");
       E e = (E)elements[size - 1];
       size--;
       return e;
    }
}