Ljava.lang.Object; 无法转换为[Ljava.lang.Integer

Question

我已经编写了一个通用类，下面是该类的构造函数。我想像这样写

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

由于我不知道运行时的泛型类型，因此它将引发异常。

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];
}
}

有什么办法解决这些问题？E的声明是

protected E[] elements = null;

这就是我要打电话的方式

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

更新 家伙，谢谢您的帮助。我在搞泛型，所以问题就被创建了。这是所有导致问题的代码-

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 ：似乎除了传递下面的答案中提到的类类型外，我们无法做到这一点。

Answer 1

这是重现异常所需的最最少的代码。

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泛型使用类型擦除来实现，因此在编译后，此html" target="_blank">代码将转换为以下内容：

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

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

显然，a new Object[]不是Integer[]。请注意演员表如何移动到您未明确放置的位置。这就是为什么(E[])new Object[size]是一个 未经检查的铸造 和显示警告信息。

相反，Object[]只有在需要将元素返回到外部世界时，才应使用并执行未经检查的强制转换。

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;
    }
}