java - 关于重载方法的泛型查询

Question

我正在阅读来自 JavaDocs 的这个例子在第一个示例中，文档说它将调用 setData(Object) 方法，但是当我尝试代码时它从不调用该方法，而是调用 setData(Integer data)方法。

但是当我实现这个例子时它给出了运行时异常

java.lang.ClassCastException at n.setData("Hello");

public class Node<T> {
    public T data;

    public Node(T data) {
        this.data = data;
    }

    public void setData(T data) {
        System.out.println("Node.setData");
        this.data = data;
    }
}

public class MyNode extends Node<Integer> {
    public MyNode(Integer data) {
        super(data);
    }

    public void setData(Integer data) {
        System.out.println("MyNode.setData");
        super.setData(data);
    }
}

但是文档说:

MyNode mn = new MyNode(5);
Node n = mn;            // A raw type - compiler throws an unchecked warning
n.setData("Hello");     
Integer x = mn.data;    // Causes a ClassCastException to be thrown.

ClassCast 异常将出现在第 4 行。它不会那样发生。

Answer 1

发生的事情已经在同一教程中进行了解释。

• n.setData("Hello"); causes the method setData(Object) to be executed on the object of class MyNode. (The MyNode class inherited setData(Object) from Node.)
• In the body of setData(Object), the data field of the object referenced by n is assigned to a String.
• The data field of that same object, referenced via mn, can be accessed and is expected to be an integer (since mn is a MyNode which is a Node<Integer>.
• Trying to assign a String to an Integer causes a ClassCastException from a cast inserted at the assignment by a Java compiler.

所以，如您所见，这就是您所经历的。尝试将字符串分配给整数时，方法调用会导致异常。

令人困惑的是代码中的注释可能放错了地方。它试图告诉您代码不会到达第 4 行，因为到那一点将抛出异常。但是，如果注释在实际抛出异常的第 3 行，会更清楚。