java-子类的访问变量（给定超类）来自另一个类

Question

Suppose I have 2 classes ClassBottom1 and ClassBottom2 , that extends classes ClassMiddle1 and ClassMiddle2 respectively, both of which extends another class ClassTop . I have 2 separate classes ClassConstruct and ClassObserve . ClassObserve extends KeyAdapter .

ClassConstruct:

LinkedList<ClassMiddle1> bottom1 = new LinkedList<ClassMiddle1>();
LinkedList<ClassMiddle2> bottom2 = new LinkedList<ClassMiddle2>();

Variables bottom1 and bottom2 contain a list of instances of ClassBottom1 and ClassBottom2 respectively.

Suppose ClassBottom1 has a boolean variable called boolVar evaluating to false . How would I access this variable from:

ClassObserver:

private ClassConstruct construct;

public ClassObserver(ClassConstruct construct) {
    this.construct = construct;
}

public void keyPressed(KeyEvent e) {
    for (int i = 0; i < construct.bottom1.size(); i++) {
        construct.bottom1.get(i).boolVar = true; // throws an error, as boolvar does't exist in class (ClassMiddle1)
    }
}

How would I solve this problem?

Thank you for your time.

Answer 1

You need to cast it if you are sure that you have the implementation you need. But basically the compiler cannot know from the declaration that you ALWAYS will have boolVar there (if you would always have it it would be in the parent class).

If you need a specific property and ALL your list items will have a specific type then maybe you should create a list of ClassBottom1 and ClassBottom2 to enforce that only objects having that type and that property are inserted. Otherwise it is not enforced and you can get to error.

That's why we have generics in the first place. Otherwise it would be just lists of Objects (since everything is an object).

Answer 2

Just putting in code, what other comments have suggested.

public void keyPressed(KeyEvent e) {
    for (int i = 0; i < construct.bottom1.size(); i++) {
        ClassMiddle1 m1 = construct.bottom1.get(i);
        if(m1 instanceof ClassBottom1)
            ((ClassBottom1)m1).boolVar = true; 
    }
}

Answer 3

You just have access to your variable boolVar directly from an instance of ClassBottom1 or a subclass of ClassBottom1 .

Casting won't be a solution because your ClassMiddle is not an instance of ClassBottom1 or a subclass of that.

// will throw a ClassCastException
((ClassBottom1) construct.bottom1.get(i)).boolVar = true;

Maybe, you can redeclare your list as LinkedList<ClassBottom1> or rethink about your hierarchy? Then you will be able to access your variable boolVar .