在Java的子类中使用父变量的扩展版本

Question

I have an extending class which needs to use and extended version of a variable in the parent class. So far I've been getting away with using it like this:

public class ParentClass
{
    protected ParentVariable variable;

    public ParentClass(){
        this.variable = new ParentVariable();
    }
    ....
}

public class ChildClass extends ParentClass
{
    public ChildClass(){
        super();
        this.variable = new ChildVariable();
        //Where ChildVariable Extends ParentVariable
        ....
    }
}

This more or less works so long as every time I use variable I cast it to ChildVarible but this is ugly and throws up some issues. Is there a better way to perform this? Thanks.

Answer 1

If it's possible to make your superclass abstract, you could solve this by using generics:

public abstract class ParentClass<T extends ParentVariable>
{
    private T variable;

    protected T getVariable(){
        return variable
    }

    protected void setVariable(T variable){
        this.variable = variable;
    }
    ....
}

public class ChildClass extends ParentClass<ChildVariable>
{
    public ChildClass() {
        setVariable(new ChildVariable());
        //Where ChildVariable Extends ParentVariable
        ChildVariable foo = getVariable() // no cast nessecary
    }
}

Notice that I made your field private. You should always access field by getter and setter methods, even in subclasses.

If this is not possible (maybe even if it was) it is an indicator that your design is flawed. Refactoring of all for classes might be needed to achieve a clean architecture. Unfortunately this depends on the whole implementation of all four classes and possibly the whole context those classes are used in. An answer to that would be too broad for stackexchange.