CODEWITHSUNDEEP
javainheritancemethodsfieldsuperclass
user810430
user810430

Reputation: 11499

How to have method use field from desendant class in java?

Code:

ClassA extends ClassBase {

 protected IntefaceA parA;
 .... }

 InterfaceA extends InterfaceBase

ClassBase {

 protected IntefaceBase parB;

 public method1() {

  parB.someMethod();
}

Code:

ClassA testClass=new ClassA();
testClass.setParA(new InterfaceA {....}; );
testClass.method1();

I receive null pointer exception because method metho1 uses class variable from ClassBase that used by method1 is parB. Does it possible have method1 use parA? If not, I need copy&paste all code of method1 from base class to descendant.

Thanks.

Upvotes: 2

Views: 93

Answers (2)

aioobe
aioobe

Reputation: 421290

You can't override fields in Java, i.e. you can't say, parA overrides parB in the way a method of a sub class would override a method of a base class.

What you need to do is to provide an access method, getPar for the parB variable, which you in ClassA override to return parA.

Then in method1, when you need to access parA you simply go through this method. Since all methods in Java are virtual the subclasses' getPar method will be invoked, even if called from within ClassBase.

interface InterfaceBase {
    void someMethod();
}

interface InterfaceA extends InterfaceBase {
    ...
}

class ClassA extends ClassBase {

    protected InterfaceA parA;

    @Override
    protected InterfaceA getPar() {
        return parA;
    }
}

class ClassBase {
    protected InterfaceBase parB;

    public void method1() {
        InterfaceBase parObj = getPar().someMethod();

        // Big method... no need for code duplication.
    }

    protected InterfaceBase getPar() {
        return parB;
    }
}

Upvotes: 2

Péter Török
Péter Török

Reputation: 116306

Overriding fields is not possible in Java, only overriding methods. So what you can do is hide your parA / parB members behind a protected virtual getter, and access them only via the getter method. Then override the getter in ClassA to return parA instead of parB:

ClassBase {
  private IntefaceBase parB;

  protected IntefaceBase getPar() {
    return parB;
  }

  public method1() {
    getPar().someMethod();
  }
}

ClassA extends ClassBase {
  private IntefaceA parA;

  protected IntefaceBase getPar() {
    return parA;
  }
}

Upvotes: 1

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