如何从Java中的超类内部调用重写方法的超类实现？

Question

Suppose I have two classes SuperClass and SubClass which extends SuperClass , such as below:

public abstract class SuperClass {
    private Object field1;

    protected SuperClass(Object obj) {
        setField1(obj);
    }

    public void setField1(Object obj) {
        // perform some check on obj and
        // then set field1 such that field1==obj
    }
}


public class SubClass extends SuperClass {
    public SubClass(Object obj) {
        super(obj);
    }

    @Override
    public void setField1(Object obj) {
        super.setField1(obj);
        // do some work that is necessary only when
        // field1 is set through SubClass.setField1()
    }
}

What I need is, when I create an object of SubClass , I need method SuperClass.setField1() to be called from inside SuperClass constructor, not SubClass.setField1() . But with the above code, method SubClass.setField1() is called which causes a NullPointerException to be thrown, since the work done in SubClass.setField1() is performed on a field that's yet null.

Is there any way to explicitly call SuperClass.setField1()‍‍‍ from inside SuperClass constructor?

Answer 1

You could move said method body to a private method and let the default method (the one which may be overridden by the subclass) delegate to the former. See this example

public abstract class SuperClass {

    private Object field1;

    protected SuperClass(Object obj){
        // call the safe implementation
        setField1Safe(obj);
    }


    public void setField1(Object obj){
        // just delegates
        setField1Safe(obj);
    }

    private void setField1Safe(Object obj){
        // perform some check on obj and
        // then set field1 such that field1==obj
    }
}

public class SubClass extends SuperClass{
     public SubClass(Object obj){
         super(obj);
     }

     @Override
     public void setField1(Object obj){
         super.setField1(obj);
         // do some work that is necessary only when
         // field1 is set through SubClass.setField1()
     }
}

That way the sub class can still override setField1 but if you really depend on the implementation then you can call the private setField1Safe method.