Crossing abstraction barrier between parent and child class

Motivation

This article is inspired by a question I received in a programming methodology class. In this class, in which we write Java code to solve programming exercises, we have the constraint that every attribute of a class should be private and final. It means there is no access to the field outside of the class, and no modification is allowed once this field is initialized. This strict requirement is put in place to enforce immutability when constructing a class object in Java.

Sooner or later, when the exercises get more complex, we tend to move on to an OOP solution whereby multiple classes are constructed and organized with the help of inheritance. The problem then arises when there is a need to access this private final field in the parent class from a subclass. What should we do then?

To give a concrete example, let's say we have the following classes:

class Parent {
	private final int value;

	Parent(int value) {
		this.value = value;
	}
}

class Child extends Parent {
	Child(int value) {
		super(value);
	}

	int add(int another) {
		return super.value + another; // UNABLE TO ACCESS!
	}
}

What should we do if the child class wants to access value from the parent?

Solutions

Change modifier

The simplest way to deal with that is to change the access modifier from private to something else - perhaps public or protected. This solution can be legitimate depending on the context. In some cases, perhaps it is perfectly normal to expose this value to other classes.

Add a getter method

From the Oracle's Java tutorial on inheritance

A subclass does not inherit the private members of its parent class. However, if the superclass has public or protected methods for accessing its private fields, these can also be used by the subclass.

So, another possible solution is to have a getter method in the parent class and make that method public. This way child classes (and technically other classes) will have access via the getter. So a quick example will be:

class Parent {
	private final int value;

	Parent(int value) {
		this.value = value;
	}

	public int getValue() {
		return this.value;
	}
}

class Child extends Parent {
	Child(int value) {
		super(value);
	}

	int add(int another) {
		return super.getValue() + another;  // CAN ACCESS!
	}
}

Having a getter method can be beneficial in the sense that even though now a "private" field is exposed, you still have one layer of abstraction over it. The users of the getter method do not need to know how that value is generated, which can be manipulated (if needed) by some complex preprocessing steps in the getter method. Also, the underlying private field could change drastically and yet the users of the getter method are unaware.

Rethink code design

Lastly, this problem may be a signal to rethink if there is a legitimate need to access a private final field. Given a parent-child relationship, sometimes it's difficult to be clear about which field/method should reside in which classes.

• Would it be better to have the field in the child class instead?
• Can we shift what the child class wanted to do with value into the parent class as a general method that the child class can inherit and possibly override?

A better code design might suggest that the private final field can stay as is, maintaining an abstraction barrier between the parent and the child class. One example solution is then:

class Parent {
	private final int value;

	Parent(int value) {
		this.value = value;
	}

	int add(int another) {
		return this.value + another;
	}
}

class Child extends Parent {
	Child(int value) {
		super(value);
	}

	int add(int another) {  // will work if this method is omitted as well,
		return super.add(another); // as it will be inherited
	}
}

Anti pattern

A problematic walkaround that some might come up with is to redeclare the same field in the child class.

class Parent {
	private final int value;

	Parent(int value) {
		this.value = value;
	}
}

class Child extends Parent {
	private final int value;
	
	Child(int value) {
		super(value);
		this.value = value;
	}

	int add(int another) {
		return this.value + another; // will work but not recommended
	}
}

This works but is arguably a bad design because it does not make use of inheritance to reduce any duplicates between shared properties. It also could result in the values (that meant to represent the same thing) going out of sync, especially if these fields were not declared as final.

Conclusion

When I was asked the motivating question, my immediate response was: "make a public getter method". To which I was then asked a follow-up question:

• Why do we resort to using a public getter method, when we want to keep the field private?

Which got me thinking:

• Why can't private fields be inherited?

This article is a reminder for me to ask the "why" questions more often, and explore the reasons for the answers.