Is this the good class design practice in Java?

Question

public class OneClass {
        private AnotherClass anotherClass = new AnotherClass();

        private void doOneJob(){
            anotherClass.doOtherJob();
        }
    }

In the above code, AnotherClass is instantiated and held as a class variable of OneClass.

• What are the consequences of this design?
• Is it a good practice?
• Will this support unit testing?

Answer 1

This is called composition and is a fundamental part of Java language. There is nothing wrong with it, and might be essential in a lot of cases.

The only change I would do is use dependency injection to make it easier for testing, ie you'll be able to supply a mock AnotherClass object this way:-

public class OneClass {
    private AnotherClass anotherClass;

    public OneClass(AnotherClass anotherClass){
        this.anotherClass = anotherClass;
    }

    private void doOneJob() {
        anotherClass.doOtherJob();
    }
}

Answer 2

It is hard to "unit" testing because you cant mock the "external" dependency in a normal way. (With groovy tests for example you can override private fields with a mock)

I think it would be better that you have a constructor that accepts a AnotherClass than you can mock it in a unit test via constructor.