Defining function with return value as another class in Java

Question

I was reading one piece of code in Java and one function definition caught my eye. Its a long piece of code so I am just putting a abstract version to know how it worked.

Consider I have a file A.java with below code.

abstract public class A
{
  public B method1()
  {
   .....
   }
  void method2() {};
}

Now I have file B.java with below sample contents

public class B extends A 
{
  .....
}

Now I am a beginner in Java but from what I have learned that I can call method2 in class B since it extends A. But how can I have return value of a class B in parent class A defined as return value of function method1. Please help me in clearing this concept. I am totally bounced over this piece of code.

Answer 1

Question

Here's the code you posted (edited slightly to tidy it up, but the structure is the same):

abstract public class A {
  public B method1() { ... }
  void method2() {};
}

public class B extends A {
  ...
}

You asked this question:

how can I have return value of a class B in parent class A defined as return value of function method1?

Specifically, you are asking how "method1()" on class A can return B, when B itself is a class which extends A:

abstract public class A {
  public B method1() { ... }

Explanation

The reason this works is that the return type of method1() – and in general, any method defined in class A – has nothing to do with the class structure of A or B.

The return type simply means if you call method1() :

• there is a result from that method call,
• the result is an object,
• and the object has type "B"

It does not say anything about how (or even if ) B relates to A.

Methods can specify any return types (so long as that type is valid.. you can't just return things which don't exist). Here's a simple edit of class A showing a few other return types. These additional return types are clearly unrelated to class A – String and List<Integer> .

abstract class A {
    abstract public B method1();
    abstract public String method2();
    abstract public List<Integer> method3();
}

In this new "A", all it says is:

• method1() returns something of type B, whatever that is
• method2() returns something of type String
• method3() returns List<Integer>

It's fine if B itself is defined in terms of A ( class B extends A ), or is altogether defined as a separate class. Here's a version of B, but without "extends A", so it's just a standalone class definition, this would work fine with your definition of A, too.

class B {
}

More reading

Here are a few snips from the Java Tutorials about method definitions:

The only required elements of a method declaration are the method's return type, name, a pair of parentheses, (), and a body between braces, {}.

and also:

The return type—the data type of the value returned by the method, or void if the method does not return a value.

There's an additional section possibly worth reading - Returning a Value from a Method – which has more discussion about how this works, what's allowed, etc.