Defining function with return value as another class in Java-CodePudding

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()
  {
   .....
   }
  abstract public boolean  method2();
  if (method2())   // Is this valid . If yes , what is the concept behind it?
  {.....}   
}

Now I have file B.java with below sample contents

public class B extends A 
{
  @Override
  public boolean method2() {
    return false;
  }
}

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.

CodePudding user response：

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 {
}

Question

Explanation

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