java subtype and subclass and true subtype

Question

I have read a lot of explanations about the java sub-typing and the true sub-typing and the subclass but every time I get more confused, please I need a perfect explanation to all the mentioned above, I am right at following statements?

1. every class that inherits from another class is a subclass
2. some class is a java sub-class to another class if it has the same methods
3. true sub-typing is when we can replace a reference between two classes

thanks in advance

Answer 1

every class the inherits from another class is a subclass

I will correct this to

"every class that extends another class is a subclass"

and FYI every class in java extends java.lang.Object class by default except Object class itself.

some class is a java sub-class to another class if it has the same methods

This is not true. consider following example

class A {
   public void someMethod(){
      // method code
   }
}

class B {
   public void someMethod(){
      // method code
   }
}

Both are independent class, B is not subclass of A or A is not subclass of B.

true sub-typing is when we can replace a reference between two classes

EDIT :

class A {
   public void someMethod(){
      // method code
   }
}

class B extends A{
   public void someMethod(){
      // method code
   }
}

You can create object of class B which is subclass of A as

A objB = new B();

This concept is Runtime Polymorphism. and you can check an article Inheritance in Java on that.

Answer 2

Only the first of these statements is true.

#2: If a class is the sub-class of another class, it will have the same methods (The reverse of this statement is not necessarily true). Consider this example:

public class Animal
{
    public void sayHello () 
    { 
         System.out.println ("Hello!");
    };
}

public class Dog extends Animal
{
    public void sayHello () 
    { 
         System.out.println ("Woof! I'm a dog.");
    };
}

public class WebApplication
{
    public void sayHello () 
    { 
         System.out.println ("Welcome!");
    };
} 

Both WebApplication and Dog have a similar sayHello method, but only Dog is a sub-class of Animal.

#3: I don't know what you mean by "replace a reference". If Dog is a sub-class of Animal, you can assign a Dog object to an Animal reference, but not vice versa.