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Why can methods be overriden but attributes can't?

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14

6

I have a class

public class A 
 public String attr ="A attribute";
 public void method() 
 System.out.println(this+" , "+this.attr);
 
 public String toString() 
 return("Object A");
 

and another class that inherits from it

public class B extends A
 public String attr = "B attribute";
 public void method() 
 super.method();
 
 public String toString() 
 return("Object B");
 

Note that the method() of B is simply a wrapper for method() of A.

When I run the following code

B b = new B();
b.method();

I get Object B , A attribute as output which means that, this and this.attr accessed different things. Why is that the case?

Shouldn't System.out.println(this) refer to the toString() method of class A ?

java inheritance this

share|improve this question

edited Mar 28 at 20:00

Ondra K.

1,0691926

asked Mar 27 at 14:44

Noah BishopNoah Bishop

1138

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  A modern IDE will have the possibility to show a warning on the second definition of attr that it 'shadows' a similarly named field of a superclass.
  
  – Mark Jeronimus
  Mar 27 at 15:49
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Noah why did you revert the edited question name? I think the edited title Why can methods be overriden but attributes can't described the content of the question better than the current one. This is important for all other people looking for an answer to their question.
  
  – Ondra K.
  Mar 28 at 12:20
  

  
  
  
  

add a comment | 

14

6

I have a class

public class A 
 public String attr ="A attribute";
 public void method() 
 System.out.println(this+" , "+this.attr);
 
 public String toString() 
 return("Object A");
 

and another class that inherits from it

public class B extends A
 public String attr = "B attribute";
 public void method() 
 super.method();
 
 public String toString() 
 return("Object B");
 

Note that the method() of B is simply a wrapper for method() of A.

When I run the following code

B b = new B();
b.method();

I get Object B , A attribute as output which means that, this and this.attr accessed different things. Why is that the case?

Shouldn't System.out.println(this) refer to the toString() method of class A ?

java inheritance this

share|improve this question

edited Mar 28 at 20:00

Ondra K.

1,0691926

asked Mar 27 at 14:44

Noah BishopNoah Bishop

1138

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  A modern IDE will have the possibility to show a warning on the second definition of attr that it 'shadows' a similarly named field of a superclass.
  
  – Mark Jeronimus
  Mar 27 at 15:49
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Noah why did you revert the edited question name? I think the edited title Why can methods be overriden but attributes can't described the content of the question better than the current one. This is important for all other people looking for an answer to their question.
  
  – Ondra K.
  Mar 28 at 12:20
  

  
  
  
  

add a comment | 

I have a class

public class A 
 public String attr ="A attribute";
 public void method() 
 System.out.println(this+" , "+this.attr);
 
 public String toString() 
 return("Object A");
 

and another class that inherits from it

public class B extends A
 public String attr = "B attribute";
 public void method() 
 super.method();
 
 public String toString() 
 return("Object B");
 

Note that the method() of B is simply a wrapper for method() of A.

When I run the following code

B b = new B();
b.method();

I get Object B , A attribute as output which means that, this and this.attr accessed different things. Why is that the case?

Shouldn't System.out.println(this) refer to the toString() method of class A ?

java inheritance this

share|improve this question

edited Mar 28 at 20:00

Ondra K.

1,0691926

asked Mar 27 at 14:44

Noah BishopNoah Bishop

1138

public class A 
 public String attr ="A attribute";
 public void method() 
 System.out.println(this+" , "+this.attr);
 
 public String toString() 
 return("Object A");
 

public class B extends A
 public String attr = "B attribute";
 public void method() 
 super.method();
 
 public String toString() 
 return("Object B");
 

B b = new B();
b.method();

share|improve this question

edited Mar 28 at 20:00

Ondra K.

1,0691926

asked Mar 27 at 14:44

Noah BishopNoah Bishop

1138

share|improve this question

edited Mar 28 at 20:00

Ondra K.

1,0691926

asked Mar 27 at 14:44

Noah BishopNoah Bishop

1138

edited Mar 28 at 20:00

Ondra K.

1,0691926

edited Mar 28 at 20:00

Ondra K.

1,0691926

asked Mar 27 at 14:44

Noah BishopNoah Bishop

1138

asked Mar 27 at 14:44

Noah BishopNoah Bishop

1138

1 Answer
1

active

oldest

votes

14

By declaring a method with a same name as parent class, you override it, that is, replace the original behaviour. But if you declare a field with a same name, you effectively hide it, making it inaccessible from that subclass, but only by super.field. See oracle docs on variable hiding, as well as using the keyword super. Note that it is not recommended to use variable hiding, as it creates exactly the kind of confusion you're experiencing.

By calling super.method(), printing this results in calling the toString method, which was in fact overridden - so that's the reason why it prints "Object B", as you've called the method on an instance of B. But the this in this.attr actually refers to the parent object, as you're calling the method from the parent class (by super.method()).

share|improve this answer

edited Mar 27 at 15:15

answered Mar 27 at 14:49

Ondra K.Ondra K.

1,0691926

add a comment | 

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14

By declaring a method with a same name as parent class, you override it, that is, replace the original behaviour. But if you declare a field with a same name, you effectively hide it, making it inaccessible from that subclass, but only by super.field. See oracle docs on variable hiding, as well as using the keyword super. Note that it is not recommended to use variable hiding, as it creates exactly the kind of confusion you're experiencing.

By calling super.method(), printing this results in calling the toString method, which was in fact overridden - so that's the reason why it prints "Object B", as you've called the method on an instance of B. But the this in this.attr actually refers to the parent object, as you're calling the method from the parent class (by super.method()).

share|improve this answer

edited Mar 27 at 15:15

answered Mar 27 at 14:49

Ondra K.Ondra K.

1,0691926

add a comment | 

14

By declaring a method with a same name as parent class, you override it, that is, replace the original behaviour. But if you declare a field with a same name, you effectively hide it, making it inaccessible from that subclass, but only by super.field. See oracle docs on variable hiding, as well as using the keyword super. Note that it is not recommended to use variable hiding, as it creates exactly the kind of confusion you're experiencing.

By calling super.method(), printing this results in calling the toString method, which was in fact overridden - so that's the reason why it prints "Object B", as you've called the method on an instance of B. But the this in this.attr actually refers to the parent object, as you're calling the method from the parent class (by super.method()).

share|improve this answer

edited Mar 27 at 15:15

answered Mar 27 at 14:49

Ondra K.Ondra K.

1,0691926

add a comment | 

By declaring a method with a same name as parent class, you override it, that is, replace the original behaviour. But if you declare a field with a same name, you effectively hide it, making it inaccessible from that subclass, but only by super.field. See oracle docs on variable hiding, as well as using the keyword super. Note that it is not recommended to use variable hiding, as it creates exactly the kind of confusion you're experiencing.

By calling super.method(), printing this results in calling the toString method, which was in fact overridden - so that's the reason why it prints "Object B", as you've called the method on an instance of B. But the this in this.attr actually refers to the parent object, as you're calling the method from the parent class (by super.method()).

share|improve this answer

edited Mar 27 at 15:15

answered Mar 27 at 14:49

Ondra K.Ondra K.

1,0691926

share|improve this answer

edited Mar 27 at 15:15

answered Mar 27 at 14:49

Ondra K.Ondra K.

1,0691926

answered Mar 27 at 14:49

Ondra K.Ondra K.

1,0691926

answered Mar 27 at 14:49

Ondra K.Ondra K.

1,0691926