Java generics - why is extends T allowed but not implements T

Question

I wonder if there is a special reason in Java for using always  extends  rather than  implements  for defining bounds of typeparameters   Example    public interface C    public class A lt B implements C gt       is prohibited but   public class A lt B extends C gt       is correct  What is the reason for that

User · Answer

It may be that the base type is a generic parameter, so the actual type may be an interface of a class. Consider:

class MyGen<T, U extends T> {

Also from client code perspective interfaces are almost indistinguishable from classes, whereas for subtype it is important.

User · Answer

Probably because for both sides  B and C  only the type is relevant  not the implementation   In your example  public class A lt B extends C gt      B can be an interface as well   extends  is used to define sub-interfaces as well as sub-classes   interface IntfSub extends IntfSuper    class ClzSub extends ClzSuper      I usually think of  Sub extends Super  as  Sub is like Super  but with additional capabilities   and  Clz implements Intf  as  Clz is a realization of Intf   In your example  this would match  B is like C  but with additional capabilities  The capabilities are relevant here  not the realization

User · Answer

It s sort of arbitrary which of the terms to use  It could have been either way  Perhaps the language designers thought of  extends  as the most fundamental term  and  implements  as the special case for interfaces   But I think implements would make slightly more sense  I think that communicates more that the parameter types don t have to be in an inheritance relationship  they can be in any kind of subtype relationship   The Java Glossary expresses a similar view

User · Answer

There is no semantic difference in the generic constraint language between whether a class  implements  or  extends   The constraint possibilities are  extends  and  super  - that is  is this class to operate with assignable to that other one  extends   or is this class assignable from that one  super

User · Answer

In fact  when using generic on interface  the keyword is also extends  Here is the code example    There are 2 classes that implements the Greeting interface    interface Greeting       void sayHello       class Dog implements Greeting        Override     public void sayHello             System out println  Greeting from Dog  Hello              class Cat implements Greeting        Override     public void sayHello             System out println  Greeting from Cat  Hello               And the test code     Test public void testGeneric         Collection lt   extends Greeting gt  animals       List lt Dog gt  dogs   Arrays asList new Dog    new Dog    new Dog         List lt Cat gt  cats   Arrays asList new Cat    new Cat    new Cat          animals   dogs      for Greeting g  animals  g sayHello         animals   cats      for Greeting g  animals  g sayHello

User · Answer

Here is a more involved example of where extends is allowed and possibly what you want   public class A lt T1 extends Comparable lt T1 gt  gt

User · Answer

The answer is in here         To declare a bounded type parameter  list the type parameter s name  followed by the extends keyword  followed by its upper bound        Note that  in this context  extends is used in a general sense to mean either extends  as in classes  or implements  as in interfaces     So there you have it  it s a bit confusing  and Oracle knows it

User · Answer

We are used to   class ClassTypeA implements InterfaceTypeA    class ClassTypeB extends ClassTypeA      and any slight deviation from these rules greatly confuses us   The syntax of a type bound is defined as   TypeBound      extends TypeVariable      extends ClassOrInterfaceType  AdditionalBound     JLS 12   4 4  Type Variables   TypeBound   If we were to change it  we would surely add the implements case  TypeBound      extends TypeVariable      extends ClassType  AdditionalBound      implements InterfaceType  AdditionalBound    and end up with two identically processed clauses   ClassOrInterfaceType      ClassType      InterfaceType    JLS 12   4 3  Reference Types and Values   ClassOrInterfaceType   except we would also need to take care of implements  which would complicate things further    I believe it s the main reason why extends ClassOrInterfaceType is used instead of extends ClassType and implements InterfaceType - to keep things simple within the complicated concept  The problem is we don t have the right word to cover both extends and implements and we definitely don t want to introduce one     lt T is ClassTypeA gt   lt T is InterfaceTypeA gt   Although extends brings some mess when it goes along with an interface  it s a broader term and it can be used to describe both cases  Try to tune your mind to the concept of extending a type  not extending a class  not implementing an interface   You restrict a type parameter by another type and it doesn t matter what that type actually is  It only matters that it s its upper bound and it s its supertype

[java] Java generics - why is "extends T" allowed but not "implements T"?

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