Geneics Question

[Amirr Rafique]

Hi all

  class Person implements Comparable<? extends Person>{//why this line gives error
int age;
 
public Person(int age_){
age = age_;
}
 
public int getAge(){
return age;
}
 
public boolean equals(Object o){
if(!(o instanceof Person)){
return false;
}
return (age == ((Person)o).getAge());
}
 
public int hashCode(){
return 123;
}
 
//implementing Comparable to add natural sort order
public int compareTo(Person p){
System.out.println("compareTo reporting");
if(age > p.getAge())
return 1;
else if (age < p.getAge())
return -1;
else
return 0;
}
}
  

Error is

TreeSetTest.java:3: unexpected type
found : ? extends Person
required: class or interface without bounds
class Person implements Comparable<? extends Person>{

[Keith Lynn]

What is the question?

[radhika holani]

question mark works when declaring a reference for a variable ,it doesn't work for generic class and method declaration.

hence the above code will give u err.

[Amirr Rafique]

Originally posted by Amir Rafique:
Hi all

  class Person implements Comparable<? extends Person>{//why this line gives error
  

Error is

[wise owen]

8.1.5 Superinterfaces:
Each InterfaceType must name an accessible (�6.6) interface type, or a compile-time error occurs. If the TypeName is followed by any type arguments, it must be a correct invocation of the type declaration denoted by TypeName, and none of the type arguments may be wildcard type arguments, or a compile-time error occurs.

[Barry Gaunt]

It may help to read:
Can I derive from a wildcard parameterized type?