cannot instantinate generic type instance in Java

Question

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Create instance of generic type in Java?

I've a little trouble. I cannot instantinate generic type instance in default constructor. here is my class

public class MyClass<U extends MyUser> {
    private U user;

    public U getUser() {
        return this.user;
    }

    public void setUser(U user) {
        this.user = user;
    }

    public MyClass() {
        this.user = new U();
    }
}

in code line this.user = new U() I'm getting exception

cannot instantinate type U

. How can I create new instance of U?

Thanks in advance

http://stackoverflow.com/questions/1090458/instantiating-a-generic-class-in-java — Collin, Aug 04 '11 at 20:17

score 0 · Answer 1 · answered Aug 04 '11 at 20:20

You need to specify a type when instantiating your User. That type needs to be a MyUser or subtype of MyUser.

public class MyClass<U extends MyUser>{
private U user;
public U getUser(){
return this.user;
}

public void setUser(U user){
  this.user=user;
}

public MyClass() {
    this.user=new MyClass<U>();
  }
}

score 0 · Answer 2 · answered Aug 04 '11 at 20:20

0

Java generics do not know the types of their templated parameters at runtime. Therefore, in your example, the runtime type of U is not known, so the constructor cannot execute new U().

You will need to pass in the type of U (something like Foo.class) to the conctructor in order for it to know how to create a new obejct of type U.

answered Aug 04 '11 at 20:20

David R Tribble

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could you write creation of type instance for my code? Thanks in advance. – pofighist Aug 04 '11 at 21:30

cannot instantinate generic type instance in Java

2 Answers2