Problem with creating objects and references of Generic class

[Zulfi Khan]

Hi,
I have created a Generic class. Now I want to create its objects and references. Following is my code

class GenericsType<T1, T2> {
        T1 id;
        T2 name;
        GenericsType next;
	GenericsType(T1 id1, T2 name1){
            id = id1;
            name = name1;
        }
	
	public T1 get_T1(){
	    return this.id;
	}
	
	public void set_T1(T1 t1){
	    this.id=t1;
	}
}


public class BuiltInLLApp {
        GenericsType<T1, T2> head, tail;//Error

        /**
        * @param args the command line arguments
        */
        public static void main(String[] args) {
            GenericsType<T1, T2> newNode = new GenericsType<>();//Error
        }
    
}

I am getting error, can't find symbol T1 and T2. Somebody please guide me.

Zulfi.

[Stephan van Hulst]

The BuiltInLLApp class doesn't know what T1 and T2 are. Are they classes? Are they type parameters? You haven't declared them.

[Zulfi Khan]

Hi,

Thanks.

I don't know how to declare them. I saw the following link:

Java Generic Example

I saw an example of Java Generic in the above link. It is not using this type of declaration. I want to create a LL. If I want to declare them how should I declare them? Why its accepting them in the "GenericsType" class? Somebody please guide me.

Zulfi.

[Stephan van Hulst]

What is an LL?

It's accepting T1 and T2 for the members of GenericsType, because you've declared those type variables in the first line when you declared GenericsType. Your BuiltInLLApp has no such declarations and they also wouldn't do you any good because you would be using them in the static main() method. Generic type parameters declared at the start of a class are only for instance members.

What you probably want to do is declare your head, tail and newNode variables in such a way that they use concrete types and not type parameters. The second code snippet under the heading "2. Java Generic Class" from the link you posted shows you how to do that with the String class.

[Campbell Ritchie]

Don't use T1 and T2 as formal type parameters (line 1, etc.). Use single letters e.g. GenericsType<T, U>. Don't call the fields id or name, because that will make you think they will have particular uses. Your users can use any type they like, so call them t and u. Try not to let the users see those field names 
Your users will declare the object reference as type GenericsType<Integer, Double> or GenericsType<List<Foo>>, Set<Bar>> or something else.

Don't use _ in your method names. Use mixedCase(). Pease be sure to indent your code correctly; you can cause yourself no end of confusion wth badly indented code.

[Zulfi Khan]

Hi,
Thanks for your reply.

import java.util.ArrayList;
import java.awt.List;
class GenericsType<T, U> {
        T t;//string
        U u;//integer
        GenericsType next;
	GenericsType(T _t, U _u){
            t = _t;
            u = _u;
        }
	
	public T get_T(){
	    return this.t;
	}
	
	public void set_T(T _t){
	    this.t=_t;
	}
}
/**
 *
 * @author zulfi
 */
public class BuiltInLLApp {

    /**
     * @param args the command line arguments
     */
        public static void main(String[] args) {
            GenericsType<String, Integer> type = new GenericsType<>(String str, Integer id);
        }
}

I am still getting syntax error:
')' expected
;  expected
can't find symbol String.

I tried to indent, given eight spaces, but 'public static...main..' not properly indented.

Somebody please guide me.
Zulfi.

[Christopher Schneider]

 public static void main(String[] args) {
      GenericsType<String, Integer> type = new GenericsType<>(String str, Integer id)
}

I think you're getting ahead of yourself. Before you look at an advanced concept like generics, you should understand constructors.

When you instantiate an object, you pass it parameters. You are passing parameters named String str and Integer id.

The two issues here are that they do not exist (the reason for "can't find symbol String.") and they have spaces.

The proper way to do this would be:

GenericsType<String, Integer> type = new GenericsType<>("baseball", 1)

Here, I am passing a string literal, "baseball", and an integer, 1. These are defined values, whereas your previous code passes undefined values to the constructor.

[Knute Snortum]

GenericsType<String, Integer> type = new GenericsType<>(String str, Integer id);

You don't use parameter types when calling a constructor or method.

See HowToFormatCode (that's a link).

Don't use variables with underscores.  See NamingConventions (that's also a link).

[Knute Snortum]

    GenericsType(T _t, U _u){
        t = _t;
        u = _u;
    }

Better would be:

    GenericsType(T t, U u){
        this.t = t;
        this.u = u;
    }

[Zulfi Khan]

Hi,
Christopher Schneider:
I am doing Java after sometime. Thanks for your help.

Knute Snortum:

Thanks for telling me formatting styles.

Zulfi.