Generic Classes question

[Suvojit Chakraborty]

Following question is from book SCJP6 by Richard F. Raposa:

What is the result of the following program?

 public class Hello < T >   {
      T t;

      public Hello(T t) {
          this.t = t;
      }

      public String toString() {
          return t.toString();
     }

   public static void main(String [] args) {
         System.out.print(new Hello < String > ("hi));
         System.out.print(new Hello("there"));
     }
 }   

A. hi
B. hi followed by a runtime exception
C. hithere
D. Compiler error on line 9
E. Compiler error on line 14

I chose Option E but the ans says Option C.

Why so???(It has explanation which I fail to understand and I am willing to type it only if someone asks)

[Sridhar Santhanakrishnan]

Why do you think there would be a compiler error?

[Seetharaman Venkatasamy]

  System.out.print(new Hello < String > ("hi));    

might be this line, missing " around hi .

[Rajeev Rnair]

Seetharaman Venkatasamy wrote:

  System.out.print(new Hello < String > ("hi));    

might be this line, missing " around hi .

    
public Hello(T t) {  
   this.t = t;  
}  

class Hello<T> is having a constructor which takes an argument of type T. This particular case you are creating an instance of Hello<String>, so you can pass a String argument to the constructor. For example, you can create

Hello<String> hello = new Hello<String>("hi");

which will assign

   hello.t = "test";

and the hello.toString() will print the hello.t, which is "hi"

[Abimaran Kugathasan]

Not only for String, but for all objects, see the code below....

public class Hello <T>	{
	T t;

	public Hello(T t)	{
		this.t = t;
	}

	public String toString()	{
		return t.toString();
	}
	public static void main(String[] arsg)	{
		System.out.println(new Hello<String>(" Hi "));
		System.out.println(new Hello("There"));
		System.out.println(new Hello(new Dog("ABI's")));
	}
}

class Dog	{
	String name;

	public String toString()	{
		return this.name.toString();
	}
	Dog(String n)	{
		name = n;
	}
}

[Suvojit Chakraborty]

Sridhar Santhanakrishnan wrote:Why do you think there would be a compiler error?

I think its a violation to have this statement in code:

          System.out.print(new Hello("there"));     

because the generic class expect a type

  public class Hello < T >   {  
      T t;  
  
      public Hello(T t) {  
          this.t = t;  
      }  
     

If you notice, we need to pass a type T so that the instance variable t and the local variable in const can have a type.

but when we say "System.out.print(new Hello("there"));" we are not suppluying the type.

[Suvojit Chakraborty]

Hi Abimaran,

I execute your code. It compiled but generated 2 warnings in following lines:

  System.out.println(new Hello("There"));  
        System.out.println(new Hello(new Dog("ABI's")));     

Output:

D:\Education\Java\JavaStudyRoom>javac -Xlint Hello.java
Hello.java:13: warning: [unchecked] unchecked call to Hello(T) as a member of the raw type Hello
System.out.println(new Hello("There"));
^
Hello.java:14: warning: [unchecked] unchecked call to Hello(T) as a member of the raw type Hello
System.out.println(new Hello(new Dog("ABI's")));
^
2 warnings

So I guess this is also a case of mixing Generics and non Generics.
Thank you so much for your code. Now I am clear with the answer.


Suvojit

[Abimaran Kugathasan]

Suvojit Chakraborty wrote:Hi Abimaran,

I execute your code. It compiled but generated 2 warnings in following lines:

  System.out.println(new Hello("There"));  
        System.out.println(new Hello(new Dog("ABI's")));     

Output:

D:\Education\Java\JavaStudyRoom>javac -Xlint Hello.java
Hello.java:13: warning: [unchecked] unchecked call to Hello(T) as a member of the raw type Hello
System.out.println(new Hello("There"));
^
Hello.java:14: warning: [unchecked] unchecked call to Hello(T) as a member of the raw type Hello
System.out.println(new Hello(new Dog("ABI's")));
^
2 warnings

So I guess this is also a case of mixing Generics and non Generics.
Thank you so much for your code. Now I am clear with the answer.


Suvojit

Exactly! But warning is not Compilation Error!

You are Welcome!