Generic Class instance Variables

[Dishi Jain]

class Test<String> {
    String str1 = (String) "Hello1";
    java.lang.String str2 = "Hello!";
    String str3 = null;
    String str4 = "MyTesting";
    String str5 = new String("MyTesting");
}

Doubts :
1. Why warning occurs for str1? (Line 02)
2. Why String variables cannot be instantiated directly? (Compilation Error occurs at line 5 and 6)

[Martin Vanyavchich]

In this case

 class Test<String>

String is used as generic type that this class will be holding when properly instantiated. Usually you would just use T or E. To fix compile time errors, you have two options. Specify when you're using Javas String or the type of class you'll be holding (holding is used very loosely here). As in this makeover.

class Test<String> {

	String str1 = (String) "Hello1";
	java.lang.String str2 = "Hello!";
	String str3 = null;
	java.lang.String str4 = "MyTesting";
	java.lang.String str5 = new java.lang.String("MyTesting");
}

Note java.lang.String usage, where we expect a Java String.

Preferably you chose another type holder such as forementioned T or E:

class Test<E> {

	String str1 = (String) "Hello1";
	java.lang.String str2 = "Hello!";
	String str3 = null;
	String str4 = "MyTesting";
	String str5 = new String("MyTesting");
}

I hope this helped.

[Matthew Brown]

It's because it's using generics in a confusing way.

Where it says class Test<String>, this means "create a class with the generic type String, where String can stand for any class". Then, within the class, String is interpreted as "whatever the generic type is", instead of interpreting it as a shorthand for java.lang.String.

This is why we usually use T or something else that doesn't clash with a real class name. The class you've written is identical to this one:

class Test<T> {
    T str1 = (T) "Hello1";
    java.lang.String str2 = "Hello!";
    T str3 = null;
    T str4 = "MyTesting";
    T str5 = new T("MyTesting");
}

And now it should be much more clear what's happening. Lines 3 and 4 are fine. On line 5 it's assigning a java.lang.String to an unknown type. It won't allow this without an explicit cast. This is what we're doing on line 2, which compiles but may throw a ClassCastException at runtime.

And on line 6 it's trying to instantiate a T directly, which isn't allowed (for a start, you don't know what constructors T will have).

Edit: snap (almost)

[Dishi Jain]

Yes,Now I get it.
It's because of confusion between what type variable is and what "String" is.
Thanks Buddies