java非字符串类型的泛型类

Question

I'm studying about java generic classes. Create one generic class with T as type parameter.

public class Genericstring<T> {
    String name;

    public void set(String name){
        this.name = name;
    }

    public static void main(String[] args){
        Genericstring<String> o = new Genericstring<String>();
        o.set("Generic");

        System.out.println(o.name);
    }
}

In above example, the type of T could string, integer or any other type. But is that possible I could create generic class with specific type argument. Like below example, but its showing error "Cannot make a static reference to the non-static type String"

public class Genericstring<String> {
    String name;

    public static void main(String[] args){     
        //Here creating object always having string type attributes. No need to specify like did in above example.
    }

Can anyone tell me, Is that possible to create generic class with String as parameter Class generictype ? If not, then why ?

Answer 1

Genericstring<String> is declaring a type variable called String .

This makes all other uses of String refer to that type variable, not java.lang.String . Where you've got the argument to main declared as String[] , it thinks you mean the type variable. And because this is a static method, you can't refer to that type parameter there, because it belongs to instances of the class.

But is that possible I could create generic class with specific type argument.

What you're describing is a non-generic class. Remove the type parameter.

public class Genericstring {
  String name;

  // Etc
}

Answer 2

No, you can't.

This code: Genericstring<String> is only naming the Generic Type Variable rather than setting as java.lang.String.

The most you can do is declaring a generic type that extends a specific Class:

public class Genericstring<T extends String>

Example:

abstract class B<T extends String> {
    protected T value;

    public T getValue() {
        return value;   
    }

    public void handleValue() {
        System.out.println(this.getValue().substring(0, 5));
    }
}

class C extends B<String>{
    public C() {
        this.value = "Hello World";
    }
}

public class A {
    public static void main(String[] args) {
        C obj = new C();
        obj.handleValue(); // prints Hello
    }
}

UPDATED: According to the comments, my answer has generated a lot of confusion because the way I've implemented.

@linuxman this is not the only way to accomplish your scenario and your right when you're mentioning Cant we mention like only B or B<T extends String> ? :

class B<T extends String> {
    protected T value;

    public T getValue() {
        return value;
    }

    public void handleValue() {
        System.out.println(this.getValue().substring(0, 5));
    }
}

public class A {
    public static void main(String[] args) {
        B obj = new B<String>();
        obj.handleValue();
    }
}

@Pshemo is right:

In this scenario only type available for is String itself, but if it is only one type then there is no point in having a generic type because it main purse is to allow us to use our class with many types.

Also, about your doubt regarding final classes:

Reference: https://stackoverflow.com/a/5181618/1715121

A final class is simply a class that can't be extended .

(This does not mean that all references to objects of the class would act as if they were declared as final .)

When it's useful to declare a class as final is covered in the answers of this question:

• Good reasons to prohibit inheritance in Java?

If Java is object oriented, and you declare a class final , doesn't it stop the idea of class having the characteristics of objects?

In some sense yes.

By marking a class as final you disable a powerful and flexible feature of the language for that part of the code. Some classes however, should not (and in certain cases can not) be designed to take subclassing into account in a good way. In these cases it makes sense to mark the class as final, even though it limits OOP. (Remember however that a final class can still extend another non-final class.)

Related article: Java: When to create a final class

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@linuxman What you're trying to accomplish doesn't make sense because the purpose of a Generic Type is to declare contracts that could implement a behavior with different types.

Hope it helps and clears your doubts.