Create instance of generic type in Java within the generic class

Question

According to the question, Create instance of generic type in Java?

At the time of writing ,the best answer is found to be...

private static class SomeContainer<E> { 
    E createContents(Class<E> clazz) { 
       return clazz.newInstance(); 
    }
 }

But the answer works only for this SomeContainer.createContents("hello");

My condition is to put the class as an inner class in a generic class,then the code should be like this.

SomeContainer<T>.createContents(T);

That will produce compile time error. There is not any created references for T,also. Is there any way to create completly new T object inside the generic class?

Thanks in advance

How is the type parameter `T` related to the type parameter of the enclosing generic class? It would be better if you show short version of your class. — Rohit Jain, Aug 22 '13 at 18:35
A better idea, instead of `Class`, use `Class extends E>`; this way, someone can pass in an instance of a subclass's class, or use the `getClass()` method on an object (as it returns `Class extends |X|>` where `|X|` is the reference type of the object) without a cast. — gparyani, Aug 22 '13 at 18:50
@ajb You can't use `.class` notation on a generic type parameter, because of type erasure. — gparyani, Aug 22 '13 at 18:51
@gparyani OK. I really wasn't clear on whether T was supposed to be a generic parameter or just a dummy name he was using to ask his question. I think some actual code would be helpful. — ajb, Aug 22 '13 at 19:00
Sorry friends. I misunderstood my problem. The comment form @gparyani can solve for problem. — johnkarka, Aug 22 '13 at 19:05
What exactly I want is `T.class`.But because of type erasure I have to create new instance of `T` to be passed into `createContents(T)` method.Then my real question should be "Is it a common practice to create another new `T` to get such object type of `T`?" — johnkarka, Aug 22 '13 at 19:14
In real Java world,is it common to create a new sample object to be passed into a generic class so that we get an new instance of such object from that generic class? — johnkarka, Aug 22 '13 at 19:35
I want to know why "type erasure" is not terminated in Java. Is there any reason? — johnkarka, Aug 22 '13 at 19:38
@John Runtime generics are on track for Java 10. The current version is Java 7, and Java 8 is on track for release in ~6 months as of writing. — gparyani, Aug 22 '13 at 23:00

score 1 · Answer 1 · edited May 23 '17 at 11:57

1

Due to implementation of generics in Java you must pass the Class<T> object for the further use.

Here is the example:

public class Outer<E> {

    private static class Inner<E> {
        E createContents(Class<E> clazz) {
            try {
                return clazz.newInstance();
            } catch (InstantiationException | IllegalAccessException e) {
                return null;
            }
        }
    }  

    private Class<E> clazz;
    private Inner<E> inner;

    public Outer(Class<E> clazz) {
        this.clazz = clazz;
        this.inner = new Inner<>();
    }


    public void doSomething() {
        E object = inner.createContents(clazz);
        System.out.println(object);
    }

}

Also you can use <? extends E>, proposed by @gparyani here

edited May 23 '17 at 11:57

Community

1
1

answered Mar 12 '14 at 12:23

Dany

1,490
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Oh great! You are the one who really understands my problem. But it's confusing for me if `Inner inner =new Inner<>();` is valid and how it is different from `Inner inner = new Inner();` – johnkarka Mar 13 '14 at 06:38
@johnkarka Here is the diamond syntax introduced in Java 7, it's some kind of type inference, compiler will substitute type by itself. For Java 6 you should use `Inner inner = new Inner();`. – Dany Mar 13 '14 at 06:44
@johnkarka BTW mutlicatch will work starting from Java 7 too. – Dany Mar 13 '14 at 06:45
`multicatch` ? please explain – johnkarka Mar 13 '14 at 07:40
@johnkarka In Java 6 you can catch only one type of exceptions, so you must write catch-block for every type, you want to catch: `catch (IOException ex) { logger.log(ex); } catch (SQLException ex) { logger.log(ex); }` In Java 7 new syntax was introduced. The example above can be rewritten as: `catch (IOException|SQLException ex) { logger.log(ex); }` – Dany Mar 13 '14 at 16:32

Create instance of generic type in Java within the generic class

1 Answers1