How to pass generic Object as a Generic parameter on other method in java?

Question

I meet a trouble in using generic method

Compiled class:

public class Something<T> {
   public static Something newInstance(Class<T> type){};
   public <T> void doSomething(T input){};
}

and my method is:

public <S> void doOtherThing(S input){
      Something smt = Something.newInstance(input.getClass());
      smt.doSomething(input); // Error here
}

It got error at Compile time:

no suitable method found for doSomething(T) T cannot be converted to capture#1 of ? extends java.lang.Object ...

I think there might be a trick to avoid this, please help

There are a lot of issues here. First there are two separate Ts. doSomething's T is not the same as the class Something's T, because doSomething is a generic method that declares its own T. You should name the two variables differently, because they are unrelated to each other. Then the fact that doSomething only uses T once as a parameter type, and T is unbounded, means that it takes anything, so it is equivalent to public void doSomething(Object input){}; the T is useless. Same thing with doOtherThing -- it is equivalent to public void doOtherThing(Object input) — newacct
– newacct, Commented Oct 9, 2016 at 18:49

Bastien · Accepted Answer · 2016-09-23 10:09:37Z

8

Pass the S class as an argument.

public class Something<T>
{
    public static <T> Something<T> newInstance(Class<T> type)
    {
        return new Something<T>();
    }

    public void doSomething(T input){;}

    public <S> void doOtherThing(Class<S> clazz, S input)
    {
        Something<S> smt = Something.newInstance(clazz);
        smt.doSomething(input);
    }
}

answered Sep 23, 2016 at 10:09

Bastien

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6 Comments

Bastien Over a year ago

But really doOtherThing is rather useless if not static. Basically it's just creating a Something instance and applying doSomething, with generic type S instead of T.

lethanh Over a year ago

2 argument from same class S to use generic, it's slight redundant

Bastien Over a year ago

Only way to avoid an unchecked cast warning.

user140547 Over a year ago

Actually, for this example, Class<T> type is unused, so you don't even need that.

Drazen Bjelovuk Over a year ago

Interesting. Does the T in the Something<T> return type reference the argument-inferred generic, or the generic of the lexical class?

|

ChenHuang · Accepted Answer · 2016-09-23 09:55:08Z

7

I think input.getClass() need be cast to Class<T>

public <S> void doOtherThing(S input){
      Something smt = Something.newInstance((Class<T>)input.getClass());
      smt.doSomething(input);
}

answered Sep 23, 2016 at 9:55

ChenHuang

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1 Comment

Bastien Over a year ago

This is unsafe because of raw type and unchecked casting.

Ivan · Accepted Answer · 2016-09-23 09:49:49Z

1

Anything like this? (generic type declaration thingy on our newInstance method)

public class Something<T> {
   public static <T> Something<T> newInstance(Class<T> type){ return null; }
   public <T> void doSomething(T input){};
}

answered Sep 23, 2016 at 9:49

Ivan

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1 Comment

lethanh Over a year ago

Nope, It work fine with non-generic variable (like String)

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