How to write a method which accepts any collection of classes which extend Throwable?

Question

I want to pass a collection of classes which extend Throwable to a method. However, my attempt fails to compile in Netbeans with jdk1.6.0_39:

package com.memex.sessionmanager;

import java.util.Collection;
import java.util.Collections;

public class GenericsDemo
{
    public void foo(Collection<Class<? extends Throwable>> c)
    {
    }

    public void bar()
    {
        foo(Collections.singleton(RuntimeException.class));
    }
}

The compiler says foo(java.util.Collection<java.lang.Class<? extends java.lang.Throwable>>) in com.memex.sessionmanager.GenericsDemo cannot be applied to (java.util.Set<java.lang.Class<java.lang.RuntimeException>>).

I can make it compile by changing the calling code to:

foo(Collections.<Class<? extends Throwable>>singleton(RuntimeException.class));

but I'd rather not force clients to write this obtuse generic type. Is it possible to write a method which will take any collection of classes which subtype Throwable?

EDIT:

Someone fixed the above example with the method signature:

public <T extends Throwable> void foo(Collection<Class<T>> c) {}

But I'm now having problems using the parameter. Specifically, I want to assign it to a field, like this:

package com.memex.sessionmanager;

import java.util.Collection;
import java.util.Collections;

public class GenericsDemo
{

    private Collection<Class<? extends Throwable>> throwables;

    public <T extends Throwable> void foo(Collection<Class<T>> c)
    {
        throwables = c;
    }

    public void bar()
    {
        foo(Collections.singleton(RuntimeException.class));
    }
}

But the compiler says:

incompatible types
found   : java.util.Collection<java.lang.Class<T>>
required: java.util.Set<java.lang.Class<? extends java.lang.Throwable>>

Related: [incompatible types and fresh type-variable](http://stackoverflow.com/questions/20543966/incompatible-types-and-fresh-type-variable) — Paul Bellora, Jan 28 '14 at 17:23

score 6 · Answer 1 · answered Jan 28 '14 at 15:32

6

Use

public <T extends Throwable> void foo(Collection<Class<T>> c) {}

answered Jan 28 '14 at 15:32

Someone

551
3
14

Sorry for so many edits; I kept messing up the formatting. I wish people would wait before downvoting. :L – Someone Jan 28 '14 at 15:34
This was very prompt and fixed my example, thank you. But I'm now having trouble using the parameter. I've edited the question. – MikeFHay Jan 28 '14 at 15:59
@MikeFHay So you want to pass it the set returned from Collections.singleton? – Someone Jan 28 '14 at 16:05
Sorry the example is contrived. I want to pass it any collection of `Throwable` Classes (including the `Set>` returned by `Collections.singleton` in my example). I then want to assign that Collection to a field. – MikeFHay Jan 28 '14 at 16:10

score 4 · Accepted Answer · answered Jan 28 '14 at 15:43

4

This should work on Java 7:

import java.util.Collection;
import java.util.Collections;

public class Generics {
  public void foo(Collection<? extends Class<? extends Throwable>> c) { 
  }

  {
    foo(Collections.singleton(RuntimeException.class));
  }
}

answered Jan 28 '14 at 15:43

McDowell

107,573
31
204
267

Looks like it works for me on Java 6 as well, just took me a while to write correctly. Thanks :) – MikeFHay Jan 28 '14 at 16:16
ok, but how to pass actual collection with more than one element there? – hi_my_name_is May 29 '20 at 09:40

How to write a method which accepts any collection of classes which extend Throwable?

2 Answers2