How to write a method which accepts any collection of classes which extend Throwable?
generics, java
Solution
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));
}
}
Problem
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>> ```