how to write a java method with generic class type?

debugcn Published at Dev

Elad Benda

I want to write a generic method (either with generics or with param)

that will replace these two calls:

private ServerEvent filterEvents() {
    return argThat(new ArgumentMatcher<ServerEvent>() {
        @Override
        public boolean matches(Object argument) {
            return argument instanceof Type1;
        }
    });

private ServerEvent filterEvents() {
    return argThat(new ArgumentMatcher<ServerEvent>() {
        @Override
        public boolean matches(Object argument) {
            return argument instanceof Type2;
        }
    });

I have tried this, but got compilation errors:

private <T> ServerEvent filterEvents() {
    return argThat(new ArgumentMatcher<ServerEvent>() {
        @Override
        public boolean matches(Object argument) {
            return argument instanceof T;
        }
    });

update:

I have tried this also, but still got compiliation error:

    verify(loggerUtilsWrapper).writeEvent(filterEvents(MatchNotFoundEvent.class));


private ServerEvent filterEvents(final Class<MatcherEvent> eventType) {
    return argThat(new ArgumentMatcher<ServerEvent>() {
        @Override
        public boolean matches(Object argument) {
            return argument instanceof eventType;
        }
    });
}

Bohemian

Due to runtime type erasure, the "T" is not available. The standard way is to pass a type token by way of a parameter of type Class<T>.

However, in this case you don't need generic, because you aren't actually using the type. Instead, just pass a Class object and compare it:

private ServerEvent filterEvents(final Class<?> c) {
    return argThat(new ArgumentMatcher<ServerEvent>() {
        @Override
        public boolean matches(Object argument) {
            return c.isInstance(argument);
        }
    });

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