How to check if a generic type implements a specific type of generic interface in java? [duplicate]

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Java implements erasure, so there’s no way to tell on runtime if genericObject is an instance of Set<String> or not. The only way to guarantee this is to use bounds on your generics, or check all elements in the set.

Compile-time Generic Bounds

Using bounds checking, which will be checked at compile-time:

public <T extends SomeInterface> void genericMethod(Set<? extends T> tSet) {
    // Do something with tSet here
}

Java 8

We can use streams in Java 8 to do this natively in a single line:

public <T> void genericMethod(T t) {
    if (t instanceof Set<?>) {
        Set<?> set = (Set<?>) t;
        if (set.stream().allMatch(String.class:isInstance)) {
            Set<String> strs = (Set<String>) set;
            // Do something with strs here
        }
    }
}

Java 7 and older

With Java 7 and older, we need to use iteration and type checking:

public <T> void genericMethod(T t) {
    Set<String> strs = new HashSet<String>();
    Set<?> tAsSet;
    if (t instanceof Set<?>) {
        tAsSet = (Set<?>) t;
        for (Object obj : tAsSet) {
            if (obj instanceof String) {
                strs.add((String) obj);
            }
        }
        // Do something with strs here
    } else {
        // Throw an exception or log a warning or something.
    }
}

Guava

As per Mark Peters’ comment below, Guava also has methods that do this for you if you can add it to your project:

public <T> void genericMethod(T t) {
    if (t instanceof Set<?>) {
        Set<?> set = (Set<?>) t;
        if (Iterables.all(set, Predicates.instanceOf(String.class))) {
            Set<String> strs = (Set<String>) set;
            // Do something with strs here
        }
    }
}

The statement, Iterables.all(set, Predicates.instanceOf(String.class)) is essentially the same thing as set instanceof Set<String>.

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