Assign mock result to variable

maybe this is a bit simple, but i am just learning

I have this code:

public RediseImpl(RedissonClient client) {
keys = client.getSetCache("keys");
}

@Override
public boolean exists(String key) {
  boolean contains = keys.contains(key);
  return contains;
}

and i want to mock what happens if the contains method is true so i have the following code:

def "When given key doesn't exists, returns false" () {

    given:
    client.getSetCache(_ as String).contains(_ as String) >> true

    when:
    def res = RediseImpl.exists("id")

    then:
    res
}

I debug the method, but the contains always give a FALSE result. How can i mock it?