@Override
  public Object respond(IMockInvocation invocation) {
    IMockInteraction interaction = DefaultJavaLangObjectInteractions.INSTANCE.match(invocation);
    if (interaction != null) return interaction.accept(invocation).get();

    Class<?> returnType = invocation.getMethod().getReturnType();
    return ReflectionUtil.getDefaultValue(returnType);
  }
A response strategy that returns zero, false, or null, depending on the method's return type.
spock-core/src/main/java/org/spockframework/mock/ZeroOrNullResponse.java