### Question:
SystemMetrics implements Serializable { public abstract T set(T b);  abstract T sum(@Nullable T b, @Nullable T output); abstract T diff(@Nullable T b, @Nullable T output); abstract T set(T b); T sum(@Nullable T b); T diff(@Nullable T b);  }### Answer:
@Test public void testSet() throws Exception { T a = createInitializedInstance(); T empty = createInstance(); empty.set(a); assertThat(empty).isEqualTo(a); }