### Question:
HealthStatsMetrics extends SystemMetrics<HealthStatsMetrics> { @VisibleForTesting static <K> SparseArray<K> op(int op, SparseArray<K> a, SparseArray<K> b, SparseArray<K> output) { output.clear(); for (int i = 0; i < a.size(); i++) { int aKey = a.keyAt(i); output.put(aKey, (K) opValues(op, a.valueAt(i), b.get(aKey))); } if (op == OP_SUM) { for (int i = 0; i < b.size(); i++) { int bKey = b.keyAt(i); if (a.get(bKey) == null) { output.put(bKey, b.valueAt(i)); } } } return output; } HealthStatsMetrics();  HealthStatsMetrics(HealthStats healthStats);  HealthStatsMetrics(HealthStatsMetrics metrics); @Override HealthStatsMetrics sum(
      @Nullable HealthStatsMetrics b, @Nullable HealthStatsMetrics output); @Override HealthStatsMetrics diff(
      @Nullable HealthStatsMetrics b, @Nullable HealthStatsMetrics output); @Override HealthStatsMetrics set(HealthStatsMetrics b); HealthStatsMetrics set(HealthStats healthStats); @Override String toString(); static String getKeyName(int key); JSONObject toJSONObject(); @Override boolean equals(Object o); @Override int hashCode(); public String dataType; final SparseArray<Long> measurement; final SparseArray<TimerMetrics> timer; final SparseArray<ArrayMap<String, Long>> measurements; final SparseArray<ArrayMap<String, TimerMetrics>> timers; final SparseArray<ArrayMap<String, HealthStatsMetrics>> stats; }### Answer:
@Test public void testSumSparseArrays() { SparseArray<Long> a = new SparseArray<>(); a.put(10, 10L); a.put(30, 30L); SparseArray<Long> b = new SparseArray<>(); b.put(10, 10L); b.put(20, 20L); SparseArray<Long> sum = new SparseArray<>(); HealthStatsMetrics.op(OP_SUM, a, b, sum); assertThat(sum.get(10)).isEqualTo(20); assertThat(sum.get(20)).isEqualTo(20); assertThat(sum.get(30)).isEqualTo(30); assertThat(sum.size()).isEqualTo(3); }
@Test public void testDiffSparseArrays() { SparseArray<Long> a = new SparseArray<>(); a.put(10, 10L); a.put(30, 30L); SparseArray<Long> b = new SparseArray<>(); b.put(10, 10L); b.put(20, 20L); SparseArray<Long> sum = new SparseArray<>(); HealthStatsMetrics.op(OP_DIFF, a, b, sum); assertThat(sum.get(10)).isEqualTo(0); assertThat(sum.get(30)).isEqualTo(30); assertThat(sum.size()).isEqualTo(2); }