Gauge API : Add LongGauge Support (Part1) (#1489)

* Gauge API : Add LongGauge Support (Part1)

* Fix review comments

* Fix review comments, removed LongGauge from registry

* Add TODO and remove unnecessary check on removeTimeSeries

7 files changed, 884 insertions, 0 deletions

diff --git a/api/src/main/java/io/opencensus/metrics/LongGauge.java b/api/src/main/java/io/opencensus/metrics/LongGauge.java
new file mode 100644
index 00000000..2294f0cf
--- /dev/null
+++ b/api/src/main/java/io/opencensus/metrics/LongGauge.java
@@ -0,0 +1,211 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import io.opencensus.internal.Utils;
+import java.util.List;
+import javax.annotation.concurrent.ThreadSafe;
+
+/**
+ * Long Gauge metric, to report instantaneous measurement of an int64 value. Gauges can go both up
+ * and down. The gauges values can be negative.
+ *
+ * <p>Example 1: Create a Gauge with default labels.
+ *
+ * <pre>{@code
+ * class YourClass {
+ *
+ *   private static final MetricRegistry metricRegistry = Metrics.getMetricRegistry();
+ *
+ *   List<LabelKey> labelKeys = Arrays.asList(LabelKey.create("Name", "desc"));
+ *   // TODO(mayurkale): Plugs-in the LongGauge into the registry.
+ *   LongGauge gauge = metricRegistry.addLongGauge("queue_size", "Pending jobs", "1", labelKeys);
+ *
+ *   // It is recommended to keep a reference of a point for manual operations.
+ *   LongPoint defaultPoint = gauge.getDefaultTimeSeries();
+ *
+ *   void doWork() {
+ *      // Your code here.
+ *      defaultPoint.add(10);
+ *   }
+ *
+ * }
+ * }</pre>
+ *
+ * <p>Example 2: You can also use labels(keys and values) to track different types of metric.
+ *
+ * <pre>{@code
+ * class YourClass {
+ *
+ *   private static final MetricRegistry metricRegistry = Metrics.getMetricRegistry();
+ *
+ *   List<LabelKey> labelKeys = Arrays.asList(LabelKey.create("Name", "desc"));
+ *   List<LabelValue> labelValues = Arrays.asList(LabelValue.create("Inbound"));
+ *
+ *   // TODO(mayurkale): Plugs-in the LongGauge into the registry.
+ *   LongGauge gauge = metricRegistry.addLongGauge("queue_size", "Pending jobs", "1", labelKeys);
+ *
+ *   // It is recommended to keep a reference of a point for manual operations.
+ *   LongPoint inboundPoint = gauge.getOrCreateTimeSeries(labelValues);
+ *
+ *   void doSomeWork() {
+ *      // Your code here.
+ *      inboundPoint.set(15);
+ *   }
+ *
+ * }
+ * }</pre>
+ *
+ * @since 0.17
+ */
+@ThreadSafe
+public abstract class LongGauge {
+
+  /**
+   * Creates a {@code TimeSeries} and returns a {@code LongPoint} if the specified {@code
+   * labelValues} is not already associated with this gauge, else returns a existing {@code
+   * LongPoint}.
+   *
+   * <p>It is recommended to keep a reference to the LongPoint instead of always calling this method
+   * for manual operations.
+   *
+   * @param labelValues the list of label values. The number of label values must be the same to
+   *     that of the label keys passed to {@link MetricRegistry#addLongGauge}.
+   * @return a {@code LongPoint} the value of single gauge.
+   * @throws NullPointerException if {@code labelValues} is null OR any element of {@code
+   *     labelValues} is null.
+   * @throws IllegalArgumentException if number of {@code labelValues}s are not equal to the label
+   *     keys passed to {@link MetricRegistry#addLongGauge}.
+   * @since 0.17
+   */
+  public abstract LongPoint getOrCreateTimeSeries(List<LabelValue> labelValues);
+
+  /**
+   * Returns a {@code LongPoint} for a gauge with all labels not set, or default labels.
+   *
+   * @return a {@code LongPoint} the value of default gauge.
+   * @since 0.17
+   */
+  public abstract LongPoint getDefaultTimeSeries();
+
+  /**
+   * Removes the {@code TimeSeries} from the gauge metric, if it is present. i.e. references to
+   * previous {@code LongPoint} objects are invalid (not part of the metric).
+   *
+   * @param labelValues the list of label values.
+   * @throws NullPointerException if {@code labelValues} is null or any element of {@code
+   *     labelValues} is null.
+   * @since 0.17
+   */
+  public abstract void removeTimeSeries(List<LabelValue> labelValues);
+
+  /**
+   * References to all previous {@code LongPoint} objects are invalid (not part of the metric).
+   *
+   * @since 0.17
+   */
+  public abstract void clear();
+
+  /**
+   * Returns the no-op implementation of the {@code LongGauge}.
+   *
+   * @return the no-op implementation of the {@code LongGauge}.
+   * @since 0.17
+   */
+  static LongGauge newNoopLongGauge(
+      String name, String description, String unit, List<LabelKey> labelKeys) {
+    return NoopLongGauge.create(name, description, unit, labelKeys);
+  }
+
+  /**
+   * The value of a single point in the Gauge.TimeSeries.
+   *
+   * @since 0.17
+   */
+  public abstract static class LongPoint {
+
+    /**
+     * Adds the given value to the current value. The values can be negative.
+     *
+     * @param amt the value to add
+     * @since 0.17
+     */
+    public abstract void add(long amt);
+
+    /**
+     * Sets the given value.
+     *
+     * @param val the new value.
+     * @since 0.17
+     */
+    public abstract void set(long val);
+  }
+
+  /** No-op implementations of LongGauge class. */
+  private static final class NoopLongGauge extends LongGauge {
+    private final int labelKeysSize;
+
+    static NoopLongGauge create(
+        String name, String description, String unit, List<LabelKey> labelKeys) {
+      return new NoopLongGauge(name, description, unit, labelKeys);
+    }
+
+    /** Creates a new {@code NoopLongPoint}. */
+    NoopLongGauge(String name, String description, String unit, List<LabelKey> labelKeys) {
+      Utils.checkNotNull(name, "name");
+      Utils.checkNotNull(description, "description");
+      Utils.checkNotNull(unit, "unit");
+      Utils.checkNotNull(labelKeys, "labelKeys should not be null.");
+      Utils.checkListElementNotNull(labelKeys, "labelKeys element should not be null.");
+      labelKeysSize = labelKeys.size();
+    }
+
+    @Override
+    public NoopLongPoint getOrCreateTimeSeries(List<LabelValue> labelValues) {
+      Utils.checkNotNull(labelValues, "labelValues should not be null.");
+      Utils.checkListElementNotNull(labelValues, "labelValues element should not be null.");
+      Utils.checkArgument(labelKeysSize == labelValues.size(), "Incorrect number of labels.");
+      return NoopLongPoint.INSTANCE;
+    }
+
+    @Override
+    public NoopLongPoint getDefaultTimeSeries() {
+      return NoopLongPoint.INSTANCE;
+    }
+
+    @Override
+    public void removeTimeSeries(List<LabelValue> labelValues) {
+      Utils.checkNotNull(labelValues, "labelValues should not be null.");
+    }
+
+    @Override
+    public void clear() {}
+
+    /** No-op implementations of LongPoint class. */
+    private static final class NoopLongPoint extends LongPoint {
+      private static final NoopLongPoint INSTANCE = new NoopLongPoint();
+
+      private NoopLongPoint() {}
+
+      @Override
+      public void add(long amt) {}
+
+      @Override
+      public void set(long val) {}
+    }
+  }
+}
diff --git a/api/src/test/java/io/opencensus/metrics/LongGaugeTest.java b/api/src/test/java/io/opencensus/metrics/LongGaugeTest.java
new file mode 100644
index 00000000..768abe06
--- /dev/null
+++ b/api/src/test/java/io/opencensus/metrics/LongGaugeTest.java
@@ -0,0 +1,87 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.metrics;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link LongGauge}. */
+@RunWith(JUnit4.class)
+public class LongGaugeTest {
+  @Rule public ExpectedException thrown = ExpectedException.none();
+
+  private static final String NAME = "name";
+  private static final String DESCRIPTION = "description";
+  private static final String UNIT = "1";
+  private static final List<LabelKey> LABEL_KEY =
+      Collections.singletonList(LabelKey.create("key", "key description"));
+  private static final List<LabelValue> LABEL_VALUES =
+      Collections.singletonList(LabelValue.create("value"));
+  private static final List<LabelKey> EMPTY_LABEL_KEYS = new ArrayList<LabelKey>();
+  private static final List<LabelValue> EMPTY_LABEL_VALUES = new ArrayList<LabelValue>();
+
+  // TODO(mayurkale): Add more tests, once LongGauge plugs-in into the registry.
+
+  @Test
+  public void noopGetOrCreateTimeSeries_WithNullLabelValues() {
+    LongGauge longGauge = LongGauge.newNoopLongGauge(NAME, DESCRIPTION, UNIT, EMPTY_LABEL_KEYS);
+    thrown.expect(NullPointerException.class);
+    thrown.expectMessage("labelValues should not be null.");
+    longGauge.getOrCreateTimeSeries(null);
+  }
+
+  @Test
+  public void noopGetOrCreateTimeSeries_WithNullElement() {
+    List<LabelValue> labelValues = Collections.singletonList(null);
+    LongGauge longGauge = LongGauge.newNoopLongGauge(NAME, DESCRIPTION, UNIT, LABEL_KEY);
+    thrown.expect(NullPointerException.class);
+    thrown.expectMessage("labelValues element should not be null.");
+    longGauge.getOrCreateTimeSeries(labelValues);
+  }
+
+  @Test
+  public void noopGetOrCreateTimeSeries_WithInvalidLabelSize() {
+    LongGauge longGauge = LongGauge.newNoopLongGauge(NAME, DESCRIPTION, UNIT, LABEL_KEY);
+    thrown.expect(IllegalArgumentException.class);
+    thrown.expectMessage("Incorrect number of labels.");
+    longGauge.getOrCreateTimeSeries(EMPTY_LABEL_VALUES);
+  }
+
+  @Test
+  public void noopRemoveTimeSeries_WithNullLabelValues() {
+    LongGauge longGauge = LongGauge.newNoopLongGauge(NAME, DESCRIPTION, UNIT, LABEL_KEY);
+    thrown.expect(NullPointerException.class);
+    thrown.expectMessage("labelValues should not be null.");
+    longGauge.removeTimeSeries(null);
+  }
+
+  @Test
+  public void noopSameAs() {
+    LongGauge longGauge = LongGauge.newNoopLongGauge(NAME, DESCRIPTION, UNIT, LABEL_KEY);
+    assertThat(longGauge.getDefaultTimeSeries()).isSameAs(longGauge.getDefaultTimeSeries());
+    assertThat(longGauge.getDefaultTimeSeries())
+        .isSameAs(longGauge.getOrCreateTimeSeries(LABEL_VALUES));
+  }
+}
diff --git a/impl_core/src/main/java/io/opencensus/implcore/internal/Utils.java b/impl_core/src/main/java/io/opencensus/implcore/internal/Utils.java
new file mode 100644
index 00000000..05a039b9
--- /dev/null
+++ b/impl_core/src/main/java/io/opencensus/implcore/internal/Utils.java
@@ -0,0 +1,41 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.implcore.internal;
+
+import java.util.List;
+
+/** General internal utility methods. */
+public final class Utils {
+
+  private Utils() {}
+
+  /**
+   * Throws a {@link NullPointerException} if any of the list elements is null.
+   *
+   * @param list the argument list to check for null.
+   * @param errorMessage the message to use for the exception. Will be converted to a string using
+   *     {@link String#valueOf(Object)}.
+   */
+  public static <T> void checkListElementNotNull(
+      List<T> list, @javax.annotation.Nullable Object errorMessage) {
+    for (T element : list) {
+      if (element == null) {
+        throw new NullPointerException(String.valueOf(errorMessage));
+      }
+    }
+  }
+}
diff --git a/impl_core/src/main/java/io/opencensus/implcore/metrics/LongGaugeImpl.java b/impl_core/src/main/java/io/opencensus/implcore/metrics/LongGaugeImpl.java
new file mode 100644
index 00000000..4ad4e354
--- /dev/null
+++ b/impl_core/src/main/java/io/opencensus/implcore/metrics/LongGaugeImpl.java
@@ -0,0 +1,172 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.implcore.metrics;
+
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.base.Preconditions.checkNotNull;
+
+import com.google.common.annotations.VisibleForTesting;
+import io.opencensus.common.Clock;
+import io.opencensus.implcore.internal.Utils;
+import io.opencensus.metrics.LabelKey;
+import io.opencensus.metrics.LabelValue;
+import io.opencensus.metrics.LongGauge;
+import io.opencensus.metrics.export.Metric;
+import io.opencensus.metrics.export.MetricDescriptor;
+import io.opencensus.metrics.export.MetricDescriptor.Type;
+import io.opencensus.metrics.export.Point;
+import io.opencensus.metrics.export.TimeSeries;
+import io.opencensus.metrics.export.Value;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.atomic.AtomicLong;
+import javax.annotation.Nullable;
+
+/** Implementation of {@link LongGauge}. */
+public final class LongGaugeImpl extends LongGauge implements Meter {
+  @VisibleForTesting static final LabelValue UNSET_VALUE = LabelValue.create(null);
+
+  private final MetricDescriptor metricDescriptor;
+  private volatile Map<List<LabelValue>, PointImpl> registeredPoints = Collections.emptyMap();
+  private final int labelKeysSize;
+  private final List<LabelValue> defaultLabelValues;
+
+  LongGaugeImpl(String name, String description, String unit, List<LabelKey> labelKeys) {
+    labelKeysSize = labelKeys.size();
+    this.metricDescriptor =
+        MetricDescriptor.create(name, description, unit, Type.GAUGE_INT64, labelKeys);
+
+    // initialize defaultLabelValues
+    defaultLabelValues = new ArrayList<LabelValue>(labelKeysSize);
+    for (int i = 0; i < labelKeysSize; i++) {
+      defaultLabelValues.add(UNSET_VALUE);
+    }
+  }
+
+  @Override
+  public LongPoint getOrCreateTimeSeries(List<LabelValue> labelValues) {
+    // lock free point retrieval, if it is present
+    PointImpl existingPoint = registeredPoints.get(labelValues);
+    if (existingPoint != null) {
+      return existingPoint;
+    }
+
+    List<LabelValue> labelValuesCopy =
+        Collections.unmodifiableList(
+            new ArrayList<LabelValue>(
+                checkNotNull(labelValues, "labelValues should not be null.")));
+    return registerTimeSeries(labelValuesCopy);
+  }
+
+  @Override
+  public LongPoint getDefaultTimeSeries() {
+    // lock free default point retrieval, if it is present
+    PointImpl existingPoint = registeredPoints.get(defaultLabelValues);
+    if (existingPoint != null) {
+      return existingPoint;
+    }
+    return registerTimeSeries(defaultLabelValues);
+  }
+
+  @Override
+  public synchronized void removeTimeSeries(List<LabelValue> labelValues) {
+    checkNotNull(labelValues, "labelValues should not be null.");
+
+    Map<List<LabelValue>, PointImpl> registeredPointsCopy =
+        new HashMap<List<LabelValue>, PointImpl>(registeredPoints);
+    if (registeredPointsCopy.remove(labelValues) == null) {
+      // The element not present, no need to update the current map of points.
+      return;
+    }
+    registeredPoints = Collections.unmodifiableMap(registeredPointsCopy);
+  }
+
+  @Override
+  public synchronized void clear() {
+    registeredPoints = Collections.emptyMap();
+  }
+
+  private synchronized LongPoint registerTimeSeries(List<LabelValue> labelValues) {
+    PointImpl existingPoint = registeredPoints.get(labelValues);
+    if (existingPoint != null) {
+      // Return a Point that are already registered. This can happen if a multiple threads
+      // concurrently try to register the same {@code TimeSeries}.
+      return existingPoint;
+    }
+
+    checkArgument(labelKeysSize == labelValues.size(), "Incorrect number of labels.");
+    Utils.checkListElementNotNull(labelValues, "labelValues element should not be null.");
+
+    PointImpl newPoint = new PointImpl(labelValues);
+    // Updating the map of points happens under a lock to avoid multiple add operations
+    // to happen in the same time.
+    Map<List<LabelValue>, PointImpl> registeredPointsCopy =
+        new HashMap<List<LabelValue>, PointImpl>(registeredPoints);
+    registeredPointsCopy.put(labelValues, newPoint);
+    registeredPoints = Collections.unmodifiableMap(registeredPointsCopy);
+
+    return newPoint;
+  }
+
+  @Nullable
+  @Override
+  public Metric getMetric(Clock clock) {
+    Map<List<LabelValue>, PointImpl> currentRegisteredPoints = registeredPoints;
+
+    int pointCount = currentRegisteredPoints.size();
+    if (pointCount > 0) {
+      List<TimeSeries> timeSeriesList = new ArrayList<TimeSeries>(pointCount);
+      for (Map.Entry<List<LabelValue>, PointImpl> entry : currentRegisteredPoints.entrySet()) {
+        timeSeriesList.add(entry.getValue().getTimeSeries(clock));
+      }
+
+      // TODO(mayurkale): optimize this for 1 timeseries (issue #1491).
+      return Metric.create(metricDescriptor, timeSeriesList);
+    }
+    return null;
+  }
+
+  /** Implementation of {@link LongGauge.LongPoint}. */
+  public static final class PointImpl extends LongPoint {
+
+    // TODO(mayurkale): Consider to use LongAdder here, once we upgrade to Java8.
+    private final AtomicLong value = new AtomicLong(0);
+    private final List<LabelValue> labelValues;
+
+    PointImpl(List<LabelValue> labelValues) {
+      this.labelValues = labelValues;
+    }
+
+    @Override
+    public void add(long amt) {
+      value.addAndGet(amt);
+    }
+
+    @Override
+    public void set(long val) {
+      value.set(val);
+    }
+
+    private TimeSeries getTimeSeries(Clock clock) {
+      return TimeSeries.createWithOnePoint(
+          labelValues, Point.create(Value.longValue(value.get()), clock.now()), null);
+    }
+  }
+}
diff --git a/impl_core/src/main/java/io/opencensus/implcore/metrics/Meter.java b/impl_core/src/main/java/io/opencensus/implcore/metrics/Meter.java
new file mode 100644
index 00000000..f5a8dc8f
--- /dev/null
+++ b/impl_core/src/main/java/io/opencensus/implcore/metrics/Meter.java
@@ -0,0 +1,34 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.implcore.metrics;
+
+import io.opencensus.common.Clock;
+import io.opencensus.metrics.export.Metric;
+import javax.annotation.Nullable;
+
+interface Meter {
+  /**
+   * Provides a {@link io.opencensus.metrics.export.Metric} with one or more {@link
+   * io.opencensus.metrics.export.TimeSeries}.
+   *
+   * @param clock the clock used to get the time.
+   * @throws NullPointerException if {@code TimeSeries} is not present in {@code Metric}.
+   * @return a {@code Metric}.
+   */
+  @Nullable
+  Metric getMetric(Clock clock);
+}
diff --git a/impl_core/src/test/java/io/opencensus/implcore/internal/UtilsTest.java b/impl_core/src/test/java/io/opencensus/implcore/internal/UtilsTest.java
new file mode 100644
index 00000000..2e0bde21
--- /dev/null
+++ b/impl_core/src/test/java/io/opencensus/implcore/internal/UtilsTest.java
@@ -0,0 +1,47 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.implcore.internal;
+
+import java.util.Arrays;
+import java.util.List;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link Utils}. */
+@RunWith(JUnit4.class)
+public class UtilsTest {
+  @Rule public ExpectedException thrown = ExpectedException.none();
+
+  @Test
+  public void checkListElementNull() {
+    List<Double> list = Arrays.asList(0.0, 1.0, 2.0, null);
+    thrown.expect(NullPointerException.class);
+    thrown.expectMessage("null");
+    Utils.checkListElementNotNull(list, null);
+  }
+
+  @Test
+  public void checkListElementNull_WithMessage() {
+    List<Double> list = Arrays.asList(0.0, 1.0, 2.0, null);
+    thrown.expect(NullPointerException.class);
+    thrown.expectMessage("list should not be null.");
+    Utils.checkListElementNotNull(list, "list should not be null.");
+  }
+}
diff --git a/impl_core/src/test/java/io/opencensus/implcore/metrics/LongGaugeImplTest.java b/impl_core/src/test/java/io/opencensus/implcore/metrics/LongGaugeImplTest.java
new file mode 100644
index 00000000..0a1c2826
--- /dev/null
+++ b/impl_core/src/test/java/io/opencensus/implcore/metrics/LongGaugeImplTest.java
@@ -0,0 +1,292 @@
+/*
+ * Copyright 2018, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.implcore.metrics;
+
+import static com.google.common.truth.Truth.assertThat;
+import static io.opencensus.implcore.metrics.LongGaugeImpl.UNSET_VALUE;
+
+import com.google.common.testing.EqualsTester;
+import io.opencensus.common.Timestamp;
+import io.opencensus.metrics.LabelKey;
+import io.opencensus.metrics.LabelValue;
+import io.opencensus.metrics.LongGauge.LongPoint;
+import io.opencensus.metrics.export.Metric;
+import io.opencensus.metrics.export.MetricDescriptor;
+import io.opencensus.metrics.export.MetricDescriptor.Type;
+import io.opencensus.metrics.export.Point;
+import io.opencensus.metrics.export.TimeSeries;
+import io.opencensus.metrics.export.Value;
+import io.opencensus.testing.common.TestClock;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link LongGaugeImpl}. */
+@RunWith(JUnit4.class)
+public class LongGaugeImplTest {
+  @Rule public ExpectedException thrown = ExpectedException.none();
+
+  private static final String METRIC_NAME = "name";
+  private static final String METRIC_DESCRIPTION = "description";
+  private static final String METRIC_UNIT = "1";
+  private static final List<LabelKey> LABEL_KEY =
+      Collections.singletonList(LabelKey.create("key", "key description"));
+  private static final List<LabelValue> LABEL_VALUES =
+      Collections.singletonList(LabelValue.create("value"));
+  private static final List<LabelValue> LABEL_VALUES1 =
+      Collections.singletonList(LabelValue.create("value1"));
+  private static final List<LabelValue> DEFAULT_LABEL_VALUES =
+      Collections.singletonList(UNSET_VALUE);
+
+  private static final Timestamp TEST_TIME = Timestamp.create(1234, 123);
+  private final TestClock testClock = TestClock.create(TEST_TIME);
+  private static final MetricDescriptor METRIC_DESCRIPTOR =
+      MetricDescriptor.create(
+          METRIC_NAME, METRIC_DESCRIPTION, METRIC_UNIT, Type.GAUGE_INT64, LABEL_KEY);
+  private final LongGaugeImpl longGaugeMetric =
+      new LongGaugeImpl(METRIC_NAME, METRIC_DESCRIPTION, METRIC_UNIT, LABEL_KEY);
+
+  @Test
+  public void getOrCreateTimeSeries_WithNullLabelValues() {
+    thrown.expect(NullPointerException.class);
+    thrown.expectMessage("labelValues should not be null.");
+    longGaugeMetric.getOrCreateTimeSeries(null);
+  }
+
+  @Test
+  public void getOrCreateTimeSeries_WithNullElement() {
+    List<LabelKey> labelKeys =
+        Arrays.asList(LabelKey.create("key1", "desc"), LabelKey.create("key2", "desc"));
+    List<LabelValue> labelValues = Arrays.asList(LabelValue.create("value1"), null);
+
+    LongGaugeImpl longGauge =
+        new LongGaugeImpl(METRIC_NAME, METRIC_DESCRIPTION, METRIC_UNIT, labelKeys);
+    thrown.expect(NullPointerException.class);
+    thrown.expectMessage("labelValues element should not be null.");
+    longGauge.getOrCreateTimeSeries(labelValues);
+  }
+
+  @Test
+  public void getOrCreateTimeSeries_WithInvalidLabelSize() {
+    List<LabelValue> labelValues =
+        Arrays.asList(LabelValue.create("value1"), LabelValue.create("value2"));
+
+    thrown.expect(IllegalArgumentException.class);
+    thrown.expectMessage("Incorrect number of labels.");
+    longGaugeMetric.getOrCreateTimeSeries(labelValues);
+  }
+
+  @Test
+  public void getOrCreateTimeSeries() {
+    LongPoint point = longGaugeMetric.getOrCreateTimeSeries(LABEL_VALUES);
+    point.add(100);
+    LongPoint point1 = longGaugeMetric.getOrCreateTimeSeries(LABEL_VALUES);
+    point1.set(500);
+
+    Metric metric = longGaugeMetric.getMetric(testClock);
+    assertThat(metric).isNotEqualTo(null);
+    assertThat(metric)
+        .isEqualTo(
+            Metric.create(
+                METRIC_DESCRIPTOR,
+                Collections.singletonList(
+                    TimeSeries.createWithOnePoint(
+                        LABEL_VALUES, Point.create(Value.longValue(500), TEST_TIME), null))));
+    new EqualsTester().addEqualityGroup(point, point1).testEquals();
+  }
+
+  @Test
+  public void getOrCreateTimeSeries_WithNegativePointValues() {
+    LongPoint point = longGaugeMetric.getOrCreateTimeSeries(LABEL_VALUES);
+    point.add(-100);
+    point.add(-33);
+
+    Metric metric = longGaugeMetric.getMetric(testClock);
+    assertThat(metric).isNotEqualTo(null);
+    assertThat(metric.getMetricDescriptor()).isEqualTo(METRIC_DESCRIPTOR);
+    assertThat(metric.getTimeSeriesList().size()).isEqualTo(1);
+    assertThat(metric.getTimeSeriesList().get(0).getPoints().size()).isEqualTo(1);
+    assertThat(metric.getTimeSeriesList().get(0).getPoints().get(0).getValue())
+        .isEqualTo(Value.longValue(-133));
+    assertThat(metric.getTimeSeriesList().get(0).getPoints().get(0).getTimestamp())
+        .isEqualTo(TEST_TIME);
+    assertThat(metric.getTimeSeriesList().get(0).getStartTimestamp()).isEqualTo(null);
+  }
+
+  @Test
+  public void getDefaultTimeSeries() {
+    LongPoint point = longGaugeMetric.getDefaultTimeSeries();
+    point.add(100);
+    point.set(500);
+
+    LongPoint point1 = longGaugeMetric.getDefaultTimeSeries();
+    point1.add(-100);
+
+    Metric metric = longGaugeMetric.getMetric(testClock);
+    assertThat(metric).isNotEqualTo(null);
+    assertThat(metric)
+        .isEqualTo(
+            Metric.create(
+                METRIC_DESCRIPTOR,
+                Collections.singletonList(
+                    TimeSeries.createWithOnePoint(
+                        DEFAULT_LABEL_VALUES,
+                        Point.create(Value.longValue(400), TEST_TIME),
+                        null))));
+    new EqualsTester().addEqualityGroup(point, point1).testEquals();
+  }
+
+  @Test
+  public void removeTimeSeries() {
+    longGaugeMetric.getOrCreateTimeSeries(LABEL_VALUES);
+    assertThat(longGaugeMetric.getMetric(testClock))
+        .isEqualTo(
+            Metric.create(
+                METRIC_DESCRIPTOR,
+                Collections.singletonList(
+                    TimeSeries.createWithOnePoint(
+                        LABEL_VALUES, Point.create(Value.longValue(0), TEST_TIME), null))));
+
+    longGaugeMetric.removeTimeSeries(LABEL_VALUES);
+    assertThat(longGaugeMetric.getMetric(testClock)).isEqualTo(null);
+  }
+
+  @Test
+  public void removeTimeSeries_WithNullLabelValues() {
+    thrown.expect(NullPointerException.class);
+    thrown.expectMessage("labelValues should not be null.");
+    longGaugeMetric.removeTimeSeries(null);
+  }
+
+  @Test
+  public void clear() {
+    LongPoint longPoint = longGaugeMetric.getOrCreateTimeSeries(LABEL_VALUES);
+    longPoint.add(-11);
+    LongPoint defaultPoint = longGaugeMetric.getDefaultTimeSeries();
+    defaultPoint.set(100);
+
+    Metric metric = longGaugeMetric.getMetric(testClock);
+    assertThat(metric).isNotEqualTo(null);
+    assertThat(metric.getMetricDescriptor()).isEqualTo(METRIC_DESCRIPTOR);
+    assertThat(metric.getTimeSeriesList().size()).isEqualTo(2);
+
+    longGaugeMetric.clear();
+    assertThat(longGaugeMetric.getMetric(testClock)).isEqualTo(null);
+  }
+
+  @Test
+  public void setDefaultLabelValues() {
+    List<LabelKey> labelKeys =
+        Arrays.asList(LabelKey.create("key1", "desc"), LabelKey.create("key2", "desc"));
+    LongGaugeImpl longGauge =
+        new LongGaugeImpl(METRIC_NAME, METRIC_DESCRIPTION, METRIC_UNIT, labelKeys);
+    LongPoint defaultPoint = longGauge.getDefaultTimeSeries();
+    defaultPoint.set(-230);
+
+    Metric metric = longGauge.getMetric(testClock);
+    assertThat(metric).isNotEqualTo(null);
+    assertThat(metric.getTimeSeriesList().size()).isEqualTo(1);
+    assertThat(metric.getTimeSeriesList().get(0).getLabelValues().size()).isEqualTo(2);
+    assertThat(metric.getTimeSeriesList().get(0).getLabelValues().get(0)).isEqualTo(UNSET_VALUE);
+    assertThat(metric.getTimeSeriesList().get(0).getLabelValues().get(1)).isEqualTo(UNSET_VALUE);
+  }
+
+  @Test
+  public void pointImpl_InstanceOf() {
+    LongPoint longPoint = longGaugeMetric.getOrCreateTimeSeries(LABEL_VALUES);
+    assertThat(longPoint).isInstanceOf(LongGaugeImpl.PointImpl.class);
+  }
+
+  @Test
+  public void multipleMetrics_GetMetric() {
+    LongPoint longPoint = longGaugeMetric.getOrCreateTimeSeries(LABEL_VALUES);
+    longPoint.add(1);
+    longPoint.add(2);
+
+    LongPoint defaultPoint = longGaugeMetric.getDefaultTimeSeries();
+    defaultPoint.set(100);
+
+    LongPoint longPoint1 = longGaugeMetric.getOrCreateTimeSeries(LABEL_VALUES1);
+    longPoint1.add(-100);
+    longPoint1.add(-20);
+
+    List<TimeSeries> timeSeriesList = new ArrayList<TimeSeries>();
+    timeSeriesList.add(
+        TimeSeries.createWithOnePoint(
+            LABEL_VALUES, Point.create(Value.longValue(3), TEST_TIME), null));
+    timeSeriesList.add(
+        TimeSeries.createWithOnePoint(
+            DEFAULT_LABEL_VALUES, Point.create(Value.longValue(100), TEST_TIME), null));
+    timeSeriesList.add(
+        TimeSeries.createWithOnePoint(
+            LABEL_VALUES1, Point.create(Value.longValue(-120), TEST_TIME), null));
+
+    Metric metric = longGaugeMetric.getMetric(testClock);
+    assertThat(metric).isNotEqualTo(null);
+    assertThat(metric.getMetricDescriptor()).isEqualTo(METRIC_DESCRIPTOR);
+    assertThat(metric.getTimeSeriesList().size()).isEqualTo(3);
+    assertThat(metric.getTimeSeriesList()).containsExactlyElementsIn(timeSeriesList);
+  }
+
+  @Test
+  public void empty_GetMetrics() {
+    assertThat(longGaugeMetric.getMetric(testClock)).isEqualTo(null);
+  }
+
+  @Test
+  public void testEquals() {
+    List<LabelKey> labelKeys =
+        Arrays.asList(LabelKey.create("key1", "desc"), LabelKey.create("key2", "desc"));
+    List<LabelValue> labelValues =
+        Arrays.asList(LabelValue.create("value1"), LabelValue.create("value2"));
+
+    LongGaugeImpl longGauge =
+        new LongGaugeImpl(METRIC_NAME, METRIC_DESCRIPTION, METRIC_UNIT, labelKeys);
+
+    LongPoint defaultPoint1 = longGauge.getDefaultTimeSeries();
+    LongPoint defaultPoint2 = longGauge.getDefaultTimeSeries();
+    LongPoint longPoint1 = longGauge.getOrCreateTimeSeries(labelValues);
+    LongPoint longPoint2 = longGauge.getOrCreateTimeSeries(labelValues);
+
+    new EqualsTester()
+        .addEqualityGroup(defaultPoint1, defaultPoint2)
+        .addEqualityGroup(longPoint1, longPoint2)
+        .testEquals();
+
+    longGauge.clear();
+
+    LongPoint newDefaultPointAfterClear = longGauge.getDefaultTimeSeries();
+    LongPoint newLongPointAfterClear = longGauge.getOrCreateTimeSeries(labelValues);
+
+    longGauge.removeTimeSeries(labelValues);
+    LongPoint newLongPointAfterRemove = longGauge.getOrCreateTimeSeries(labelValues);
+
+    new EqualsTester()
+        .addEqualityGroup(defaultPoint1, defaultPoint2)
+        .addEqualityGroup(longPoint1, longPoint2)
+        .addEqualityGroup(newDefaultPointAfterClear)
+        .addEqualityGroup(newLongPointAfterClear)
+        .addEqualityGroup(newLongPointAfterRemove)
+        .testEquals();
+  }
+}