content: /*
   * Licensed to the Apache Software Foundation (ASF) under one or more
   * contributor license agreements.  See the NOTICE file distributed with
   * this work for additional information regarding copyright ownership.
   * The ASF licenses this file to You 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 com.alibaba.metrics;
  
  /**
   * A gauge whose value is derived from the value of another gauge.
   *
   * @param <F> the base gauge's value type
   * @param <T> the derivative type
   */
  public abstract class DerivativeGauge<F, T> implements Gauge<T> {
  
      private final Gauge<F> base;
      private long lastUpdate;
  
      /**
       * Creates a new derivative with the given base gauge.
       *
       * @param base the gauge from which to derive this gauge's value
       */
      protected DerivativeGauge(Gauge<F> base) {
          this.base = base;
      }
  
      @Override
      public T getValue() {
          T result = transform(base.getValue());
          lastUpdate = System.currentTimeMillis();
          return result;
      }
  
      @Override
      public long lastUpdateTime() {
          return lastUpdate;
      }
  
      /**
       * Transforms the value of the base gauge to the value of this gauge.
       *
       * @param value the value of the base gauge
       * @return this gauge's value
       */
      protected abstract T transform(F value);
  }
  