code: /*
   * #%L
   * Simmetrics Core
   * %%
   * Copyright (C) 2014 - 2016 Simmetrics 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.
   * #L%
   */
  package org.simmetrics;
  
  /**
   * Measures the similarity between two arbitrary objects of the same type . The
   * measurement results in a value between 0 and 1 inclusive. A value of zero
   * indicates that the sets are dissimilar, a value of 1 indicates they are
   * similar.
   * <p>
   * The similarity measure should be consistent with equals such that
   * {@code a.equals(b) => compare(a,b) == 1.0}.
   * <p>
   * The similarity measure should be reflexive such that
   * {@code compare(a,a) == 1.0}.
   * <p>
   * The similarity measure should be symmetric such that
   * {@code compare(a,b) == compare(b,a)}.
   * <p>
   * The similarity measure may be transitive such that
   * {@code 1.0 - compare(a, c) ≤ 1.0 - compare(a, b) + 1.0 - compare(b, c)}
   * <p>
   * The similarity measure may satisfy the coincidence axiom such that
   * {@code compare(a, b) = 0 if and only if a.equals(b)}
   * <p>
   * Implementations may not modify the arguments. Arguments should be treated as
   * if they were unmodifiable.
   * 
   * @see <a href="https://en.wikipedia.org/wiki/Metric_(mathematics)">Wikipedia -
   *      Metric</a>
   * 
   * @param <T>
   *            type of the elements compared
   * 
   */
  
  public interface Metric<T> {
  	/**
  	 * Measures the similarity between a and b. The measurement results in a
  	 * value between 0 and 1 inclusive. A value of {@code 0.0} indicates that
  	 * {@code a} and {@code b} are dissimilar, a value of {@code 1.0} indicates
  	 * they are similar.
  	 * 
  	 * @param a
  	 *            object a to compare
  	 * @param b
  	 *            object b to compare
  	 * @return a value between 0 and 1 inclusive indicating similarity
  	 * @throws NullPointerException
  	 *             when either a or b is null
  	 */
  	public float compare(T a, T b);
  
  }
  