Java tutorial
/* * 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 org.apache.commons.collections4.functors; import java.io.Serializable; import java.util.HashSet; import java.util.Set; import org.apache.commons.collections4.Predicate; /** * Predicate implementation that returns true the first time an object is * passed into the predicate. * * @since 3.0 * @version $Id: UniquePredicate.java 1476582 2013-04-27 14:13:54Z tn $ */ public final class UniquePredicate<T> implements Predicate<T>, Serializable { /** Serial version UID */ private static final long serialVersionUID = -3319417438027438040L; /** The set of previously seen objects */ private final Set<T> iSet = new HashSet<T>(); /** * Factory to create the predicate. * * @param <T> the type that the predicate queries * @return the predicate * @throws IllegalArgumentException if the predicate is null */ public static <T> Predicate<T> uniquePredicate() { return new UniquePredicate<T>(); } /** * Constructor that performs no validation. * Use <code>uniquePredicate</code> if you want that. */ public UniquePredicate() { super(); } /** * Evaluates the predicate returning true if the input object hasn't been * received yet. * * @param object the input object * @return true if this is the first time the object is seen */ public boolean evaluate(final T object) { return iSet.add(object); } }