Here you can find the source of putIfAbsent(Map
public static <K, V> void putIfAbsent(Map<K, V> from, Map<K, ? super V> to)
//package com.java2s; /*/* ww w .j a v a 2 s.c o m*/ * Copyright 2011-2013 HTTL Team. * * 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. */ import java.util.*; public class Main { public static <K, V> void putIfAbsent(Map<K, V> from, Map<K, ? super V> to) { if (from == null || to == null) { return; } for (Map.Entry<K, V> entry : from.entrySet()) { if (!to.containsKey(entry.getKey())) { to.put(entry.getKey(), entry.getValue()); } } } }