| /** |
| * 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.taobao.weex.utils; |
| |
| import java.util.HashMap; |
| |
| public class WXDataStructureUtil { |
| |
| /** |
| * The largest power of two that can be represented as an {@code int}. |
| */ |
| private static final int MAX_POWER_OF_TWO = 1 << (Integer.SIZE - 2); |
| |
| /** |
| * Creates a {@code HashMap} instance, with a high enough "initial capacity" that it <i>should</i> |
| * hold {@code expectedSize} elements without growth. This behavior cannot be broadly guaranteed, |
| * but it is observed to be true for OpenJDK 1.7. It also can't be guaranteed that the method |
| * isn't inadvertently <i>oversizing</i> the returned map. |
| * |
| * @param expectedSize the number of entries you expect to add to the returned map |
| * @return a new, empty {@code HashMap} with enough capacity to hold {@code expectedSize} entries |
| * without resizing |
| * @throws IllegalArgumentException if {@code expectedSize} is negative |
| */ |
| public static <K, V> HashMap<K, V> newHashMapWithExpectedSize(int expectedSize) { |
| return new HashMap<>(capacity(expectedSize)); |
| } |
| |
| /** |
| * Returns a capacity that is sufficient to keep the map from being resized as long as it grows no |
| * larger than expectedSize and the load factor is >= its default (0.75). |
| */ |
| private static int capacity(int expectedSize) { |
| if (expectedSize < 3) { |
| checkNonnegative(expectedSize, "expectedSize"); |
| return expectedSize + 1; |
| } |
| if (expectedSize < MAX_POWER_OF_TWO) { |
| // This is the calculation used in JDK8 to resize when a putAll |
| // happens; it seems to be the most conservative calculation we |
| // can make. 0.75 is the default load factor. |
| return (int) ((float) expectedSize / 0.75F + 1.0F); |
| } |
| return Integer.MAX_VALUE; // any large value |
| } |
| |
| private static int checkNonnegative(int value, String name) { |
| if (value < 0) { |
| throw new IllegalArgumentException(name + " cannot be negative but was: " + value); |
| } |
| return value; |
| } |
| } |