Initializing HashSet at the Time of Construction – 在构建时初始化HashSet

最后修改: 2018年 7月 22日

1. Overview


In this quick tutorial, we’ll introduce various methods of initializing the HashSet with values, at the time of its construction.


To instead explore the features of HashSet, refer to this core article here.


We’ll dive into Java built-in methods since Java 5 and before, followed by new mechanisms introduced since Java 8.

我们将深入研究Java的内置方法自Java 5及以前的,然后是自Java 8引入的新机制。

We’ll also see a custom utility method and finally explore the features provided by third-party collection libraries, Google Guava in particular.

我们还将看到一个定制的实用方法,最后探索第三方集合库提供的功能,特别是Google Guava。

If we’ve already migrated to JDK9+, we can simply use collection factory methods.


2. Java Built-In Methods


Let’s begin by examining three built-in mechanisms available since Java 5 or before.

让我们首先检查一下自Java 5或之前就有的三种内置机制。

2.1. Using Another Collection Instance


We can pass an existing instance of another collection to initialize the Set.


Here we are using an inline-created List:


Set<String> set = new HashSet<>(Arrays.asList("a", "b", "c"));

2.2. Using Anonymous Class


In yet another approach, we can use the anonymous class to add an element to HashSet.


Note the use of double curly braces. This approach is technically very expensive because it creates an anonymous class each time it’s called.


So, depending on how frequently we need to initialize Set, we can try to avoid using this approach:


Set<String> set = new HashSet<String>(){{

2.3. Using Collections Utility Method Since Java 5

2.3.从Java 5开始使用集合实用方法

The Java’s Collections utility class provides the method named singleton to create a Set with a single element. The Set instance created with the singleton method is immutable, meaning that we cannot add more values to it.

Java的Collections utility类提供了名为singleton的方法来创建一个具有单个元素的Set。用singleton方法创建的Set实例是immutable,意味着我们不能向它添加更多的值。

There are situations especially in unit testing where we need to create a Set with a single value:


Set<String> set = Collections.singleton("a");

3. Defining Custom Utility Method


We can define a static final method as below. The method accepts variable arguments.

我们可以定义一个static final方法,如下所示。该方法接受变量参数。

Using Collections.addAll, which accepts the collection object and an array of values, is best among others because of the low overhead of copying values.


The method uses generics so we can pass values of any type:


public static final <T> Set<T> newHashSet(T... objs) {
    Set<T> set = new HashSet<T>();
    Collections.addAll(set, objs);
    return set;

Here’s how we can use the utility method in our code:


Set<String> set = newHashSet("a","b","c");

4. Using Stream Since Java 8

4.使用Stream自Java 8开始

With the introduction of Stream API in Java 8, we have additional options such as Stream with Collectors:

随着Stream API在Java 8中的引入,我们有了更多的选择,比如StreamCollectors

Set<String> set = Stream.of("a", "b", "c")

5. Using Third-Party Collection Library


There are multiple third-party collections libraries including Google Guava, Apache Commons Collections and Eclipse Collections just to name a few.

有多个第三方集合库,包括Google Guava、Apache Commons Collections和Eclipse Collections,仅举几例。

These libraries provide convenient utility methods to initialize collections like Set. Since Google Guava is one of the most commonly used, we’ve included an example from it.

这些库提供了方便的实用方法来初始化像Set这样的集合。由于Google Guava是最常用的一个,我们已经包括了它的一个例子。

Guava has convenient methods for mutable and immutable Set objects:


Set<String> set = Sets.newHashSet("a", "b", "c");

Similarly, Guava has a utility class for creating immutable Set instances:


Set<String> set = ImmutableSet.of("a", "b", "c");

6. Conclusion


In this article, we saw multiple ways in which a HashSet can be initialized while it is constructed.


These approaches don’t necessarily cover all possible ways by any means. This article is just an attempt to showcase the most common ways.


For example, one approach not covered here could be using the object builder to construct a Set.


As always, the working code example is available over on GitHub.