Guava – Join and Split Collections – Guava – 连接和拆分集合

1. Overview

1.概述

In this tutorial, we will learn how to use the Joiner and Splitter in the Guava library. We’ll convert collections into a String with the Joiner and we’ll split a String into a collection with the Splitter.

在本教程中，我们将学习如何使用Guava库中的Joiner和Splitter 。我们将用Joiner将集合转换成一个字符串，用Splitter将一个字符串分割成一个集合。

2. Convert List into String Using Joiner

2.使用Joiner将List转换为String

Let’s start with a simple example to join a List into a String using Joiner. In the following example, we join a List of names into one String using the comma “,” as a separator:

让我们从一个简单的例子开始，使用Joiner将一个List连接成一个String。在下面的例子中，我们用逗号”, “作为分隔符，将一个名字的List连接成一个String。

@Test
public void whenConvertListToString_thenConverted() {
    List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom");
    String result = Joiner.on(",").join(names);

    assertEquals(result, "John,Jane,Adam,Tom");
}

3. Convert Map to String Using Joiner

3.使用Joiner将Map转换为String

Next – let’s see how to use Joiner to convert a Map to a String. In the following example, we use withKeyValueSeparator() to join the key with its value:

接下来–让我们看看如何使用Joiner来将Map转换为String。在下面的例子中，我们使用withKeyValueSeparator()来连接键和它的值。

@Test
public void whenConvertMapToString_thenConverted() {
    Map<String, Integer> salary = Maps.newHashMap();
    salary.put("John", 1000);
    salary.put("Jane", 1500);
    String result = Joiner.on(" , ").withKeyValueSeparator(" = ")
                                    .join(salary);

    assertThat(result, containsString("John = 1000"));
    assertThat(result, containsString("Jane = 1500"));
}

4. Join Nested Collections

4.加入嵌套的集合

Now – let’s see how to join nested collections into a String. In the following example, we join the result of transforming each List to a String:

现在–让我们看看如何将嵌套的集合连接成一个String。在下面的例子中，我们将每个List转化为String的结果进行连接。

@Test
public void whenJoinNestedCollections_thenJoined() {
    List<ArrayList<String>> nested = Lists.newArrayList(
      Lists.newArrayList("apple", "banana", "orange"),
      Lists.newArrayList("cat", "dog", "bird"),
      Lists.newArrayList("John", "Jane", "Adam"));
    String result = Joiner.on(";").join(Iterables.transform(nested,
      new Function<List<String>, String>() {
          @Override
          public String apply(List<String> input) {
              return Joiner.on("-").join(input);
          }
      }));

    assertThat(result, containsString("apple-banana-orange"));
    assertThat(result, containsString("cat-dog-bird"));
    assertThat(result, containsString("John-Jane-Adam"));
}

5. Handle Null Values While Using Joiner

5.使用Joiner时处理空值

Now – let’s see different ways to Handle Null Values While Using Joiner.

现在–让我们看看在使用 Joiner 时处理空值的不同方法。

To skip null values while joining collection use skipNulls() as in the following example:

要在连接集合时跳过空值，请使用skipNulls()，如下例。

@Test
public void whenConvertListToStringAndSkipNull_thenConverted() {
    List<String> names = Lists.newArrayList("John", null, "Jane", "Adam", "Tom");
    String result = Joiner.on(",").skipNulls().join(names);

    assertEquals(result, "John,Jane,Adam,Tom");
}

If you don’t want to skip null values and want to replace them instead, use useForNull() as in the following example:

如果你不想跳过空值，而想用替换它们，请使用useForNull()，如下面的例子。

@Test
public void whenUseForNull_thenUsed() {
    List<String> names = Lists.newArrayList("John", null, "Jane", "Adam", "Tom");
    String result = Joiner.on(",").useForNull("nameless").join(names);

    assertEquals(result, "John,nameless,Jane,Adam,Tom");
}

Note that useForNull() doesn’t change the original list, it only affect the output of the join.

注意，useForNull()不会改变原始列表，它只影响连接的输出。

6. Create List From String Using Splitter

6.使用Splitter从String创建List

Now – let’s see how to split a String into a List. In the following example, we use “-” separator to split the input String to List:

现在–让我们看看如何将一个字符串分割成列表。在下面的例子中，我们使用”-“分隔符将输入的字符串分割成列表。

@Test
public void whenCreateListFromString_thenCreated() {
    String input = "apple - banana - orange";
    List<String> result = Splitter.on("-").trimResults()
                                          .splitToList(input);

    assertThat(result, contains("apple", "banana", "orange"));
}

Note that trimResults() removes the leading and trailing whitespace from the resulting substrings.

请注意，trimResults()会从生成的子串中删除前导和尾部的空白。

7. Create Map From String Using Splitter

7.使用Splitter从String创建Map

Next – let’s see how Create Map from String Using Splitter. In the following example, we use withKeyValueSeparator() to split a String into a Map:

接下来–让我们看看如何使用分割器从字符串创建地图。在下面的例子中，我们使用withKeyValueSeparator()将一个字符串分割成一个地图。

@Test
public void whenCreateMapFromString_thenCreated() {
    String input = "John=first,Adam=second";
    Map<String, String> result = Splitter.on(",")
                                         .withKeyValueSeparator("=")
                                         .split(input);

    assertEquals("first", result.get("John"));
    assertEquals("second", result.get("Adam"));
}

8. Split String With Multiple Separators

8.用多个分隔符分割字符串

Now – let’s see how to split a String with multiple separators. In the following example, we use both “.” and “,” to split our String:

现在–让我们看看如何用多个分隔符分割一个字符串。在下面的例子中，我们同时使用”. “和”, “来分割我们的字符串。

@Test
public void whenSplitStringOnMultipleSeparator_thenSplit() {
    String input = "apple.banana,,orange,,.";
    List<String> result = Splitter.onPattern("[.,]")
                                  .omitEmptyStrings()
                                  .splitToList(input);

    assertThat(result, contains("apple", "banana", "orange"));
}

Note that omitEmptyStrings() ignores empty strings and doesn’t add them to the resulting List.

请注意，omitEmptyStrings()会忽略空字符串，不会将它们添加到生成的List中。

9. Split a String at Specific Length

9.以特定长度分割一个字符串

Next – let’s take a look on splitting a String at specific length. In the following example, we split our String every 3 characters:

接下来–让我们看看如何按特定长度分割String。在下面的例子中，我们每隔3个字符分割一次我们的String。

@Test
public void whenSplitStringOnSpecificLength_thenSplit() {
    String input = "Hello world";
    List<String> result = Splitter.fixedLength(3).splitToList(input);

    assertThat(result, contains("Hel", "lo ", "wor", "ld"));
}

10. Limit the Split Result

10.限制分割结果

Finally – let’s see how to Limit the Split Result. If you want the Splitter to stop splitting after specific number of items – use limit() as in the following example:

最后–让我们看看如何限制分割结果。如果你想让Splitter在特定数量的项目后停止分割 – 使用limit()，如下例所示。

@Test
public void whenLimitSplitting_thenLimited() {
    String input = "a,b,c,d,e";
    List<String> result = Splitter.on(",")
                                  .limit(4)
                                  .splitToList(input);

    assertEquals(4, result.size());
    assertThat(result, contains("a", "b", "c", "d,e"));
}

11. Conclusion

11.结论

In this tutorial we illustrated how to use both the Joiner and Splitter in Guava to do a variety of transformations between collections and Strings.

在本教程中，我们说明了如何在Guava中使用Joiner和Splitter来进行集合和字符串之间的各种转换。

The implementation of all these examples and code snippets can be found in my Guava github project – this is an Eclipse based project, so it should be easy to import and run as it is.

所有这些例子和代码片段的实现可以在我的Guava github项目中找到 – 这是一个基于Eclipse的项目，所以它应该很容易导入和运行，如实。