Guava - Lists工具类

1. Overview

In this tutorial – we will illustrate the most common and useful ways to work with Lists using the Guava library.

Let's start simple – and take a look at just creating a new ArrayList using Guava syntax – without new:

List<String> names = Lists.newArrayList("John", "Adam", "Jane");

2. Reverse a List

First, let's reverse a List using Lists.reverse() as in the following example:

@Test
public void whenReverseList_thenReversed() {
    List<String> names = Lists.newArrayList("John", "Adam", "Jane");

    List<String> reversed = Lists.reverse(names);
    assertThat(reversed, contains("Jane", "Adam", "John"));
}

3. Generate Character List from a String

Now – let's see how to break a String apart into a list of Characters.

In the following example – we use the Lists.CharactersOf() API to create a Character List from a String “John”:

@Test
public void whenCreateCharacterListFromString_thenCreated() {
    List<Character> chars = Lists.charactersOf("John");

    assertEquals(4, chars.size());
    assertThat(chars, contains('J', 'o', 'h', 'n'));
}

4. Partition a List

Next – Let's see how to partition a List.

In the following example – we use Lists.partition() to get consecutive sublists each of size two:

@Test
public void whenPartitionList_thenPartitioned(){
    List<String> names = Lists.newArrayList("John","Jane","Adam","Tom","Viki","Tyler");

    List<List<String>> result = Lists.partition(names, 2);

    assertEquals(3, result.size());
    assertThat(result.get(0), contains("John", "Jane"));
    assertThat(result.get(1), contains("Adam", "Tom"));
    assertThat(result.get(2), contains("Viki", "Tyler"));
}

5. Remove Duplicates From List

Now – let's use a simple trick to remove duplicates from a List.

In the following example – we copy the elements into a Set and then we create a List back out of the remaining elements:

@Test
public void whenRemoveDuplicatesFromList_thenRemoved() {
    List<Character> chars = Lists.newArrayList('h','e','l','l','o');
    assertEquals(5, chars.size());

    List<Character> result = ImmutableSet.copyOf(chars).asList();
    assertThat(result, contains('h', 'e', 'l', 'o'));
}

6. Remove Null Values from List

Next – let's see how to remove null values from a List.

In the following example – we remove all null values using the highly useful Iterables.removeIf() API and a predicate provided by the library itself:

@Test
public void whenRemoveNullFromList_thenRemoved() {
    List<String> names = Lists.newArrayList("John", null, "Adam", null, "Jane");
    Iterables.removeIf(names, Predicates.isNull());

    assertEquals(3, names.size());
    assertThat(names, contains("John", "Adam", "Jane"));
}

7. Convert a List to an ImmutableList

Finally – let's see how to create an immutable copy of a List – an ImmutableList – using the ImmutableList.copyOf() API:

@Test
public void whenCreateImmutableList_thenCreated() {
    List<String> names = Lists.newArrayList("John", "Adam", "Jane");

    names.add("Tom");
    assertEquals(4, names.size());

    ImmutableList<String> immutable = ImmutableList.copyOf(names);
    assertThat(immutable, contains("John", "Adam", "Jane", "Tom"));
}

8. Conclusion

And here we are – a quick tutorial going over most of the useful things you can do with Lists using Guava.

To dig into Lists even further, check out the Predicates and Functions guide to lists as well as the in-depth guide to Joining and Splitting lists in Guava.

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.