Guava Collections简介

codingman included in Guava Java Collections

2019-05-04 605 words 2 minutes

Contents

1. 简介

这篇文章被组织成小而集中的示例和代码片段，用于使用 Guava 风格的集合。

格式是**不断增长的代码示例列表，**无需额外解释 - 它旨在使 API 的常见用法在开发过程中易于访问。

2. 示例

将 List<Parent> 向下转换为 List<Child>

注意：这是 Java 中非协变泛型集合的解决方法

class CastFunction<F, T extends F> implements Function<F, T> {
    @Override
    public final T apply(final F from) {
        return (T) from;
    }
}
List<TypeParent> originalList = Lists.newArrayList();
List<TypeChild> theList = Lists.transform(originalList, 
    new CastFunction<TypeParent, TypeChild>());

没有 Guava 的更简单的替代方案 - 涉及 2 个演员操作

List<Number> originalList = Lists.newArrayList();
List<Integer> theList = (List<Integer>) (List<? extends Number>) originalList;

向集合中添加一个可迭代对象

Iterable<String> iter = Lists.newArrayList();
Collection<String> collector = Lists.newArrayList();
Iterables.addAll(collector, iter);

根据自定义匹配规则检查集合是否包含元素

Iterable<String> theCollection = Lists.newArrayList("a", "bc", "def");
    boolean contains = Iterables.any(theCollection, new Predicate<String>() {
    @Override
    public boolean apply(final String input) {
        return input.length() == 1;
    }
});
assertTrue(contains);

使用搜索的替代解决方案

Iterable<String> theCollection = Sets.newHashSet("a", "bc", "def");
boolean contains = Iterables.find(theCollection, new Predicate<String>() {
    @Override
    public boolean apply(final String input) {
       return input.length() == 1;
    }
}) != null;
assertTrue(contains);

仅适用于 Set 的替代解决方案

Set<String> theCollection = Sets.newHashSet("a", "bc", "def");
boolean contains = !Sets.filter(theCollection, new Predicate<String>() {
    @Override
    public boolean apply(final String input) {
        return input.length() == 1;
    }
}).isEmpty();
assertTrue(contains);

NoSuchElementException on Iterables.find当什么都没有找到时

Iterable<String> theCollection = Sets.newHashSet("abcd", "efgh", "ijkl");
Predicate<String> inputOfLengthOne = new Predicate<String>() {
    @Override
    public boolean apply(final String input) {
        return input.length() == 1;
    }
};
String found = Iterables.find(theCollection, inputOfLengthOne);

这将抛出NoSuchElementException异常：

java.util.NoSuchElementException
	at com.google.common.collect.AbstractIterator.next(AbstractIterator.java:154)
	at com.google.common.collect.Iterators.find(Iterators.java:712)
	at com.google.common.collect.Iterables.find(Iterables.java:643)

解决方案：有一个重载的find方法，它将默认返回值作为参数，并且可以使用null来调用所需的行为：

String found = Iterables.find(theCollection, inputOfLengthOne, null);

从集合中删除所有空值

List<String> values = Lists.newArrayList("a", null, "b", "c");
Iterable<String> withoutNulls = Iterables.filter(values, Predicates.notNull());

直接创建不可变的 List/Set/Map

ImmutableList<String> immutableList = ImmutableList.of("a", "b", "c");
ImmutableSet<String> immutableSet = ImmutableSet.of("a", "b", "c");
ImmutableMap<String, String> imuttableMap = 
    ImmutableMap.of("k1", "v1", "k2", "v2", "k3", "v3");

从标准集合创建不可变的 List/Set/Map

List<String> muttableList = Lists.newArrayList();
ImmutableList<String> immutableList = ImmutableList.copyOf(muttableList);
Set<String> muttableSet = Sets.newHashSet();
ImmutableSet<String> immutableSet = ImmutableSet.copyOf(muttableSet);
Map<String, String> muttableMap = Maps.newHashMap();
ImmutableMap<String, String> imuttableMap = ImmutableMap.copyOf(muttableMap);

使用构建器的替代解决方案

List<String> muttableList = Lists.newArrayList();
ImmutableList<String> immutableList = 
    ImmutableList.<String> builder().addAll(muttableList).build();
Set<String> muttableSet = Sets.newHashSet();
ImmutableSet<String> immutableSet = 
    ImmutableSet.<String> builder().addAll(muttableSet).build();
Map<String, String> muttableMap = Maps.newHashMap();
ImmutableMap<String, String> imuttableMap = 
    ImmutableMap.<String, String> builder().putAll(muttableMap).build();