1. Introduction
1.介绍
This cookbook article is organized into small and focused recipes and code snippets for using Guava style collections.
这篇食谱文章被组织成小而集中的食谱和代码片断，用于使用Guava风格的集合。
The format is that of a growing list of code examples with no additional explanation necessary – it is meant to keep common usages of the API easy to access during development.
其格式是一个不断增长的代码示例列表，不需要额外的解释–其目的是在开发过程中保持API的常见用法易于访问。
2. The Recipes
2.菜谱
downcast a List<Parent> to a List<Child>
将一个List<Parent>下传到一个List<Child>
– note: this is a workaround for non-covariant generified collections in Java
– 注：这是对Java中的非变异生成集合的一种变通方法
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>());simpler alternative without Guava – involving 2 cast operations
没有Guava的更简单的替代方案–涉及2个铸造操作。
List<Number> originalList = Lists.newArrayList();
List<Integer> theList = (List<Integer>) (List<? extends Number>) originalList;adding an iterable to a collection
向一个集合添加一个可迭代的对象
Iterable<String> iter = Lists.newArrayList();
Collection<String> collector = Lists.newArrayList();
Iterables.addAll(collector, iter);check if collection contains element(s) according to a custom matching rule
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);alternative solution using search
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);alternative solution only applicable to Sets
仅适用于集合的替代解决方案。
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 when nothing is found
NoSuchElementException on Iterables.find when nothing is found
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);– this will throw the NoSuchElementException exception:
– 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)– solution: there is an overloaded find method that takes the default return value as an argument and can be called with null for the desired behavior:
– 解决方案：有一个重载的find方法，它将默认的返回值作为参数，可以用null来调用所需的行为。
String found = Iterables.find(theCollection, inputOfLengthOne, null);remove all null values from a collection
从一个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());create immutable List/Set/Map directly
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");create immutable List/Set/Map from a standard collection
从一个标准集合中创建不可变的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);alternative solution using builders
使用建设者的替代解决方案。
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();3. More Guava Cookbooks
3.更多的番石榴烹饪书
Guava is a comprehensive and fantastically useful library – here’s a few more APIs covered in cookbook form:
Guava是一个全面的、非常有用的库–这里还有一些以菜谱形式介绍的API。
Enjoy.
请享受。
4. Going Forward
4.前进
As I mentioned in the beginning, I am experimenting with this different format – the cookbook – to try to gather simple common tasks of using Guava Collections in a single place. The focus of this format is simplicity and speed, so most recipes have no additional explanation other than the code example itself.
正如我在一开始提到的，我正在尝试使用这种不同的格式–菜谱–来尝试将使用Guava集合的简单常用任务集中在一个地方。这种格式的重点是简单和快速，所以大多数菜谱除了代码例子本身外，没有其他解释。
Finally – I am looking at this as a living document – I’m going to keep adding recipes and examples as I run into them. Feel free to provide more in the comments and I’ll look to incorporate them into the cookbook.
最后–我把它看作是一份活的文件–我将不断增加我所遇到的食谱和例子。欢迎在评论中提供更多信息，我将寻求将它们纳入食谱中。
The implementation of all these examples and code snippets can be found over on GitHub – this is a Maven-based project, so it should be easy to import and run as it is.