Google Guava API学习笔记(2):集合

最新推荐文章于 2025-11-30 18:24:05 发布

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[b][size=x-large]不可变集合 Immutable Collections[/size][/b]

[b][size=large]例子[/size][/b]

public static final ImmutableSet<String> COLOR_NAMES = ImmutableSet.of(
  "red",
  "orange",
  "yellow",
  "green",
  "blue",
  "purple");

class Foo {
  Set<Bar> bars;
  Foo(Set<Bar> bars) {
    this.bars = ImmutableSet.copyOf(bars); // defensive copy!
  }
}

[b][size=large]为啥？[/size][/b]

不可变对象拥有众多好处：

[list]
[*]Safe for use by untrusted libraries.
[*]Thread-safe: can be used by many threads with no risk of race conditions.
[*]Doesn't need to support mutation, and can make time and space savings with that assumption. All immutable collection implementations are more memory-efficient than their mutable siblings (analysis)
[*]Can be used as a constant, with the expectation that it will remain fixed
[/list]

防止可变对象拷贝是一个良好的编程习惯，请看这儿：[url]http://www.javapractices.com/topic/TopicAction.do?Id=15[/url]

Making immutable copies of objects is a good defensive programming technique. Guava provides simple, easy-to-use immutable versions of each standard Collection type, including Guava's own Collection variations.

接下来，Guava说居然JDK的Colletions提供了一套Collections.unmodifiableXXX方法，但不太好用，因此Guava不是在重复造轮子。又强调说，如果你不希望对一个集合进行update，那么最好采用防御性拷贝方式，将它们拷贝到一个不可变的集合中。Guava的集合实现都不支持null的元素（Guva通过研究，发现95%的情况不需要使用null的集合元素），所以如果需要null集合元素，就不能用Guava了。

[b][size=large]咋整？[/size][/b]

可使用如下方法创建不可变集合：
[list]
[*]using the copyOf method, for example, ImmutableSet.copyOf(set)
[*]using the of method, for example, ImmutableSet.of("a", "b", "c") or ImmutableMap.of("a", 1, "b", 2)
[*]using a Builder, 看示例：
[/list]


public static final ImmutableSet<Color> GOOGLE_COLORS =
       ImmutableSet.<Color>builder()
           .addAll(WEBSAFE_COLORS)
           .add(new Color(0, 191, 255))
           .build();

[b]使用copeOf(0[/b]


ImmutableSet<String> foobar = ImmutableSet.of("foo", "bar", "baz");
thingamajig(foobar);

void thingamajig(Collection<String> collection) {
   ImmutableList<String> defensiveCopy = ImmutableList.copyOf(collection);
   ...
}


Map<String, Integer> counts = new HashMap<String, Integer>();
for (String word : words) {
  Integer count = counts.get(word);
  if (count == null) {
    counts.put(word, 1);
  } else {
    counts.put(word, count + 1);
  }
}

现在：


Multiset<String> wordsMultiset = HashMultiset.create();
wordsMultiset.addAll(words);

[b][size=large]Table[/size][/b]

Table<Vertex, Vertex, Double> weightedGraph = HashBasedTable.create();
weightedGraph.put(v1, v2, 4);
weightedGraph.put(v1, v3, 20);
weightedGraph.put(v2, v3, 5);

weightedGraph.row(v1); // returns a Map mapping v2 to 4, v3 to 20
weightedGraph.column(v3); // returns a Map mapping v1 to 20, v2 to 5

List<TypeThatsTooLongForItsOwnGood> list = new ArrayList<TypeThatsTooLongForItsOwnGood>();

Guava提供如下方式：


List<TypeThatsTooLongForItsOwnGood> list = Lists.newArrayList();
Map<KeyType, LongishValueType> map = Maps.newLinkedHashMap();

JDK 7.0才支持：

List<TypeThatsTooLongForItsOwnGood> list = new ArrayList<>();

Guava提供更多：

Set<Type> copySet = Sets.newHashSet(elements);
List<String> theseElements = Lists.newArrayList("alpha", "beta", "gamma");

指定大小：


List<Type> exactly100 = Lists.newArrayListWithCapacity(100);
List<Type> approx100 = Lists.newArrayListWithExpectedSize(100);
Set<Type> approx100Set = Sets.newHashSetWithExpectedSize(100);

类似的：

Multiset<String> multiset = HashMultiset.create();

对象工具类提供的方法，如：


List<Integer> countUp = Ints.asList(1, 2, 3, 4, 5);
List<Integer> countDown = Lists.reverse(theList); // {5, 4, 3, 2, 1}

List<List<Integer>> parts = Lists.partition(countUp, 2); // {{1, 2}, {3, 4}, {5}}

不可变工具类提供的方法，如：


Set<String> wordsWithPrimeLength = ImmutableSet.of("one", "two", "three", "six", "seven", "eight");
Set<String> primes = ImmutableSet.of("two", "three", "five", "seven");

SetView<String> intersection = Sets.intersection(primes, wordsWithPrimeLength); // contains "two", "three", "seven"
// I can use intersection as a Set directly, but copying it can be more efficient if I use it a lot.
return intersection.immutableCopy();

[b][size=x-large]扩展工具类[/size][/b]

略了，看：[url]http://code.google.com/p/guava-libraries/wiki/CollectionHelpersExplained[/url]