不可变集合 Immutable Collections

例子 

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!
  }
}

为啥？ 
不可变对象拥有众多好处： 

• 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

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

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。 
咋整？ 
可使用如下方法创建不可变集合： 

• 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：

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

使用copeOf(0 

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

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

asList 
All immutable collections provide an ImmutableList view via asList(), so -- for example -- even if you have data stored as an ImmutableSortedSet, you can get the kth smallest element with sortedSet.asList().get(k). 

通过asList构造的List通常比直接new ArrayList要快，毕竟它是指定大小的。 


在哪？ 

Interface	JDK or Guava?	Immutable Version
Collection	JDK	ImmutableCollection
List	JDK	ImmutableList
Set	JDK	ImmutableSet
SortedSet/NavigableSet	JDK	ImmutableSortedSet
Map	JDK	ImmutableMap
SortedMap	JDK	ImmutableSortedMap
Multiset	Guava	ImmutableMultiset
SortedMultiset	Guava	ImmutableSortedMultiset
Multimap	Guava	ImmutableMultimap
ListMultimap	Guava	ImmutableListMultimap
SetMultimap	Guava	ImmutableSetMultimap
BiMap	Guava	ImmutableBiMap
ClassToInstanceMap	Guava	ImmutableClassToInstanceMap
Table	Guava	ImmutableTable

创建集合 
简化集合创建 
原来多麻烦： 

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);

Table 
Java代码  

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

工具类 

Interface	JDK or Guava?	Corresponding Guava utility class
Collection	JDK	Collections2 (avoiding conflict with java.util.Collections)
List	JDK	Lists
Set	JDK	Sets
SortedSet	JDK	Sets
Map	JDK	Maps
SortedMap	JDK	Maps
Queue	JDK	Queues
Multiset	Guava	Multisets
Multimap	Guava	Multimaps
BiMap	Guava	Maps
Table	Guava	Tables

JDK 7.0之前： 

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();