guava集合类(二）

有经验的同学都知道，如果Abc是一个类，那么它没有实现的功能，可能会出现在Abcs类中。

比如Arrays是对数组功能的扩展，Collections是对集合功能的扩展。Files、Paths是对File、Path类相关操作的扩展。

guava由此延伸，构建了更多对原生集合的加强操作，都集中在Abcs类中。

一、构造

这些工具类中，首先会有基于构建者模式的静态构造方法。

//Iterables
Iterable concatenated = Iterables.concat(
  Ints.asList(1, 2, 3),
  Ints.asList(4, 5, 6));
// concatenated has elements 1, 2, 3, 4, 5, 6

String lastAdded = Iterables.getLast(myLinkedHashSet);

String theElement = Iterables.getOnlyElement(thisSetIsDefinitelyASingleton);

//Sets
Set wordsWithPrimeLength = ImmutableSet.of("one", "two", "three", "six", "seven", "eight");
Set primes = ImmutableSet.of("two", "three", "five", "seven");

SetView intersection = Sets.intersection(primes, wordsWithPrimeLength);

//Lists
List countDown = Lists.reverse(theList); // {5, 4, 3, 2, 1}

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

//Maps
ImmutableMap stringsByIndex = Maps.uniqueIndex(strings, new Function () {
    public Integer apply(String string) {
      return string.length();
    }
  });

Map left = ImmutableMap.of("a", 1, "b", 2, "c", 3);
Map right = ImmutableMap.of("b", 2, "c", 4, "d", 5);
MapDifference diff = Maps.difference(left, right);

//Multisets
Multiset multiset1 = HashMultiset.create();
multiset1.add("a", 2);

Multiset multiset2 = HashMultiset.create();
multiset2.add("a", 5);

multiset1.containsAll(multiset2); // returns true: all unique elements are contained,
  // even though multiset1.count("a") == 2 < multiset2.count("a") == 5
Multisets.containsOccurrences(multiset1, multiset2); // returns false

Multisets.removeOccurrences(multiset2, multiset1); // multiset2 now contains 3 occurrences of "a"

multiset2.removeAll(multiset1); // removes all occurrences of "a" from multiset2, even though multiset1.count("a") == 2
multiset2.isEmpty(); // returns true

//Multimaps
ImmutableSet digits = ImmutableSet.of(
    "zero", "one", "two", "three", "four",
    "five", "six", "seven", "eight", "nine");
Function lengthFunction = new Function() {
  public Integer apply(String string) {
    return string.length();
  }
};
ImmutableListMultimap digitsByLength = Multimaps.index(digits, lengthFunction);

//Tables
Table table = Tables.newCustomTable(
  Maps.>newLinkedHashMap(),
  new Supplier> () {
    public Map get() {
      return Maps.newLinkedHashMap();
    }
  });

二、反转

ArrayListMultimap multimap = ArrayListMultimap.create();
multimap.putAll("b", Ints.asList(2, 4, 6));
multimap.putAll("a", Ints.asList(4, 2, 1));
multimap.putAll("c", Ints.asList(2, 5, 3));

TreeMultimap inverse = Multimaps.invertFrom(multimap, TreeMultimap. create());
// note that we choose the implementation, so if we use a TreeMultimap, we get results in order
/*
 * inverse maps:
 *  1 => {"a"}
 *  2 => {"a", "b", "c"}
 *  3 => {"c"}
 *  4 => {"a", "b"}
 *  5 => {"c"}
 *  6 => {"b"}
 */


Map map = ImmutableMap.of("a", 1, "b", 1, "c", 2);
SetMultimap multimap = Multimaps.forMap(map);
// multimap maps ["a" => {1}, "b" => {1}, "c" => {2}]
Multimap inverse = Multimaps.invertFrom(multimap, HashMultimap. create());
// inverse maps [1 => {"a", "b"}, 2 => {"c"}]



ImmutableSet digits = ImmutableSet.of(
    "zero", "one", "two", "three", "four",
    "five", "six", "seven", "eight", "nine");
Function lengthFunction = new Function() {
  public Integer apply(String string) {
    return string.length();
  }
};
ImmutableListMultimap digitsByLength = Multimaps.index(digits, lengthFunction);
/*
 * digitsByLength maps:
 *  3 => {"one", "two", "six"}
 *  4 => {"zero", "four", "five", "nine"}
 *  5 => {"three", "seven", "eight"}
 */