Google Guava-集合类型

Guava引入了很多JDK没有的、但我们发现明显有用的新集合类型。

Multiset

统计一个词在文档中出现了多少次，传统的做法是这样的：（这里举个简单的传统方法，下面不在说明）

@Test
public void test20(){
List<String> words = new ArrayList<String>();
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);
}
}
}

这种写法很笨拙，也容易出错，并且不支持同时收集多种统计信息，如总词数。我们可以做的更好。

Guava提供了一个新集合类型 Multiset，它可以多次添加相等的元素。很好使用例如：

@Test
public void test10() {
Multiset<String> multiset1 = HashMultiset.create();
multiset1.add("a", 2);
multiset1.add("b");
multiset1.add("b");
int count = multiset1.count("b");
System.out.println(count);
Multiset<String> multiset2 = HashMultiset.create();
multiset2.add("a", 5);

multiset1.containsAll(multiset2); // 返回true；因为包含了所有不重复元素，
// 虽然multiset1实际上包含2个"a"，而multiset2包含5个"a"
Multisets.containsOccurrences(multiset1, multiset2); // returns false
// multiset2.removeOccurrences(multiset1); // multiset2 现在包含3个"a"
multiset2.removeAll(multiset1);// multiset2移除所有"a"，虽然multiset1只有2个"a"
multiset2.isEmpty(); // returns true
}

下面我用测试用例说明一下guava集合类型的新功能

// 在JDK的基础上增加了map的功能
@Test
public void test8() {
Map<String, Integer> left = ImmutableMap.of("a", 1, "b", 2, "c", 3, "d", 4);
Map<String, Integer> right = ImmutableMap.of("a", 2, "b", 2, "c", 3);
MapDifference<String, Integer> diff = Maps.difference(left, right);
System.out.println(diff.entriesDiffering());// 键相同 但是值不相同的映射项
System.out.println(diff.entriesInCommon());// 键值都相同的映射项
System.out.println(diff.entriesOnlyOnLeft()); // {"a" => 1}
System.out.println(diff.entriesOnlyOnRight()); // {"d" => 5}
}

// 一个非常有用的集合，取得交集，并集，第一个集合中有的数据第二个集合中没有的数据等方法
@Test
public void test7() {
List<String> list = Lists.newArrayList();
// list.
Set<String> wordsWithPrimeLength = ImmutableSet.of("one", "two", "three", "six", "seven", "eight");
Set<String> primes = ImmutableSet.of("two", "three", "five", "seven");
// primes.add("hello");
SetView<String> intersection = Sets.intersection(primes, wordsWithPrimeLength);
// intersection包含"two", "three", "seven"
ImmutableSet<String> immutableCopy = intersection.immutableCopy();// 可以使用交集，但不可变拷贝的读取效率更高
// Sets.difference(set1, set2);
SetView<String> difference = Sets.difference(primes, wordsWithPrimeLength);
SetView<String> union = Sets.union(primes, wordsWithPrimeLength);
System.out.println(union);
System.out.println(difference);
System.out.println(immutableCopy);
Set<String> animals = ImmutableSet.of("gerbil", "hamster");
Set<String> fruits = ImmutableSet.of("apple", "orange", "banana");

Set<List<String>> product = Sets.cartesianProduct(animals, fruits);
// {{"gerbil", "apple"}, {"gerbil", "orange"}, {"gerbil", "banana"},
// {"hamster", "apple"}, {"hamster", "orange"}, {"hamster", "banana"}}

Set<Set<String>> animalSets = Sets.powerSet(animals);
// {{}, {"gerbil"}, {"hamster"}, {"gerbil", "hamster"}}
}
// 比较器，能够比较数据大小按照自己的要求
@Test
public void test6() {
Ordering<String> order = new Ordering<String>() {
@Override
public int compare(String left, String right) {
return Ints.compare(left.length(), right.length());
}
};
order.compare("a", "b");
String min = order.min("a", "b1", "c11", "d111");
System.out.println(min);
}
@Test
public void test11() {
Multiset<String> multiset = HashMultiset.create();
multiset.add("a", 3);
multiset.add("b", 5);
multiset.add("c", 1);
multiset.add("a");
multiset.add("a");
multiset.add("a");
multiset.add("a");
ImmutableMultiset<String> highestCountFirst = Multisets.copyHighestCountFirst(multiset);
System.out.println(highestCountFirst.elementSet());
// highestCountFirst，包括它的entrySet和elementSet，按{"b", "a", "c"}排列元素
}

// 根据特征进行筛选集合中的数据
@Test
public void test12() {
ImmutableSet<String> digits = ImmutableSet.of("zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine");
Function<String, Integer> lengthFunction = new Function<String, Integer>() {
public Integer apply(String string) {
return string.length();
}
};
ImmutableListMultimap<Integer, String> digitsByLength = Multimaps.index(digits, lengthFunction);
System.out.println(digitsByLength);
/*
* digitsByLength maps: 3 => {"one", "two", "six"} 4 => {"zero", "four",
* "five", "nine"} 5 => {"three", "seven", "eight"}
*/
}

@Test
public void test13() {
ArrayListMultimap<String, Integer> 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<Integer, String> create = TreeMultimap.create();
TreeMultimap<Integer, String> inverse = Multimaps.invertFrom(multimap, create);
System.out.println(inverse);
// 注意我们选择的实现，因为选了TreeMultimap，得到的反转结果是有序的
/*
* inverse maps: 1 => {"a"} 2 => {"a", "b", "c"} 3 => {"c"} 4 => {"a",
* "b"} 5 => {"c"} 6 => {"b"}
*/
}
//map中的key和value值进行反转
@Test
public void test14() {
Map<String, Integer> map = ImmutableMap.of("a", 1, "b", 1, "c", 2);
SetMultimap<String, Integer> multimap = Multimaps.forMap(map);
// multimap：["a" => {1}, "b" => {1}, "c" => {2}]
System.out.println(multimap);
HashMultimap<Integer, String> dest = HashMultimap.create();
Multimap<Integer, String> inverse = Multimaps.invertFrom(multimap, dest);
System.out.println(inverse);
// inverse：[1 => {"a","b"}, 2 => {"c"}]
/*
* Map<String, Integer> map = ImmutableMap.of("a", 1, "b", 1, "c", 2);
* SetMultimap<String, Integer> multimap = Multimaps.forMap(map);
*/
// multimap：["a" => {1}, "b" => {1}, "c" => {2}]
// Multimap<Integer, String> inverse = Multimaps.invertFrom(multimap,
// Multimap<Integer, String>.create());
// inverse：[1 => {"a","b"}, 2 => {"c"}]
}

// 多功能的map
@Test
public void test15() {
ArrayListMultimap<String, Integer> multimap = ArrayListMultimap.create();
multimap.put("a", 1);
multimap.put("a", 2);
System.out.println(multimap);
Map<String, Integer> map = new HashMap<String, Integer>();
map.put("a", 1);
map.put("a", 2);
System.out.println(map);
Map<String, Integer> tree = new TreeMap<String, Integer>();
tree.put("a", 1);
tree.put("a", 2);
System.out.println(tree);
Map<String, Integer> table = new Hashtable<String, Integer>();
table.put("a", 1);
table.put("a", 2);
System.out.println(table);
/**
* {a=[1, 2]} {a=2} {a=2} {a=2}
*/
}