Google Guava API学习笔记(2):集合

不可变集合 Immutable Collections 


例子 

Java代码  

1. public static final ImmutableSet<String> COLOR_NAMES = ImmutableSet.of(  
2.   "red",  
3.   "orange",  
4.   "yellow",  
5.   "green",  
6.   "blue",  
7.   "purple");  
8.   
9. class Foo {  
10.   Set<Bar> bars;  
11.   Foo(Set<Bar> bars) {  
12.     this.bars = ImmutableSet.copyOf(bars); // defensive copy!  
13.   }  
14. }  

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：

Java代码  

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

使用copeOf(0 

Java代码  

1. ImmutableSet<String> foobar = ImmutableSet.of("foo", "bar", "baz");  
2. thingamajig(foobar);  
3.   
4. void thingamajig(Collection<String> collection) {  
5.    ImmutableList<String> defensiveCopy = ImmutableList.copyOf(collection);  
6.    ...  
7. }  

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？ 

InterfaceJDK or Guava?Immutable VersionCollectionJDKImmutableCollectionListJDKImmutableListSetJDKImmutableSetSortedSet/NavigableSetJDKImmutableSortedSetMapJDKImmutableMapSortedMapJDKImmutableSortedMapMultisetGuavaImmutableMultisetSortedMultisetGuavaImmutableSortedMultisetMultimapGuavaImmutableMultimapListMultimapGuavaImmutableListMultimapSetMultimapGuavaImmutableSetMultimapBiMapGuavaImmutableBiMapClassToInstanceMapGuavaImmutableClassToInstanceMapTableGuavaImmutableTable

创建集合 

简化集合创建 
原来多麻烦： 

Java代码  

1. Map<String, Integer> counts = new HashMap<String, Integer>();  
2. for (String word : words) {  
3.   Integer count = counts.get(word);  
4.   if (count == null) {  
5.     counts.put(word, 1);  
6.   } else {  
7.     counts.put(word, count + 1);  
8.   }  
9. }  

现在： 

Java代码  

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

Table 

Java代码  

1. Table<Vertex, Vertex, Double> weightedGraph = HashBasedTable.create();  
2. weightedGraph.put(v1, v2, 4);  
3. weightedGraph.put(v1, v3, 20);  
4. weightedGraph.put(v2, v3, 5);  
5.   
6. weightedGraph.row(v1); // returns a Map mapping v2 to 4, v3 to 20  
7. weightedGraph.column(v3); // returns a Map mapping v1 to 20, v2 to 5  

工具类 

InterfaceJDK or Guava?Corresponding Guava utility classCollectionJDKCollections2 (avoiding conflict with java.util.Collections)ListJDKListsSetJDKSetsSortedSetJDKSetsMapJDKMapsSortedMapJDKMapsQueueJDKQueuesMultisetGuavaMultisetsMultimapGuavaMultimapsBiMapGuavaMapsTableGuavaTables

JDK 7.0之前： 

Java代码  

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

Guava提供如下方式： 

Java代码  

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

JDK 7.0才支持： 

Java代码  

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

Guava提供更多： 

Java代码  

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

指定大小： 

Java代码  

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

类似的： 

Java代码  

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

对象工具类提供的方法，如： 

Java代码  

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

Java代码  

1. Set<String> wordsWithPrimeLength = ImmutableSet.of("one", "two", "three", "six", "seven", "eight");  
2. Set<String> primes = ImmutableSet.of("two", "three", "five", "seven");  
3.   
4. SetView<String> intersection = Sets.intersection(primes, wordsWithPrimeLength); // contains "two", "three", "seven"  
5. // I can use intersection as a Set directly, but copying it can be more efficient if I use it a lot.  
6. return intersection.immutableCopy();  

扩展工具类 

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

转自:http://stamen.iteye.com/blog/1669120