Google collections api an introduction

By Jitendra Gosain Google Collections API

Introduction What is Google Collections API? Its a utility API. Its an extension to the Java Collections API. Its a set of new Collection Types and implementations. Part of the Project called “ Guava ” Requires “ JDK 1.5”

Multimap A collection similar to a Map, but which may associate multiple values with a single key. Keys and values can be null. Adding a new key-value pair equal to an existing key-value pair has no effect.

HashMultimap Example HashMultimap<Integer, String> hm = HashMultimap.create (); hm.put(1, "first");//Add Values to keys hm.put(2, "second"); hm.put(1, "third"); //Same key is added a new value hm.put(1, "fourth"); //hm is {1=[fourth, third, first], 2=[second]}

HashMultimap Example hm.remove(1,"first");//{1=[fourth, third], 2=[second]} hm.removeAll(1);//{2=[second]} Multimap safeMultimap = Multimaps. synchronizedMultimap (hm); SetMultimap safeSetMultimap = Multimaps. synchronizedSetMultimap (hm); Map mp = hm. asMap (); //Converts to Java Map

Multiset A Multiset can have duplicates unlike a Java Set. Also called a “ Bag ”. The total number of occurrences of an element in a multiset is called the count of that element.

HashMultiset Example HashMultiset ms = HashMultiset. create (); ms.add("1"); ms.add("2"); ms.add("1"); ms.add("3", 3); //HashMultiset ms is [3 x 3, 2, 1 x 2]

HashMultiset Set emSet = ms. elementSet (); //[3, 2, 1] – Above returns set of distinct elements ms.remove("1", 1); //Set is now [3 x 3, 2, 1] ms.remove("1", ms.count("1")); //[3 x 3, 2] Set set = new HashSet(); set.add("2"); ms. retainAll (set); //[2] – Retains the elements of Collection “set”

BiMap A bimap (or " bidirectional map ") is a map that preserves the uniqueness of its values as well as that of its keys and looking up a key from a value is possible . Also called “ unique-valued map ”. A HashBiMap and its inverse are both serializable.

BiMap Example HashBiMap<Integer, String> hmBiMap = HashBiMap. create (); hmBiMap.put(1, "first"); hmBiMap.put(2, "second"); hmBiMap.put(2, “newsecond"); //2 holds this hmBiMap.put(3, "second"); //Above throws an error (value already present)

BiMap Example hmBiMap. inverse (); //Above returns the inverse view of this bimap, value mapped to key now. Set set = hmBiMap. values (); //Returns a collection of values of the bimap

ImmutableMap An instance of ImmutableMap contains its own data that will never change . Provides read-only access of data. The Java “Collections.unmodifiableMap” which is a view of a separate map which can still change, an instance of ImmutableMap contains its own data and will never change. Also called the “ Constant Maps ”.

ImmutableMap Example ImmutableMap<String,Integer> map1 = new ImmutableMap. Builder <String,Integer>() .put("one", 1) .put("two", 2) .put("three", 3) . build ();//Build an Immutable map

ImmutableMap Example map1.put("four", 4); //Not allowed map1.remove(4); //Not allowed //Above throws “UnsupportedOperationException” map1. containsKey ("one"); //true map1. containsValue (3); //true

ImmutableMap Example Map<String, String> javaMap = new HashMap<String, String>(); javaMap.put("key1", "value1"); javaMap.put("key2", "value2"); ImmutableMap<String,String> map = ImmutableMap.of ("key1", "value1", "key2", "value2");

ImmutableSet An instance of the ImmutableSet contains its own data that will not change . ImmutableSet set1 = new ImmutableSet. Builder ().add("1").add("2") .build(); set1.add(“3”); //Its not Allowed set1.remove(“1”); //Not Allowed ImmutableSet set2 = ImmutableSet.of ("1", "2", "3");

ImmutableList An instance of ImmutableList contains its own data that will never change . List<String> myList = new ArrayList<String>(); myList.add("first"); myList.add("second");

ImmutableList Example List<String> readOnlyList = Collections.unmodifiableList (myList); readOnlyList.add("fourth"); //Will throw Exception ImmutableList list1 = new ImmutableList. Builder ().add("1").add("2").build(); list1.add("5"); //Not Allowed List.remove(“1”); //Not Allowed

ImmutableList Example List<String> constList = new ArrayList<String>(); constList.add("a"); constList.add("b"); ImmutableList<String> immutableList = ImmutableList.of ("a", "b");

MapDifference An interface representing the differences between the two given maps. ImmutableMap<String,Integer> map1 = new ImmutableMap.Builder <String,Integer>() .put("one", 1).put("two", 2).put("three", 3).build(); //Above creates an ImmutableMap // map1 is now {one=1, two=2, three=3}

MapDifference ImmutableMap<String,Integer> map2 = new ImmutableMap.Builder <String,Integer>() .put("five", 5) .put("four", 4) .put("three", 3) .put("one", 10) .build(); // map2 is {five=5, four=4, three=3, one=10}

MapDifference MapDifference <String, Integer> difference = Maps.difference (map1, map2); difference. entriesInCommon (); difference. entriesDiffering (); difference. entriesOnlyOnLeft (); difference. entriesOnlyOnRight ();

MapDifference difference. areEqual (); MapDifference.ValueDifference <Integer> val = (MapDifference.ValueDifference<Integer>) difference.entriesDiffering(). get ("one");

Preconditions A final class (com.google.common.base) which provides static methods to verify correct arguments and state. String str = null; Preconditions. checkNotNull( str); int value = -1; Preconditions. checkArgument (value >= 0, "negative value");

Function Interface A Function (com.google.common.base.Function) is used in transformation of one object to the another. Represented as public interface Function<F,T> First argument is the “function input” and second argument is the “function output”.

Function Interface Example Function<String, String> extractUserName = new Function<String, String>(){ public String apply (String emailAddress) { String userName = emailAddress.substring(0, emailAddress.indexOf('@')); return userName; } };

Function Example List<String> emailList = Lists.newArrayList("abc@gmail.com", "xyz@gmail.com", "aaa@gmail.com"); List<String> userNameList = Lists. transform (emailList, extractUserName );

Function Example Function<String, Integer> functionStringToInteger = new Function<String, Integer>(){ public Integer apply (String str){ return Integer.parseInt(str); } }; List strTemp = Lists.newArrayList("1", "2", "3", "4"); List intList = Lists.transform(strTemp, functionStringToInteger );

Functions Class Function toStringFunction = Functions.toStringFunction (); List intList = Lists.newArrayList(1,2,3,4,5); Lists.transform( intList, toStringFunction );

Predicate A predicate is a way to specify some conditions based on an instance of a class The defined predicate then can be used to selectively filter out matching instances of that class from a collection. Predicate<Integer> salaryCut = new Predicate <Integer>() { public boolean apply (Integer salary) { return salary > 50000; } };

Predicate Example List<Integer> salaryList = Lists.newArrayList(); salaryList.add(new Integer(20000)); salaryList.add(new Integer(60000)); salaryList.add(new Integer(70000)); salaryList.add(new Integer(40000)); List<Integer> filteredSalary = Lists.newArrayList( Iterables.filter (salaryList, salaryCut ));

Predicate Example List<String> list1 = Lists.newArrayList("1", "2", "3"); List<String> list2 = Lists.newArrayList("1", "4", "5"); List<String> list3 = Lists.newArrayList("1", "4", "6"); boolean result = and ( in (list1),in(list2), in(list3)).apply("4"); result = or (in(list1),in(list2), in(list3)).apply("4");

Predicates ImmutableMap<Integer,String> map = new ImmutableMap.Builder<Integer,String>() .put(10, "Ten") .put(20, "Twenty") .put(30, "Thirty") .build(); Map<Integer,String> filtered11 = Maps.filterKeys (map,Predicates. or ( Predicates. equalTo (10), Predicates. equalTo (30)));

Miscellaneous Joiner (com.google.common.base) ImmutableSet s1 = ImmutableSet.of("1", "2", "3"); String joinedStr = Joiner.on (":"). join (s1) ; Joiner joiner = Joiner.on("* ").skipNulls(); String strWithStar = joiner.join(“A", null, “B", “C");

Joiner joiner = Joiner.on("* "). useForNull ("Default"); String str = joiner. join (“A", null, “B", “C"); Map<String, String> mp = new LinkedHashMap<String, String>(); mp.put("1", "1"); mp.put("2", "2"); String str = Joiner.on (", "). withKeyValueSeparator ("=").join(mp); //1=1, 2=2

Collections2 Collections2 Predicate<Integer> salaryCut = new Predicate <Integer>() { public boolean apply (Integer salary) { return salary > 50000; } };

Collections2 List<Integer> salaryList = Lists. newArrayList(); salaryList.add(new Integer(20000)); salaryList.add(new Integer(60000)); salaryList.add(new Integer(70000)); salaryList.add(new Integer(40000)); Collection<Integer> newSalaryList = Collections2.filter (salaryList, salaryCut); newSalaryList.add(1000);//Exception salaryList. clear ();//newSalaryList gets empty too

Maps Static Utility API’s for Java Map. Map<Integer, String> tmpMap = new HashMap<Integer, String> (); Map<Integer, String> tmpMap1 = Maps.newHashMap (); //{10=Ten, 20=Twenty, 30=Thirty} Maps. filterKeys (map, Predicates.equalTo(10)); Maps. filterValues (map, Predicates.equalTo("Ten"));

Map<Integer,String> transformed = Maps.transformValues (map, new Function<String,String>() { public String apply(String from) { return "A " + from; } }); //10=A Ten, 20=A Twenty, 30=A Thirty}

Lists List<Integer> listA = Lists.newArrayList (); listA.add(new Integer(1)); listA.add(new Integer(2)); listA.add(new Integer(3)); listA.add(new Integer(4)); listA.add(new Integer(5)); List finalLists = Lists.partition (listA, 3);

Sets HashSet<String> hSet = Sets.newHashSet() ; ImmutableSet<String> set1 = ImmutableSet.of("1", "2", "3"); ImmutableSet<String> set2 = ImmutableSet.of("2", "3", "4"); Sets.union (set1, set2); Sets.intersection (set1, set2); Sets.difference (set1, set2); Sets.filter (set1, Predicates.equalTo("1"))

Where can I refer the API’s? Reference: http://guava-libraries.googlecode.com/svn/trunk/javadoc/index.html http:// code.google.com/p/google -collections/

Google collections api an introduction

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