Google collections api an introduction

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By Jitendra Gosain

Google Collections API

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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”

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Key Interfaces and Classes

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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.

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HashMultimap ExampleHashMultimap<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]}

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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

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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.

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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]

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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”

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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.

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BiMap ExampleHashBiMap<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)

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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

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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”.

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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

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ImmutableMap Examplemap1.put("four", 4); //Not allowedmap1.remove(4); //Not allowed//Above throws “UnsupportedOperationException”

map1.containsKey("one"); //truemap1.containsValue(3); //true

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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");

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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");

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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");

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ImmutableList Example

List<String> readOnlyList = Collections.unmodifiableList(myList);

readOnlyList.add("fourth"); //Will throw ExceptionImmutableList list1 = new

ImmutableList.Builder().add("1").add("2").build();

list1.add("5"); //Not AllowedList.remove(“1”); //Not Allowed

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ImmutableList Example

List<String> constList = new ArrayList<String>();

constList.add("a");

constList.add("b");

ImmutableList<String> immutableList = ImmutableList.of("a", "b");

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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}

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MapDifferenceImmutableMap<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}

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MapDifferenceMapDifference<String, Integer> difference =

Maps.difference(map1, map2);

difference.entriesInCommon();

difference.entriesDiffering();

difference.entriesOnlyOnLeft();

difference.entriesOnlyOnRight();

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MapDifference

difference.areEqual();

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

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PreconditionsA 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");

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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”.

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Function Interface ExampleFunction<String, String> extractUserName = new Function<String, String>(){ public String apply(String emailAddress) {

String userName = emailAddress.substring(0, emailAddress.indexOf('@')); return userName;

} };

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Function Example

List<String> emailList = Lists.newArrayList("[email protected]", "[email protected]", "[email protected]");

List<String> userNameList = Lists.transform(emailList, extractUserName);

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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);

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Functions Class

Function toStringFunction = Functions.toStringFunction();

List intList = Lists.newArrayList(1,2,3,4,5);

Lists.transform(intList, toStringFunction);

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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; }};

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Predicate ExampleList<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));

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Predicate ExampleList<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");

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PredicatesImmutableMap<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)));

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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");

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Joinerjoiner = 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

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Collections2

Collections2

Predicate<Integer> salaryCut = new Predicate<Integer>() {

public boolean apply(Integer salary) {

return salary > 50000;

}

};

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Collections2List<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);//ExceptionsalaryList.clear();//newSalaryList gets empty too

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MapsStatic 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"));

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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}

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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);

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SetsHashSet<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"))

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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/

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Thank You!