Pairs in Java

Learn to work with key value pairs in Java using Pair classes e.g. javafx.util.Pair, ImmutablePair, MmutablePair (common langs) and io.vavr.Tuple2 class.

Read More : Tuples in Java

1. Why we need pairs?

A pair provide a convenient way of associating a simple key to value. In Java, maps are used to store key-value pairs. Maps store a collection of pairs and operate them as a whole.

Sometimes, we need to work on requirements where a key-value pair shall exist on it’s own. e.g.

• A pair need to be passed in a method as argument
• Method need to return two values in form of a pair

2. javafx.util.Pair class

Java core APIs have javafx.util.Pair as closest match which serve the purpose of having two values as name-value pair. Follow this link to learn to add JavaFx support in eclipse.

Pair class provides following methods.

• boolean equals​(Object o) – Test this Pair for equality with another Object.
• K getKey() – Gets the key for this pair.
• V getValue() – Gets the value for this pair.
• int hashCode() – Generate a hash code for this Pair.
• String toString() – String representation of this Pair.

Let’s see a java program to create and use pair.

Pair<Integer, String> pair = new Pair<>(100, "howtodoinjava.com");
		
Integer key = pair.getKey();		//100
String value = pair.getValue();		//howtodoinjava.com

pair.equals(new Pair<>(100, "howtodoinjava.com"));	//true - same name and value

pair.equals(new Pair<>(222, "howtodoinjava.com"));	//false	- different name

pair.equals(new Pair<>(100, "example.com"));		//false	- different value

3. Pair, ImmutablePair and MutablePair – Apache commons lang

Commons lang library has a useful class which can used as pair i.e. org.apache.commons.lang3.tuple.Pair. It has two subclasses which can also be used for same purpose i.e. ImmutablePair and MutablePair.

• Pair class is a pair consisting of two elements.
• Pair refers to the elements as ‘left’ and ‘right’.
• Pair also implements the Map.Entry interface where the key is ‘left’ and the value is ‘right’.
• ImmutablePair is immutable representation on Pair. If mutable objects are stored in the pair, then the pair itself effectively becomes mutable. The class is also not final, so a subclass could add undesirable behavior.
• ImmutablePair is thread-safe if the stored objects are thread-safe.

ImmutablePair<Integer, String> pair = ImmutablePair.of(100, "howtodoinjava.com");
		
Integer key = pair.getKey();			//100
String value = pair.getValue();			//howtodoinjava.com

//Integer key = pair.getLeft();			//100
//String value = pair.getRight();		//howtodoinjava.com

pair.equals(ImmutablePair.of(100, "howtodoinjava.com"));	//true - same name and value

pair.equals(ImmutablePair.of(222, "howtodoinjava.com"));	//false	- different name

pair.equals(ImmutablePair.of(100, "example.com"));		//false	- different value

Do not forget to import the library into application classpath.

<dependency>
	<groupId>org.apache.commons</groupId>
	<artifactId>commons-lang3</artifactId>
	<version>3.8.1</version>
</dependency>

4. io.vavr.Tuple2 – Vavr

Another useful class for storing key-value pair is Tuple2.

Tuple2 provide lots of useful method to work on data stored in it. e.g.

• T1 _1() – Getter of the 1st element of this tuple.
• T2 _2() – Getter of the 2nd element of this tuple.
• Tuple2 update1(T1 value) – Sets the 1st element of this tuple to the given value.
• Tuple2 update2(T2 value) – Sets the 2nd element of this tuple to the given value.
• Map.Entry toEntry() – Converts the tuple to java.util.Map.Entry Tuple.
• Tuple2 swap() – Swaps the elements of this Tuple.
• Tuple2 map(BiFunction mapper) – Maps the components of this tuple using a mapper function.
• int compareTo(Tuple2 that) – Compare two Tuple2 instances.

Tuple2<Integer, String> pair = new Tuple2<>(100, "howtodoinjava.com");
		
Integer key = pair._1();			//100
String value = pair._2();			//howtodoinjava.com

pair.equals(new Tuple2<>(100, "howtodoinjava.com"));	//true - same name and value

pair.equals(new Tuple2<>(222, "howtodoinjava.com"));	//false	- different name

pair.equals(new Tuple2<>(100, "example.com"));		//false	- different value

Do not forget to import the library into application classpath.

<dependency>
	<groupId>io.vavr</groupId>
	<artifactId>vavr</artifactId>
	<version>0.10.2</version>
</dependency>

Drop me your questions related to working with name-value pairs in Java.

Happy Learning !!