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Required fields*

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Required fields*

What is the equivalent of the C++ Pair<L,R> in Java?

Is there a good reason why there is no Pair<L,R> in Java? What would be the equivalent of this C++ construct? I would rather avoid reimplementing my own.

It seems that 1.6 is providing something similar (AbstractMap.SimpleEntry<K,V>), but this looks quite convoluted.

Answer*

Another way to implement Pair with.

 - Public immutable fields, i.e. simple data structure.
 - Comparable.
 - Simple hash and equals.
 - Simple factory so you don't have to provide the types. e.g. Pair.of("hello", 1);

        public class Pair<FIRST, SECOND> implements Comparable<Pair<FIRST, SECOND>> {
        
        	public final FIRST first;
        	public final SECOND second;
        
        	private Pair(FIRST first, SECOND second) {
        		this.first = first;
        		this.second = second;
        	}
        
        	public static <FIRST, SECOND> Pair<FIRST, SECOND> of(FIRST first,
        			SECOND second) {
        		return new Pair<FIRST, SECOND>(first, second);
        	}
        
        	@Override
        	public int compareTo(Pair<FIRST, SECOND> o) {
        		int cmp = compare(first, o.first);
        		return cmp == 0 ? compare(second, o.second) : cmp;
        	}
        
        	// todo move this to a helper class.
        	private static int compare(Object o1, Object o2) {
        		return o1 == null ? o2 == null ? 0 : -1 : o2 == null ? +1
        				: ((Comparable) o1).compareTo(o2);
        	}
        
        	@Override
        	public int hashCode() {
        		return 31 * hashcode(first) + hashcode(second);
        	}
        
        	// todo move this to a helper class.
        	private static int hashcode(Object o) {
        		return o == null ? 0 : o.hashCode();
        	}
        
        	@Override
        	public boolean equals(Object obj) {
        		if (!(obj instanceof Pair))
        			return false;
        		if (this == obj)
        			return true;
        		return equal(first, ((Pair) obj).first)
        				&& equal(second, ((Pair) obj).second);
        	}
        
        	// todo move this to a helper class.
        	private boolean equal(Object o1, Object o2) {
        		return o1 == null ? o2 == null : (o1 == o2 || o1.equals(o2));
        	}
        
        	@Override
        	public String toString() {
        		return "(" + first + ", " + second + ')';
        	}
        }

Edit Summary*

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10

I like static factory method of. It reminds of Google Guava immutable collections.

Jarek Przygódzki
– Jarek Przygódzki

2011-04-05 19:35:16 +00:00
Commented Apr 5, 2011 at 19:35
7

You are at some point casting o1 to Comparable, even though nothing indicates it will actually implement that interface. If that is a requirement, the FIRST type parameter should be FIRST extends Comparable<?>.

G_H
– G_H

2011-04-20 17:36:49 +00:00
Commented Apr 20, 2011 at 17:36
I'm not a java guy, so please forgive me for my ignorance, but what sort of helper classes were you thinking of in the TODO comments?

user220878
– user220878

2012-03-29 14:41:28 +00:00
Commented Mar 29, 2012 at 14:41
3

31 is a bad constant for hashCode. For example, if you use HashMap keyed by Pair<Integer,Integer> for 2D map, you will get many collisions. For example (a*65497)^b would be better suited.

Michał Zieliński
– Michał Zieliński

2013-06-29 18:58:16 +00:00
Commented Jun 29, 2013 at 18:58
1

@MarioCarneiro ^ is xor, not power

Michał Zieliński
– Michał Zieliński

2016-01-13 21:03:15 +00:00
Commented Jan 13, 2016 at 21:03

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What is the equivalent of the C++ Pair<L,R> in Java?

Answer*