How to implement Pair class in Java like in C++

Question

How to realize class pair in Java like in C++, for using like in C++?

Answer 1

When I did it I did something similar to the Map.Entry<K, V> interface from the standard library. You should probably override Object.equals(Object) and Object.hashCode() so that two pairs where the key and value are logically equal to each other are logically equal and hash the same - see item 9 of Bloch's Effective Java for some pointers on how to do good implementations.

Here's what I did:

@Override
public String toString() {
    //same convention as AbstractMap.SimpleEntry
    return key + "=" + value;
}

@Override
public boolean equals(Object o) {
    if(o == this) return true;
    if(!(o instanceof Pair)) return false;

    Object otherKey = ((Pair<?, ?>)o).getKey();
    Object otherValue = ((Pair<?, ?>)o).getValue();
    return (key == null ? otherKey == null : key.equals(otherKey))
            && (value == null ? otherValue == null
                    : value.equals(otherValue));
}

@Override
public int hashCode() {
    return 17 + 55555 * (key == null ? 72 : key.hashCode())
            + 232323 * (value == null ? 73 : value.hashCode());
}

Answer 2

class Pair<F,S> {
    private F first;
    private S second;

    public Pair(F first, S second) { 
        this.first = first;
        this.second = second;
    }

    public F getFirst() { return first }
    public S getSecond() { return second }
}

Answer 3

You just have to include the right header

#include <utility>
#include <string>

using std::string;
using std::makepair;
using std::pair;

void foo()
{
    string hello("Hello");
    float  value(42);

    auto p = makepair(hello, value); // or: pair<string, float> = ...
}