Generic pair class

Question

Just attempting this question I found in a past exam paper so that I can prepare for an upcoming Java examination.

Provide a generic class Pair for representing pair

Answer 1

You can look to implementation of standard Java classes AbstractMap.SimpleEntry and AbstractMap.SimpleImmutableEntry. It is pretty easy to google sources:

• http://www.docjar.com/html/api/java/util/AbstractMap.java.html
• http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/util/AbstractMap.java

Answer 2

I think No. Quote:

"the class should be parameterised over two types..."

I think they are expecting in terms of :

public class Pair<ThingA, ThingB>

Answer 3

Getters are broken

public thing getFirst() {
  return thing.first;
}

public thing getSecond() {
  return thing.second;
}

thing should be replaced with this

Answer 4

thing is a Type Variable in an unsual notation - we usually use one uppercase latter (like T). Then: a type variable does not have any methods, so your getters won't compile.

Quick improvement: replace all thing with T

Quick fix for getters:

public T getFirst() {
 return first;
}

public T getSecond() {
 return second;
}

---

One requirement was to allow two different types for the pair members. So the class signature should look like:

public Pair<S,T> {
  private S first;
  private T second;
  //...
}

Answer 5

I implemented something similar but with static builder and chained setters

public class Pair<R, L> {

private R left;
private L right;

public static <K,V> Pair<K, V> of(K k, V v) {
    return new Pair<K,V>(k, v);
}

public Pair() {}

public Pair(R key, L value) {
    this.left(key);
    this.right(value);
}

public R left() {
    return left;
}

public Pair<R, L> left(R key) {
    this.left = key;
    return this;
}

public L right() {
    return right;
}

public Pair<R, L> right(L value) {
    this.right = value;
    return this;
}
}

Answer 6

Apache Commons Lang has a generic pair implementation

https://commons.apache.org/proper/commons-lang/apidocs/org/apache/commons/lang3/tuple/Pair.html