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2008-11-03T05:21:04Z

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== Writing a generic Pair ==

=== Description ===
One of the drawbacks of Java is there isn't an easy way to return multiple values from a method. It would be nice to return say a pair of values, each with a specifiable type. So, for this exercise, you are to write a generic ''Pair'' class to hold two objects of parameterisable types. For example, a variable of type <code>Pair<String,Date></code> will hold a String object and a Date object. It should define/override the following public methods:
* T1 getFirst() - access the first element of the pair
* void setFirst(T1) - update the first element of the pair, where T1 is the type variable for the first element
* T2 getSecond() - access the second element of the pair
* void setFirst(T2) - update the second element of the pair, where T2 is the type variable for the second element
* int hashCode() - returns the sum of the hashCodes of the elements. I.e. returns first.hashCode() + second.hashCode(). The reason for overriding this is a little tricky. It's mainly for our ''Pair'' to work nicely with HashSets and HashMaps.
* boolean equals(Object) - returns true if and only if the given Object is instace of ''Pair'' and the individual elements of this ''Pair'' are equal to the respective elements of the given ''Pair''
* String toString() - returns the String representation of the pair in the form of "<first, second>"

Below is an example of how we can benefit from your ''Pair'' class:
<pre>
public static void main(String[] args) {
int[] nums = {1,4,2,4,3,4,9,8,7,6,6,2};
Pair<Integer,Double> result = medianAndMean(nums);
System.out.println("Median & Mean: "+result);

int[] nums2 = {6,4,2,3,5,9,2,8,3};
Pair<Integer,Double> result2 = medianAndMean(nums);
System.out.println("Same? " + result.equals(result2));
}

public static Pair<Integer,Double> medianAndMean(int[] numbers) {
Integer median = calculateMedian(numbers); //assume this method exists
Double mean = calculateMean(numbers); //assume this method exists
return new Pair<Integer,Double>(median,mean);
}
</pre>

And the corresponding output:
<pre>
Median & Mean: <4, 4.6667>
Same? true
</pre>

=== Test Cases ===
* Use [[SE251Ex:Generic_Pair_TestPair|TestPair]] to test the correctness of your Pair

=== Tips ===
If you're still not friendly with generics, first write a non-generic ''Pair'' that stores both its items as Objects.
Then pretty much replace every occurrence of ''Object'' with the appropriate type variable.

== Discussion ==
Here you can ask questions and discuss stuff

SE251Ex:Generic Pair - Revision history

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