SE251Ex:Generic Pair

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 Pair<String,Date> 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:

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);
}

And the corresponding output:

Median & Mean: <4, 4.6667>
Same? true

Test Cases

• Use 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

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