Intermediate Java

06 - Data Structures: Sets

Anca Todirica

Goals

  • learn what are sets and math theory behind it
  • learn when sets can be used
  • how sets are different from the lists
  • get familiar with two implementations coming in JDK: HashSet, TreeSet
  • get familiar with the most commonly used methods on sets

Tips

How to generate a random number?

To return a random number from range between 0 and 50 (exclusive):


Random r = new Random();
int randomNumber = r.nextInt(50);

Slides

Exercises

Exercise 1

In main method

  1. Create a new HashSet of strings, add banana, apple, orange.
  2. Print content of sets using for loop
  3. Clear the set.
  4. Add one more element grapes.
  5. Check if set is empty, and if it’s not empty print “I am not empty!”. What is the order of elements after adding them to HashSet?

😎 Toggle Solution 
   public static void main(String[] args) {
        HashSet<String> fruits = new HashSet<>();
        fruits.add("banana");
        fruits.add("apple");
        fruits.add("oranges");

        for (String fruit : fruits) {
            System.out.println(fruit);
        }

        fruits.clear();
        System.out.println("Printing cleared set: ");
        for (String fruit : fruits) {
            System.out.println(fruit);
        }

        fruits.add("grapes");

        System.out.println("Printing after adding grapes:");
        for (String fruit : fruits) {
            System.out.println(fruit);
        }

        if (!fruits.isEmpty()) {
            System.out.println("I am not empty!");
        }
    }

Exercise 2

Write a static method randomSet that will return a HashSet containing a collection of n unique random numbers in range 0-100. In the main method iterate over the returned Set and print each number multiplied by 2.


😎 Toggle Solution 
   public static HashSet<Integer> randomSet(int nr) {
        HashSet<Integer> randomSet = new HashSet<>();

        Random r = new Random();

        while (randomSet.size() < nr) {
            randomSet.add(r.nextInt(100));
        }

        return randomSet;
    }

Where n is a number of random elements in set.

Exercise 3

Change data type in previous exercise from HashSet to TreeSet. What is the difference?


😎 Toggle Solution 
    public static TreeSet<Integer> randomSet(int nr) {
        TreeSet<Integer> randomSet = new TreeSet<>();

        Random r = new Random();

        while (randomSet.size() < nr) {
            randomSet.add(r.nextInt(100));
        }

        return randomSet;
    }

Exercise 4

Write a static method intersection that will return an intersection of two sets given by parameters. Note - sets given by parameters may not be modified.


😎 Toggle Solution 
    public static Set<Integer> intersection(Set<Integer> set1, Set<Integer> set2) {
        Set<Integer> intersection = new HashSet<>(set1);

        intersection.retainAll(set2);

        return intersection;
    }

Exercise 5

Write a static method union that will return an union of two sets given by parameters. Note - sets given by parameters may not be modified.


😎 Toggle Solution 
    public static Set<Integer> union(Set<Integer> set1, Set<Integer> set2) {
        Set<Integer> union = new HashSet<>(set1);
        union.addAll(set2);

        return union;
    }

Exercise 6

Write a static method difference that will return a difference between two sets given by parameters. Note - sets given by parameters may not be modified.


😎 Toggle Solution 
    public static Set<Integer> difference(Set<Integer> set1, Set<Integer> set2) {
        Set<Integer> difference = new HashSet<>(set1);
        difference.removeAll(set2);

        return difference;
    }

Exercise 7

Write a static method symetricDifference that will return the symetric difference between two sets given by parameters. Note - sets given by parameters may not be modified.


😎 Toggle Solution 
    public static Set<Integer> symetricDifference(Set<Integer> set1, Set<Integer> set2) {
        Set<Integer> union = new HashSet<>(set1);
        union.retainAll(set2);

        Set<Integer> symetricDifference = new HashSet<>(set1);
        symetricDifference.addAll(set2);
        symetricDifference.removeAll(union);

        return symetricDifference;
    }

Exercise 8

Create a TreeSet containing integers that will maintain descending order of numbers. Hint: Look at alternative TreeSet constructors.


😎 Toggle Solution 
    public static void main(String[] args) {
        //Creates a tree set by using a comparator. The comparator is used to sort the elements,
        Comparator<Integer> comp = (Integer o1, Integer o2) -> (o2.compareTo(o1));
        Set<Integer> ts = new TreeSet<>(comp);
        ts.addAll(Arrays.asList(1, 4, 7, 8, 5));
        System.out.println(ts);

        //Another method is to use the methode "descendingSet()".
        //This methods will return the items from the set in a descending order.
        TreeSet<Integer> testSet = new TreeSet<>(Arrays.asList(1, 4, 7, 8, 5));
        TreeSet<Integer> descendingOrderTreeSet = new TreeSet<>(testSet.descendingSet());

        System.out.println(descendingOrderTreeSet);
    }

Additional Resources

Last Updated: 2/9/2019, 11:51:34 PM

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