Collections. Practical Tasks

Add two boxes of the same weight to a TreeSet and only one of them stays in the set. If compareTo() compares nothing but the weight field, the second box counts as a duplicate and is silently dropped — no exception, no warning:

Set<HeavyBox> boxes = new TreeSet<>(); // compareTo() compares only weight
boxes.add(new HeavyBox(10, 20, 30, 5));
boxes.add(new HeavyBox(40, 50, 60, 5));
System.out.println(boxes.size()); // Output: 1

This page has 10 Java collections exercises — practice problems on List, Set, Queue and Map with ArrayList, TreeSet, HashSet, ArrayDeque, Iterator, HashMap and Comparable. The tasks follow the lessons of the Collections module: from lists to sets, queues and maps, and the last task continues the Online Store project from the earlier modules. Some tasks come with sample input and hints on version-specific APIs.

Which Collection Each Exercise Needs

Each exercise practices one collection and two or three of its key methods. If a method in the table is new to you, review the lesson in the right column first — then a task takes 20–40 minutes instead of an evening with the Javadoc.

ArrayList of HeavyBox Objects

1. Create a dynamic array (ArrayList) of HeavyBox objects.
2. The class HeavyBox extends the class Box and has a field int weight.
3. The class Box has the fields double width, double height, double depth.
4. Print the contents of the list with a for-each loop.
5. Change the weight of the first box to 1.
6. Remove the last box.
7. Get an array of the boxes from the list in three different ways and print it to the console.
8. Remove all boxes.

TreeSet of Boxes Sorted by Weight

1. Create a TreeSet of HeavyBox objects.
2. The class HeavyBox extends the class Box and has a field int weight.
3. The class Box has the fields double width, double height, double depth.
4. HeavyBox must implement the Comparable interface; the boxes are sorted by the field weight.
5. Print the contents of the set with a for-each loop.

Remove Duplicate Numbers from a String with a Set

1. The user enters a set of numbers as a single line: "1, 2, 3, 4, 4, 5, 10, 11".
2. Remove the duplicate numbers from the line.
3. Print the result to the console.
4. Use the Set interface.

Set Union and Intersection: union() and intersect()

1. Write the methods static <T> Set<T> union(Set<T> set1, Set<T> set2) and static <T> Set<T> intersect(Set<T> set1, Set<T> set2) that return the union and the intersection of two sets.
2. The methods must return a new set and must not modify set1 and set2.
3. Test both methods on sets filled with sample data.

ArrayDeque Queue: offer() and poll()

1. Create a queue of HeavyBox objects.
2. The class HeavyBox extends the class Box and has a field int weight.
3. The class Box has the fields double width, double height, double depth.
4. Use the class ArrayDeque.
5. Add the boxes to the queue with the method offer().
6. Remove them with the method poll().

Split a Collection with an Iterator

1. Create a collection of Box objects.

   public class Box {
       private double width;
       private double height;
       private double depth;
   
       public Box(double width, double height, double depth) {
           this.width = width;
           this.height = height;
           this.depth = depth;
       }
   
       public double getWidth() {
           return width;
       }
   
       // getHeight(), getDepth() and toString() work the same way
   }

2. Write a method that goes through the collection and checks the width of each box.
3. If a box is wider than 30 cm, move it to another collection.
4. Use an iterator to go through the elements.

HashMap of Books: entrySet(), keySet() and values()

1. Create a HashMap that maps a book title to a book object (class Book).
2. The class Book has the fields name, cost and author.
3. Iterate over the key-value pairs and print them — entrySet().
4. Iterate over the book titles and print them — keySet().
5. Iterate over the books and print them — values().
6. Write a separate method for each kind of iteration.

Students: Filter by Average Grade and Promote to the Next Year

1. Create a class Student with a name, a group, a year of study and grades in several subjects (on a 5-point scale).
2. Create a Set<Student> of Student objects. Override equals() and hashCode() in Student.
3. Write a method that removes the students whose average grade is < 3.
4. Write a method that promotes a student to the next year if the average grade is >= 3.
5. Write a method printStudents(Set<Student> students, int year) that takes a set of students and a year of study and prints the names of the students in that year.

Map of Pets: Name to Pet Object

1. Create a class Pet with a field name and an abstract method makeNoise().
2. Create subclasses of Pet — Cat, Dog and Parrot. Each class implements its own version of makeNoise().
3. Create a map of pets where the key is the name of the pet and the value is a Pet object.
4. Add different pets to the map.
5. Write a method that prints all keys of the map.
6. Write a method that calls makeNoise() for every pet.

Collections Instead of Arrays, Sorted Catalog

Online Store · part 6 of 6 · ← part 5

1. Replace the arrays in your Online Store with collections.
2. Keep the products of the catalog sorted, for example by price. Sort them with a Comparator and List.sort() or Collections.sort().

Where Collection Code Goes Wrong

Most mistakes in collections exercises do not break compilation: the code runs, but it loses elements, keeps duplicates or throws an exception only on certain data. These are the errors that show up most often in solutions to the tasks on this page.