13 - Data Structures: Compare objects

Ahmed

Goals

learn how to use Data structures like: Maps or Set with your objects
learn how compare objects together
Practice!

Tips

you can create a Set of your own object like this

Set<Robot> robotSet = new HashSet<>();

Slides

Robot Class

public class Robot {

    public int id;

    public String name;

    public String color;

    public int weigh;

    public Robot(){

    }

    public Robot(int id, String name, String color, int weigh) {
        this.id = id;
        this.name = name;
        this.color = color;
        this.weigh = weigh;
    }

    @Override
    public boolean equals(Object o) {
        Robot anotherRobot = (Robot) o;
        return id == anotherRobot.id &&
                name.equals(anotherRobot.name) &&
                color.equals(anotherRobot.color) &&
                weigh == anotherRobot.weigh;
    }

    @Override
    public int hashCode() {
        return id * 17;
    }
}

check if two objects are equal

public class Main {

    public static void main(String [] args) {


        Robot robot = new Robot(1, "Robot1", "Blue", 20);
        Robot sameRobot  = new Robot(1, "Robot1", "Blue", 20);

        System.out.println(robot.equals(sameRobot));
    }
}

Exercises

Exercise 1

Create student class with fields: first name, last name, age, course. then override hashcode and equal functions and create 3 objects from this class two of them are the same. then create a HashSet of these students and print the size of the list

😎 Toggle Solution


public class Student {

    public String firstName;

    public String lastName;

    public String course;

    public int age;

    public Student() {

    }

    public Student(String firstName, String lastName, String course, int age) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.course = course;
        this.age = age;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return Objects.equals(firstName, student.firstName) &&
                Objects.equals(lastName, student.lastName);
    }

    @Override
    public int hashCode() {
        return Objects.hash(firstName, lastName);
    }
}

public class Main {

    public static void main(String [] args) {


        Student student1 = new Student("Harry", "Potter", "java", 20);
        Student student2 = new Student("Harry", "Potter", "java", 20);
        Student student3 = new Student("John", "Wesley", "java", 21);

        Set<Student> students = new HashSet<>(3);

        students.add(student1);
        students.add(student2);
        students.add(student3);

        System.out.println(students.size());
    }
}

### Exercise 2

Use the same student class and and create a lookup . the key is the student and the value is the number of project score he got for example harry potter ==> 20

try to define two object with same first name and last name but with different age and try to add it to the lookup

😎 Toggle Solution

public class Main{

	public static void main(String [] args) {


        Student student1 = new Student("Harry", "Potter", "java", 20);
        Student student2 = new Student("Harry", "Potter", "java", 30);
        Student student3 = new Student("John", "Wesley", "java", 21);

        HashMap<Student, Integer> studentsScore = new HashMap<>();

        studentsScore.put(student1, 10);
        System.out.println(studentsScore.put(student2, 20));
        studentsScore.put(student3, 30);



    }
}

### Exercise 3

Write a program that creates a map of students and their score like previouse example. Add 5 students from our class to this map.

loop over the map entries and print all students first name and scores
add students with same first name and last name and print the map again
use HashMap
print the the average score in the classe

hint

average of scores is the sum of all scores divided by the number of studens

😎 Toggle Solution


public class Main{

    public static void main(String [] args) {


        Student student1 = new Student("Harry", "Potter", "java", 20);
        Student student2 = new Student("Harry", "Potter", "java", 30);
        Student student3 = new Student("John", "Wesley", "java", 10);

        HashMap<Student, Integer> studentsScore = new HashMap<>();

        studentsScore.put(student1, 10);
        studentsScore.put(student2, 20);
        studentsScore.put(student3, 30);


        double totalScores = 0;
        for(Map.Entry<Student, Integer> entry: studentsScore.entrySet()) {
            totalScores += entry.getValue();
        }
        System.out.println(totalScores/studentsScore.size());
    }
}

## Additional Resources

[📖 HashCode and equals (https://www.journaldev.com/21095/java-equals-hashcode)

# 13 - Data Structures: Compare objects

# Goals

# Tips

# Slides

# Robot Class

# Exercises

# Exercise 1

# hint