Week 3: Objects, files and exceptions/1a Objects and references

ArrayList


import java.util.ArrayList;

public class Example {
    public static void main(String[] args){
       ArrayList<Double> results = new ArrayList<>();

       //  variable results refers to an object,
       // so list can be manipulated through it
       results.add(2.5);
       results.add(5.75);
       results.add(4.5);

       for (double result : results) {
           System.out.println(result);
       }
    }
}

Program outputs:
2.5
5.75
4.5



================================



Several references to the same object


import java.util.ArrayList;

public class Example {
    public static void main(String[] args){
        ArrayList<Integer> numbers = new ArrayList<>();
        ArrayList<Integer> numbersTwo = numbers; // refer to the same 'numbers' list
        
        numbers.add(5);
        numbers.add(9);
        numbers.add(15);

        numbersTwo.add(17);
        numbersTwo.add(21);

        System.out.println(numbers);
        System.out.println(numbersTwo);
    }
}


Program outputs:
[5, 9, 15, 17, 21]
[5, 9, 15, 17, 21]







================================


Easy to match same content for BASIC TYPEs

Scanner reader = new Scanner(System.in);
while (true) {
    System.out.print("Give a number: ");
    int num = Integer.valueOf(reader.nextLine());

    if (num == 10) {
        System.out.println("You gave a ten!");
        break;
    }
}

Example execution:
Give a number: 5
Give a number: 6
Give a number: 3
Give a number: 10
You gave a ten!




VS




Difficult to match same object for REFERENCE/OBJECT TYPEs
- match references
- not matching content




ArrayList<Integer> list1 = new ArrayList<>();
ArrayList<Integer> list2 = new ArrayList<>();
ArrayList<Integer> list3 = list1;

list1.add(1);
list1.add(2);
list1.add(3);

list2.add(1);
list2.add(2);
list2.add(3);

System.out.println("list1 == list2? " + (list1 == list2));
System.out.println("list1 == list3? " + (list1 == list3));
System.out.println("list2 == list3? " + (list2 == list3));


Program outputs:
list1 == list2? false     // DIFFERENT REFERENCE BUT SAME CONTENT
list1 == list3? true      // SAME REFERENCE, cuz list3 was refererring to list1, and its contents
list2 == list3? false     // DIFFERENT REFERENCE BUT SAME CONTENT









================================



The 'equals' operator can be used to compare the SIMILARITY of OBJECTS.
The operator returns true if the objects have the SAME CONTENT.



ArrayList<Integer> list1 = new ArrayList<>();
ArrayList<Integer> list2 = new ArrayList<>();
ArrayList<Integer> list3 = list1;

list1.add(1);
list1.add(2);
list1.add(3);

list2.add(1);
list2.add(2);
list2.add(3);

System.out.println("list1.equals(list2) " + (list1.equals(list2)));
System.out.println("list1.equals(list3) " + (list1.equals(list3)));
System.out.println("list2.equals(list3) " + (list2.equals(list3)));


Program outputs:
list1.equals(list2) true
list1.equals(list3) true
list2.equals(list3) true








================================


Since STRINGS are Java OBJECTS, their EQUALITY must also be compared using the EQUALS operator, for example

Scanner reader  = new Scanner(System.in);
System.out.print("Give a name: ");
String name = reader.nextLine();


if (name == "Jack Java") {
    System.out.println("nimi == Jack Java"); // THIS DID NOT RUN
}
if (name.equals("Jack Java")){
    System.out.println("the name is Jack Java"); // THIS RAN
}


Program outputs:
Give a name: Jack Java
the name is Jack Java