obj1.equals(obj2)

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

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+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
+
+
+