The equals() method resides within the object class. This is used to determine if two object instances are the same instance (based on the instance signature) If you want to determine if instances are instead based on the signature but by viewing the variables passed within the parenthesis, you need to override the equals() method.

Example below

public class EqualsOverride {

 private int x;
 private int y;

 public EqualsOverride(int x, int y){
  this.x = x;
  this.y = y;

 }

 public boolean equals(EqualsOverride e){

  if ((x == e.x)&&(y==e.y))
  return true;
  else
  return false;
 }

 public static void main(String[] args) {

  EqualsOverride e1 = new EqualsOverride(20, 30);
  EqualsOverride e2 = new EqualsOverride(20, 30);

  System.out.println(e1.equals(e2));

 }

}

This equals true

regarding the equals() method, you must remember:

• The base class when declaring the class instance must be must be the class with the equals() override method within

  EqualsOverride e1 = new EqualsOverride(20, 30);
  EqualsOverride e2 = new EqualsOverride(20, 30);

Having another class as the base will not invoke the override equals() method

  Object e1 = new EqualsOverride(20, 30);
  Object e2 = new EqualsOverride(20, 30);

You can call the super class constructor from within the derived constructor

super() method...

• super() can only be called within the derived class constructor only
• Must be the first line within the derived class constructor

public class Apples extends Fruit{
 
public Apples (int x, int y){
 super(10,20);
 }
}

This example below will throw an error

public Apples (int x, int y){
int x = 100;
 super(10,20);
 }

• Upcasting -- Changing the instance of the class to it's superclass
• Downcasting -- Changing from it's class instance to a derived class (requires explicit casting)

String s1 = "Alright!";
Object o = s1; //No explicit casting needed

Downcasting requires you to explicitly highlight the casting (Start with an object then converts into a String instance)

Object o = new Object();
String s1 = (String) o;//requires casting

Upcast then downcast
Int -+ Obj --+ String
You cannot upcast then downcast because you'll get a ClassCastException error

There's no ClassCastException exception handling, so the best way is to use the instanceOf boolean operator. This helps the java program determine if downcasting is safe to do.

if (obj instanceOf String){
String str2 = (String) obj;
{

+++
+++ package1
+++
 Class1

+++
+++ package2
+++
 Class2

package1.Class1 = new package1.Class1();

If you want to create an instance of Class1 from within Class2, you need to declare the FQN next to the base class.

Alternatively, you can just import the class from by using the import

package 2:
import package1.Class1;

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