Constructor

After creating an object for a class, a constructor is called and can set initial values for the object attributes.

public class Main {
    int x;  // Create a class attribute
  
    // Create a class constructor for the Main class
    public Main() {
      x = 5;  // Set the initial value for the class attribute x
    }
  
    public static void main(String[] args) {
      Main myObj = new Main(); // Create an object of class Main (This will call the constructor)
      System.out.println(myObj.x); // Print the value of x
    }
  }

Accesssors and Mutators

Accessor and mutator methods can be used to return the value of a private variable and change the value of the variable respectively.

public class CheckingAccount{
    private int balance;
    
    //An accessor method
    public int getBalance(){
      return this.balance;
    }

    //A mutator method
    public void setBalance(int newBalance){
        this.balance = newBalance;
      }
  }

"This"

The this keyword is the current object in a method or constructor. It is used to invoke the current class, return the current class objet, or pass an argument.

public class Main {
    int x;
  
    // Constructor with a parameter
    public Main(int x) {
      this.x = x;
    }
  
    // Call the constructor
    public static void main(String[] args) {
      Main myObj = new Main(5);
      System.out.println("Value of x = " + myObj.x);
    }
  }public class Main {
  int x;

  // Constructor with a parameter
  public Main(int x) {
    this.x = x;
  }

  // Call the constructor
  public static void main(String[] args) {
    Main myObj = new Main(5);
    System.out.println("Value of x = " + myObj.x);
  }
}

The toString() will represent any object as a string.

class Student{  
    int rollno;  
    String name;  
    String city;  
     
    Student(int rollno, String name, String city){  
    this.rollno=rollno;  
    this.name=name;  
    this.city=city;  
    }  
     
    public static void main(String args[]){  
      Student s1=new Student(101,"Raj","lucknow");  
      Student s2=new Student(102,"Vijay","ghaziabad");  
        
      System.out.println(s1);//compiler writes here s1.toString()  
      System.out.println(s2);//compiler writes here s2.toString()  
    }  
   }

Standard Methods

The equals() method will return true or false depending on whether two strings are equal.

String myStr1 = "Hello";
String myStr2 = "Hello";
String myStr3 = "Another String";
System.out.println(myStr1.equals(myStr2)); // Returns true because they are equal
System.out.println(myStr1.equals(myStr3)); // false

true
false

A late binding of an object is when the object is determined when the code is ran.

class Animal {  
    void eat(){System.out.println("animal is eating...");}  
}  
     
class Dog extends Animal{  
    void eat() {
        System.out.println("dog is eating...");
    }  
public static void main(String args[]) {  
    Animal a=new Dog();  
    a.eat();  
}  
}

Polymorphism is how a class is able to provide different implementations of a method. It allows you to make the same action in different ways.

Big O notation is what the set of all the different algorithms that is able to run at a certain speed.

To combine two different strings, you can use concatenation which can combine two or more strings to create one new string.

Homework