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LESSON 8: WRAPPER CLASSES IN JAVA



✅Wrapper classes provide a way to use primitive data types (int, boolean, etc..) as objects.

✳️Sometimes you must use wrapper classes, for example when working with Collection objects, such as ArrayList, where primitive types cannot be used (the list can only store objects):

example:
ArrayList<Integer> myNumbers = new ArrayList<Integer>(); // Valid

Creating Wrapper Objects

To create a wrapper object, use the wrapper class instead of the primitive type. To get the value, you can just print the object:
Example

public class Main {
  public static void main(String[] args) {
    Integer myInt = 5;
    Double myDouble = 5.99;
    Character myChar = 'A';
    System.out.println(myInt);
    System.out.println(myDouble);
    System.out.println(myChar);
  }
}

//

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LESSON 9: CONSTRUCTORS in Java

✳️ A constructor in Java is a special method that is used to initialize objects. The constructor is called when an object of a class is created. It can be used to set initial values for object attributes.


How Constructors are Different From Methods in Java?

❤️ Constructors must have the same name as the class within which it is defined while it is not necessary for the method in Java.
❤️Constructors do not return any type while method(s) have the return type or void if does not return any value.

❤️Constructors are called only once at the time of Object creation while method(s) can be called any number of times.

Example: look the sample code having two constructors;

package ubJapro1;
//import java.util.*;


public class Firsta {

  public static void main(String[] args) {
    
  Firsta2 firs  = new Firsta2("ashe",34,76.4);// constructor 1
  Firsta2 firs2  = new Firsta2("asegid",32,70.4);// constructor 2
  
   System.out.println(firs.name+" is your name.");
  
    firs2.home();
   
  }
  
    
    
    
      
  }

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LESSON 10: Creating objects in java

✳️An object is a basic unit of Object-Oriented Programming and represents the real life entities. It consists of:-
⚜️State means the attributes.
⚜️Behavior means the methods of an object.
⚜️Identity means the unique name to an object.


the four ways of creating objects in java

1. using new key word

//Test t = new Test();
2. Using Class.forName( String className) method;

  Test obj = (Test)Class.forName("com.p1.Test").newInstance();

3. using clone() method

/Test t = new Test();
Test t2 = (Test)t1.clone();

4. Deserialization
it is is technique of reading an object from the saved state in a file.

✅example using the first way:

 class Animal {}

class Dog extends Animal {}
class Cat extends Animal {}

public class Test
{
    // using Dog object
    Animal obj = new Dog();

    // using Cat object
    obj = new Cat();
}       

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LESSON 11: ENCAPSULATION

✅Encapsulation is defined as the wrapping up of data under a single unit. It is the mechanism that binds together code and the data it manipulates.
Another way to think about encapsulation is, it is a protective shield that prevents the data from being accessed by the code outside this shield.
Following is an example that demonsrates how to achieve encapsualtion in java:

/* File name : EncapTest.java */

public class EncapTest {
   private String name;
   private String idNum;
   private int age;

   public int getAge() {
      return age;
   }

   public String getName() {
      return name;
   }

   public String getIdNum() {
      return idNum;
   }

   public void setAge( int newAge) {
      age = newAge;
   }

   public void setName(String newName) {
      name = newName;
   }

   public void setIdNum( String newId) {
      idNum = newId;
   }
}

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LESSON 12: POLYMORPHISM

The word polymorphism means having many forms. In simple words, we can define polymorphism as the ability of a message to be displayed in more than one form.

Types of polymorphism

In Java , polymorphism is mainly divided into two types:

i.Compile-time Polymorphism
💎This type of polymorphism is achieved by function overloading or operator overloading. But Java doesn’t support the Operator Overloading.

ii.Runtime Polymorphism
❤️‍🔥It is a process in which a function call to the overridden method is resolved at Runtime. This type of polymorphism is achieved by Method Overriding.

>💗 Java program to demostrate polymorphism

class Parent {
 
    // Method of parent class
    void Print()
    {
 
        // Print statement
        System.out.println("parent class");
    }
}
 

// Helper class
class subclass2 extends Parent {
 
    // Method
    void Print()
    {
 
        // Print statement
        System.out.println("subclass2");
    }
}
 
// Class 4
// Main class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
 
        // Creating object of class 1
        Parent a;
 
        // Now we will be calling print methods
        // inside main() method
 
        a = new subclass1();
        a.Print();
 
      
    }
}