Day-70 My Java Basics Revision

Today, instead of attending regular classes, we had a test to revise everything we learned since the beginning of our Java journey. Here's a detailed summary of all the key concepts we've discussed so far.

1.Class

A class is a blueprint or template used to create objects.

• It is declared using the class keyword.
• Inside a class, we can declare variables, methods, and constructors.
• Example:

  class Main {
      // variables, methods, constructors
  }

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2. Global and Local Variables

• Local Variables: Declared inside a method, only accessible within that method.

• Global Variables: Declared outside methods but inside a class; accessible anywhere in the class. Also called fields.

• Global variables are of two types:

1. Static (class variables)
2. Non-static (instance variables)

3. Data Types in Java

Java has two types of data types:

1.Primitive Data Types (fixed size):

• byte, short, int, long, float, double, char, boolean

• Default: int for integers, double for floating points.

2.Non-Primitive Data Types (reference types):

• Arrays, Strings, Classes...

4.Packages

Packages act like folders to organize classes.
There are two types:

1. Built-in packages (like from Java API)
2. User-defined packages

Example:

package myPackage;

Use javac -d . filename.java to compile and java packagename.classname to run.

5. Object

An object is an instance of a class that contains both state and behavior.
We create objects using the new keyword.

Example:

Main obj = new Main();

6. Constructor and Constructor Chaining

• A constructor has the same name as the class and no return type.
• Used to initialize objects with specific values.
• Java provides a default constructor automatically.
• Constructor Chaining is calling one constructor from another using this() as the first statement.

7.Inheritance

Inheritance allows one class to access the properties and methods of another class.

• Use extends keyword.
• Parent class: Super class
• Child class: Sub class
• Main benefit: Code reusability

8. Abstraction

Abstraction means showing only essential details and hiding unnecessary ones.

• Use abstract keyword for classes or methods.
• Cannot instantiate abstract classes directly.
• Abstract methods have no body — must be implemented by subclass.

9. Polymorphism

Polymorphism = One interface, many forms.

Two types:

1.Compile-time Polymorphism (Method Overloading)

• Same method name with different arguments or data types.

2.Runtime Polymorphism (Method Overriding)

• Same method in child and parent class.

10. Encapsulation

Encapsulation = Binding data and code together.

• Declare variables as private.
• Access and update them using getter and setter methods.
• Increases data security.

11. Interface

Another way to achieve abstraction.

• An interface is not a class; all methods are abstract by default.
• Use implements instead of extends.
• Interfaces don’t have constructors.

12 & 13. this vs super / this() vs super()

• this: Refers to current object

• super: Refers to parent class object

• this() is used to call another constructor in the same class.

• super() is used to call the constructor of the parent class.

14. final Keyword

• final can be used with class, method and variables.
• Final class → cannot be inherited
• Final method → cannot be overridden
• Final variable → value cannot be changed

15. static vs non-static

• static: Belongs to class; shared across all objects
• non-static: Belongs to object; memory allocated at object creation

Static variables can be accessed using ClassName.variableName.
Non-static variables require an object reference.