Java Day 5: Local & Global Variables, Static vs Non-Static, Naming Rules, Comments, Methods vs Constructors

Local and Global Variables Local Variables Definition: A local variable is declared inside a method, constructor, or block and can only be accessed within that scope. Lifetime: Exists only while the method/block is executing. Example: public class Example { void display() { int localVar = 10; // Local variable System.out.println(localVar); } } Global (Instance) Variables Definition: A variable declared inside a class but outside any method, constructor, or block. Lifetime: Exists as long as the object exists. Example: public class Example { int globalVar = 20; // Global variable void display() { System.out.println(globalVar); } } Using Static and Non-Static Elements 1. Static to Static Direct Access: Static variables, methods, and objects can be accessed inside static methods without creating an object. Example: class Test { static int num = 10; static void display() { System.out.println(num); } public static void main(String[] args) { display(); } } 2. Non-Static to Static Requires Object: A non-static variable/method must be accessed through an instance inside a static method. Example: class Test { int num = 20; void display() { System.out.println(num); } public static void main(String[] args) { Test obj = new Test(); // Creating object obj.display(); // Accessing non-static method } } 3. Static to Non-Static Direct Access: A non-static method can directly access a static variable/method. Example: class Test { static int num = 30; void display() { System.out.println(num); // Direct access } } 4. Non-Static to Non-Static Direct Access: Non-static variables/methods can be accessed inside non-static methods directly. Example: class Test { int num = 40; void display() { System.out.println(num); // Direct access } } Naming Rules Class Names Start with an uppercase letter. Use CamelCase. Example: StudentDetails, EmployeeData Variable and Object Names Start with a lowercase letter. Use camelCase. Example: studentAge, employeeSalary Method Names Use verbs. Start with lowercase. Example: getName(), calculateSalary() Comments in Java Single-line Comment // This is a single-line comment Multi-line Comment /* This is a multi-line comment. Used to describe more information. */ Methods vs Constructors Methods Used to define behavior. Can return values. Need to be called explicitly. Example: class Test { void display() { System.out.println("Hello"); } } Constructors Used to initialize objects. No return type. Automatically called when an object is created. Example: class Test { Test() { System.out.println("Constructor Called"); } } This blog covers all aspects of Java local/global variables, static vs non-static elements, naming rules, comments, constructors, methods, and object creation in depth.

Apr 1, 2025 - 17:16

Java Day 5: Local & Global Variables, Static vs Non-Static, Naming Rules, Comments, Methods vs Constructors

Local and Global Variables

Local Variables

Definition: A local variable is declared inside a method, constructor, or block and can only be accessed within that scope.
Lifetime: Exists only while the method/block is executing.
Example:

  public class Example {
      void display() {
          int localVar = 10; // Local variable
          System.out.println(localVar);
      }
  }

Global (Instance) Variables

Definition: A variable declared inside a class but outside any method, constructor, or block.
Lifetime: Exists as long as the object exists.
Example:

  public class Example {
      int globalVar = 20; // Global variable
      void display() {
          System.out.println(globalVar);
      }
  }

Using Static and Non-Static Elements

1. Static to Static

Direct Access: Static variables, methods, and objects can be accessed inside static methods without creating an object.
Example:

  class Test {
      static int num = 10;
      static void display() {
          System.out.println(num);
      }
      public static void main(String[] args) {
          display();
      }
  }

2. Non-Static to Static

Requires Object: A non-static variable/method must be accessed through an instance inside a static method.
Example:

  class Test {
      int num = 20;
      void display() {
          System.out.println(num);
      }
      public static void main(String[] args) {
          Test obj = new Test(); // Creating object
          obj.display(); // Accessing non-static method
      }
  }

3. Static to Non-Static

Direct Access: A non-static method can directly access a static variable/method.
Example:

  class Test {
      static int num = 30;
      void display() {
          System.out.println(num); // Direct access
      }
  }

4. Non-Static to Non-Static

Direct Access: Non-static variables/methods can be accessed inside non-static methods directly.
Example:

  class Test {
      int num = 40;
      void display() {
          System.out.println(num); // Direct access
      }
  }

Naming Rules

Class Names

Start with an uppercase letter.
Use CamelCase.
Example: StudentDetails, EmployeeData

Variable and Object Names

Start with a lowercase letter.
Use camelCase.
Example: studentAge, employeeSalary

Method Names

Use verbs.
Start with lowercase.
Example: getName(), calculateSalary()

Comments in Java

Single-line Comment

// This is a single-line comment

Multi-line Comment

/*
 This is a multi-line comment.
 Used to describe more information.
*/

Methods vs Constructors

Methods

Used to define behavior.
Can return values.
Need to be called explicitly.
Example:

  class Test {
      void display() {
          System.out.println("Hello");
      }
  }

Constructors

Used to initialize objects.
No return type.
Automatically called when an object is created.
Example:

  class Test {
      Test() {
          System.out.println("Constructor Called");
      }
  }

This blog covers all aspects of Java local/global variables, static vs non-static elements, naming rules, comments, constructors, methods, and object creation in depth.