Java Base Concepts

1. Java Data Types

Explanation:
In Java, a data type defines what kind of value a variable can hold. It tells the compiler how much memory to reserve and how to interpret the data.

Primitive Data Types:

These are the most basic data types. Java has 8 of them:

• int – for integers like 10, 200, -5
• float, double – for decimal values like 3.14
• char – for single characters like 'A', 'z'
• boolean – only true or false

Non-Primitive Data Types:

These are more complex types built using primitives:

• String – stores text
• Array – stores multiple values
• Class, Interface – used in object-oriented programming

Official Reference:
https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html

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2. Static and Non-Static

Explanation:
In Java, variables and methods can be static or non-static (also called instance members).

Static:

• Shared across all objects.
• You don’t need to create an object to access it.

class Example {
    static int x = 10;  // static variable

    static void show() {
        System.out.println(x);
    }
}

You can call it like:
Example.show();

Non-Static:

• Each object gets its own copy.
• You must create an object to use it.

class Example {
    int y = 20;

    void display() {
        System.out.println(y);
    }
}

Official Reference:
https://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html

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3. Java Methods

Explanation:
A method is a block of code that performs a specific task. It helps avoid repeating code and makes your program organized.

Syntax:

returnType methodName(parameters) {
    // method body
}

Example:

void greet() {
    System.out.println("Hello!");
}

This method prints a greeting when called.

Official Reference:
https://docs.oracle.com/javase/tutorial/java/javaOO/methods.html

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4. Return Types

Explanation:
A return type defines what value a method will give back after it's done working.

• If it returns nothing, use void.
• If it returns an int, String, etc., mention the type.

Examples:

int square(int x) {
    return x * x;
}

void greet() {
    System.out.println("Hi!");
}

In the first example, square returns a number.
In the second, greet returns nothing — it just prints a message.

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5. Objects in Java

Explanation:
Java is an object-oriented language, meaning you write classes and create objects from them.

A class is like a blueprint.
An object is the real thing built from that blueprint.

Example:

class Dog {
    void bark() {
        System.out.println("Woof!");
    }
}

public class Main {
    public static void main(String[] args) {
        Dog d = new Dog();  // Object created
        d.bark();           // Calling method
    }
}

Official Reference:
https://docs.oracle.com/javase/tutorial/java/javaOO/objects.html

6. Local and Global Variables

Explanation:
A variable is used to store data. Depending on where it's declared, it can be either local or global (also called instance variable).

Local Variable:

• Declared inside a method
• Can be used only in that method

Example:

void show() {
    int a = 10;  // local variable
    System.out.println(a);
}

Global Variable:

• Declared outside all methods but inside a class
• Can be used by all methods in the class

Example:

class Test {
    int x = 50;  // global variable

    void printX() {
        System.out.println(x);
    }
}

Official Reference:
https://docs.oracle.com/javase/tutorial/java/nutsandbolts/variables.html