public class Main {
  public static void main(String[] args) {
    System.out.println("Hello World");
  }
}

• Every Java program must be inside a class.
• The class name should start with an uppercase letter (e.g., Main).
• The file name must match the class name (e.g., Main.java).

• Java is case-sensitive: MyClass and myclass are different.

• Save the file as Main.java.
• Compile and run using Java tools (ensure Java is installed).

Hello World

• Required entry point of every Java program.

public static void main(String[] args)

• Starts with //

// This is a comment
System.out.println("Hello World");

• Variables are containers for data.
• Each variable has a type and name.

• String → Text, "Hello"
• int → Integer numbers, 123
• float → Decimal numbers, 19.99f
• char → Single character, 'A'
• boolean → True/false values

• All Java variables must be identified with unique names.
• These unique names are called identifiers.
• Identifiers can be:
  • Short names (like x and y)
  • Descriptive names (like age, sum, totalVolume)

• It is recommended to use descriptive names to make code understandable and maintainable.

// Good
int minutesPerHour = 60;

// OK, but not so easy to understand what m actually is
int m = 60;

Java has two main groups of primitive number types:

Store whole numbers (positive or negative) without decimals.

• byte
• short
• int
• long

• Stores numbers from -128 to 127
• Useful to save memory if values fit in this range

byte myNum = 100;
System.out.println(myNum);

• Stores numbers from -32,768 to 32,767

short myNum = 5000;
System.out.println(myNum);

• Stores numbers from -2,147,483,648 to 2,147,483,647
• Most commonly used integer type

int myNum = 100000;
System.out.println(myNum);

• Stores numbers from -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
• Use when int range is exceeded
• End literal with L

long myNum = 15000000000L;
System.out.println(myNum);

double myNum = 19.99d;
System.out.println(myNum);

• The precision of a floating point value indicates how many digits it can have after the decimal point.
• float precision: about 6-7 decimal digits.
• double precision: about 16 decimal digits.
• Therefore, it is safer to use double for most calculations where precision matters.

• Floating point numbers can be represented in scientific notation using an "e" or "E" to indicate powers of 10.

float f1 = 35e3f;
double d1 = 12E4d;
System.out.println(f1);
System.out.println(d1);

Very often in programming, you will need a data type that can only have one of two values, like:

• YES / NO
• ON / OFF
• TRUE / FALSE

For this, Java has a boolean data type, which can only take the values true or false.

boolean isJavaFun = true;
boolean isFishTasty = false;
System.out.println(isJavaFun);     // Outputs true
System.out.println(isFishTasty);   // Outputs false

The char data type is used to store a single character.
The character must be surrounded by single quotes, like 'A' or 'c'.

char myGrade = 'B';
System.out.println(myGrade);

Alternatively, if you are familiar with ASCII values, you can use those to display certain characters:

char myVar1 = 65, myVar2 = 66, myVar3 = 67;
System.out.println(myVar1);  // Outputs: A
System.out.println(myVar2);  // Outputs: B
System.out.println(myVar3);  // Outputs: C

The String data type is used to store a sequence of characters (text).
String values must be surrounded by double quotes.

String greeting = "Hello World";
System.out.println(greeting);

• The String type is so commonly used and integrated in Java, some call it "the special ninth type."

• A String in Java is actually a non-primitive data type, because it refers to an object.

• The String object has methods to perform operations on strings.

• Don’t worry if you don’t understand objects yet — this will be covered in later chapters.