Java Syntax Explained for Beginners: Program Structure, Data Types, Loops, Methods and Examples

Java is strict, which makes it a good first language for learning how programs are built. Learn the handful of rules below and most beginner errors disappear.

Your first Java program

Save this as Hello.java. The file name must match the public class name exactly.

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

• public class Hello declares a class. All Java code lives in classes.
• public static void main(String[] args) is where the program starts.
• System.out.println(...) prints a line of text.
• Curly braces { } mark the start and end of a block, and each statement ends with ;.

Compile and run from a terminal, with a JDK installed:

javac Hello.java
java Hello

Output: Hello, Masai!. From Java 11 you can also run a single file with java Hello.java.

The core syntax rules

• Case-sensitive. Main and main are different names.
• Semicolons end statements.
• Class names start with a capital letter (BankAccount); methods and variables with a lowercase letter (totalMarks).
• One public class per file, and the file name matches it.
• Declare before use. Every variable has a type.
• Comments: // one line, /* many lines */, /** documentation */.

Data types and variables

int age = 21;                  // whole number
double cgpa = 8.7;              // decimal number
char grade = 'A';               // one character, single quotes
boolean placed = false;         // true or false
String name = "Asha";           // text, double quotes
final int MAX_ATTEMPTS = 3;     // constant: cannot change

Operators

Use == to compare numbers and .equals() to compare the contents of strings.

Decisions and loops

int marks = 72;
if (marks >= 90) {
    System.out.println("Distinction");
} else if (marks >= 60) {
    System.out.println("First class");
} else {
    System.out.println("Keep going");
}

for (int i = 1; i <= 3; i++) {
    System.out.println("Round " + i);
}

int n = 3;
while (n > 0) {
    System.out.println(n--);
}

Output: First class, then Round 1, Round 2, Round 3, then 3, 2, 1.

Methods

A method has a return type, a name and parameters. void means it returns nothing.

public class Calculator {
    static int add(int a, int b) {        // return type, name, parameters
        return a + b;
    }

    public static void main(String[] args) {
        int sum = add(4, 5);
        System.out.println("Sum = " + sum);   // Sum = 9
    }
}

Arrays

int[] marks = {72, 85, 90};
System.out.println(marks.length);      // 3
System.out.println(marks[1]);          // 85
for (int m : marks) {                  // enhanced for loop
    System.out.println(m);
}

An array has a fixed size, and indexes start at 0.

Reading input

import java.util.Scanner;

public class Greeting {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.print("Your name: ");
        String name = in.nextLine();
        System.out.println("Welcome, " + name);
        in.close();
    }
}

Classes and objects

A class is a blueprint and new makes an object. The full example, with the four OOP ideas, is on the OOP page.

The most common Java errors for beginners

Where to practise

• Oracle's free Java tutorials on the language basics.
• Write the practice programs: a calculator, a number guessing game, a marks report.
• Then continue with OOP, hash maps and the software development path. The Masai guide to Java basic syntax has more examples.

Frequently asked questions

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