Topic 3. Fundamental Programming Structures in Java

This topic covers the building blocks used in almost every Java program: program structure, comments, data types, variables, operators, strings, and arrays.

A Simple Java Program

Start with a small example and read it step by step.

public class Demo {
    public static void main(String[] args) {
        System.out.println("Start");

        int x = 10;
        int y = 5;
        int sum = x + y;

        System.out.println(sum);
    }
}

What happens:

1. JVM calls main as the entry point.
2. Variables x and y are created.
3. sum is computed.
4. Result is printed.

Comments

Comments should explain intent and constraints. Java has 3 comment types.

1. Single-line comment

// Use UTC to avoid timezone-dependent behavior
Instant now = Instant.now();

2. Multi-line comment

/*
 Legacy constraint:
 id must contain exactly 8 characters
*/
boolean valid = id.length() == 8;

3. Documentation comment (Javadoc)

/**
 * Returns price after discount.
 * @param amount original amount
 * @param percent discount in percent
 * @return final amount after discount
 */
public double applyDiscount(double amount, double percent) {
    return amount - amount * percent / 100.0;
}

Good rule:

• Add comments when “why” is not obvious.
• Do not comment obvious syntax.

Data Types

Java is strongly typed: every value has a type checked at compile time.

• Primitive types: int, long, double, boolean, char.
• Reference types: String, arrays, classes, interfaces.

int age = 20;
String name = "Alex";

age stores a value directly; name stores a reference to an object.

Variables, Assignments and Initializations

A variable is a named storage slot.

• Initialization: first value assignment.
• Assignment: later value update.

int balance = 100;       // initialization
balance = balance + 50;  // assignment
System.out.println(balance);

Output:

150

Operators

Main operator groups:

• Arithmetic: +, -, *, /, %.
• Comparison: ==, !=, >, <, >=, <=.
• Logical: &&, ||, !.

int age = 20;
boolean canBuy = age >= 18 && age < 65;
System.out.println(canBuy);

String

String in Java is a data type for text: user names, emails, messages, URLs, file paths, and any sequence of characters.

A string is an ordered sequence of characters. Example:

String name = "Alex";
String city = "Prague";
String message = "Hello, world!";

Most application input/output is text-based: form fields, JSON payloads, query params, logs, and error messages.

String is also immutable in Java. Once created, a specific string object cannot be changed; operations create a new string object.

What to understand early

1. Methods like toUpperCase() return a new string.
2. Use equals for content comparison, not ==.
3. For repeated concatenation in loops, prefer StringBuilder.

Example 1. Immutability

String s = "java";
String upper = s.toUpperCase();

System.out.println(s);     // java
System.out.println(upper); // JAVA

Example 2. String comparison

String a = "test";
String b = new String("test");

System.out.println(a == b);      // false (reference comparison)
System.out.println(a.equals(b)); // true  (content comparison)

Example 3. Concatenation in loops

StringBuilder sb = new StringBuilder();
for (int i = 1; i <= 3; i++) {
    sb.append(i).append(" ");
}
String result = sb.toString();
System.out.println(result); // 1 2 3

Arrays

An array is a fixed-size sequence of elements of the same type.

• Single element type.
• Size is fixed at creation time.
• Zero-based indexing.

Create an array

int[] scores = {90, 75, 88};

Read an element

System.out.println(scores[1]);

Output:

75

Update an element

scores[1] = 80;
System.out.println(scores[1]);

Output:

80

Safe iteration

for (int i = 0; i < scores.length; i++) {
    System.out.println(scores[i]);
}

Out-of-range access throws ArrayIndexOutOfBoundsException.

Practice after this topic

1. Write a program that computes average score from an array.
2. Compare strings using == and equals and observe the difference.
3. Replace loop concatenation with StringBuilder.

These are foundational skills for conditions, loops, collections, and OOP topics that follow.