Java Basics: Hello World, Variables, Arrays, Expressions, Control Structures, and Object‑Oriented Concepts

Hello World

The first example shows a simple HelloWorld.java program that prints the string "Hello World!" on the console, illustrating the basic structure of a Java class, the main method, and a statement.

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

Variables

Java is a statically‑typed language; variables must be declared with a type before use. The tutorial demonstrates declaration, assignment, and printing of an integer variable.

public class Test {
    public static void main(String[] args) {
        System.out.println("Declare in the middle:");
        int a;
        a = 5;
        System.out.println(a); // print an integer
    }
}

It also notes that a variable can be declared and initialized in a single statement, e.g., int a = 5;, and that comments are introduced with //.

Arrays

Arrays store multiple values of the same type. Declaration syntax ( int[] a;) and allocation with new are shown, as well as inline initialization.

int[] a; // declaration
int[] a = new int[100]; // allocate 100 ints
int[] a = new int[] {1, 3, 5, 7, 9}; // allocate and initialize

Expressions

Expressions combine variables, constants, and operators to produce a value. Examples include arithmetic ( 1 + 1), relational ( a > 4.2), boolean ( true && false), and bitwise operators ( &, |, ^, ~, <<, >>).

Control Structures

The tutorial covers the standard C‑style control flow constructs in Java: if‑else, while, do…while, for, break, continue, and switch. Sample syntax is provided for each.

if (condition1) {
    // statements
} else if (condition2) {
    // statements
} else {
    // statements
}

while (condition) {
    // statements
}

do {
    // statements
} while (condition);

for (initial; condition; update) {
    // statements
}

switch (expression) {
    case 1:
        // statements
        break;
    default:
        // statements
        break;
}

Object‑Oriented Basics

Objects are abstractions with identity, state (data members/fields), and behavior (methods). Classes define the blueprint; instances are created with new. The tutorial defines a simple Human class with a breath() method and an int height field, then shows how to instantiate and use it.

class Human {
    void breath() {
        System.out.println("hu...hu...");
    }
    int height;
}

public class Test {
    public static void main(String[] args) {
        Human aPerson = new Human();
        aPerson.breath();
        System.out.println(aPerson.height);
    }
}

Summary

Java’s syntax resembles C/C++ but has its own nuances; careful attention to details such as type declarations, array allocation, and object identity is required. The language is fully object‑oriented, and mastering these fundamentals provides a solid foundation for further Java development.