Java: Short Tutorial

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Java Tutorial (Easy & Modern Features)

Java is a robust, object-oriented language widely used for enterprise applications, Android development, and large-scale systems.

1. "Hello, World!"

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

Explanation:

public class HelloWorld: Defines a class named HelloWorld. In Java, all code resides within classes.
public static void main(String[] args): This is the entry point of the Java program.
- public: The method is accessible from anywhere.
- static: The method belongs to the class itself, not to any specific object of the class.
- void: The method doesn't return any value.
- main: The special name for the entry point method.
- String[] args: An array of strings to receive command-line arguments.
System.out.println(...): Prints output to the console, followed by a newline.

2. var for Local Variable Type Inference (Java 10+):

Allows the compiler to infer the type of a local variable based on its initialization, similar to C++ auto or Python's typing.

auto or Python's typing.

import java.util.ArrayList;
import java.util.List;

public class VarExample {
    public static void main(String[] args) {
        var message = "Java is fun!"; // Compiler infers 'message' is String
        var count = 10;             // Compiler infers 'count' is int
        var names = new ArrayList<String>(); // Compiler infers 'names' is ArrayList<String>

        names.add("Alice");
        names.add("Bob");

        System.out.println(message);
        System.out.println("Count: " + count);
        System.out.println("Names: " + names);
    }
}

Explanation:

var message = ...: Declares message using var, letting the compiler determine its type (String).

3. Records (Java 14+):

A concise way to create classes that primarily hold data. They automatically generate constructors, getters, equals(), hashCode(), and toString().

public record Point(int x, int y) {
    public static void main(String[] args) {
        Point p1 = new Point(10, 20);
        Point p2 = new Point(10, 20);
        Point p3 = new Point(30, 40);

        System.out.println("Point 1: " + p1); // Uses generated toString()
        System.out.println("Point 1 X: " + p1.x()); // Uses generated getter
        System.out.println("p1 equals p2? " + p1.equals(p2)); // Uses generated equals()
        System.out.println("p1 equals p3? " + p1.equals(p3)); // Uses generated equals()
    }
}

Explanation:

public record Point(int x, int y): Defines a record named Point with two integer components, x and y.
p1.x(): Automatically generates a getter method for the x component.

4. Text Blocks (Java 15+):

A convenient way to define multi-line strings without needing escape sequences like \n.

strings without needing escape sequences like \n.

public class TextBlockExample {
    public static void main(String[] args) {
        String html = """
                <html>
                <head>
                    <title>My Page</title>
                </head>
                <body>
                    <h1>Welcome!</h1>
                    <p>This is a simple paragraph.</p>
                </body>
                </html>
                """;

        System.out.println(html);
    }
}

Explanation:

""" ... """: Uses triple double quotes to define a text block. Newlines and indentation are generally preserved.

5. Switch Expressions (Java 14+, enhanced in Java 17+):

A more concise way to write switch statements, allowing them to return a value.

public class SwitchExpressionExample {
    public static void main(String[] args) {
        int dayOfWeek = 3;

        String dayName = switch (dayOfWeek) {
            case 1 -> "Monday";
            case 2 -> "Tuesday";
            case 3 -> "Wednesday";
            case 4 -> "Thursday";
            case 5 -> "Friday";
            case 6 -> "Saturday";
            case 7 -> "Sunday";
            default -> "Invalid day";
        };

        System.out.println("The day is: " + dayName);
    }
}

Explanation:

switch (dayOfWeek) { ... }: Defines the switch expression.
case 1 -> "Monday";: Uses the -> operator to specify the value to return for each case.
default -> "Invalid day";: The default case.

Running Java Code:

Save the code in a .java file (e.g., MyProgram.java). Open your terminal or command prompt, navigate to the directory where you saved it.

Compile:

javac MyProgram.java

Run:

java MyProgram

0:00

Java Tutorial (Easy & Modern Features)

Java is a robust, object-oriented language widely used for enterprise applications, Android development, and large-scale systems.

1. "Hello, World!"

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

Explanation:

public class HelloWorld: Defines a class named HelloWorld. In Java, all code resides within classes.
public static void main(String[] args): This is the entry point of the Java program.
- public: The method is accessible from anywhere.
- static: The method belongs to the class itself, not to any specific object of the class.
- void: The method doesn't return any value.
- main: The special name for the entry point method.
- String[] args: An array of strings to receive command-line arguments.
System.out.println(...): Prints output to the console, followed by a newline.

2. var for Local Variable Type Inference (Java 10+):

Allows the compiler to infer the type of a local variable based on its initialization, similar to C++ auto or Python's typing.

auto or Python's typing.

import java.util.ArrayList;
import java.util.List;

public class VarExample {
    public static void main(String[] args) {
        var message = "Java is fun!"; // Compiler infers 'message' is String
        var count = 10;             // Compiler infers 'count' is int
        var names = new ArrayList<String>(); // Compiler infers 'names' is ArrayList<String>

        names.add("Alice");
        names.add("Bob");

        System.out.println(message);
        System.out.println("Count: " + count);
        System.out.println("Names: " + names);
    }
}

Explanation:

var message = ...: Declares message using var, letting the compiler determine its type (String).

3. Records (Java 14+):

A concise way to create classes that primarily hold data. They automatically generate constructors, getters, equals(), hashCode(), and toString().

public record Point(int x, int y) {
    public static void main(String[] args) {
        Point p1 = new Point(10, 20);
        Point p2 = new Point(10, 20);
        Point p3 = new Point(30, 40);

        System.out.println("Point 1: " + p1); // Uses generated toString()
        System.out.println("Point 1 X: " + p1.x()); // Uses generated getter
        System.out.println("p1 equals p2? " + p1.equals(p2)); // Uses generated equals()
        System.out.println("p1 equals p3? " + p1.equals(p3)); // Uses generated equals()
    }
}

Explanation:

public record Point(int x, int y): Defines a record named Point with two integer components, x and y.
p1.x(): Automatically generates a getter method for the x component.

4. Text Blocks (Java 15+):

A convenient way to define multi-line strings without needing escape sequences like \n.

strings without needing escape sequences like \n.

public class TextBlockExample {
    public static void main(String[] args) {
        String html = """
                <html>
                <head>
                    <title>My Page</title>
                </head>
                <body>
                    <h1>Welcome!</h1>
                    <p>This is a simple paragraph.</p>
                </body>
                </html>
                """;

        System.out.println(html);
    }
}

Explanation:

""" ... """: Uses triple double quotes to define a text block. Newlines and indentation are generally preserved.

5. Switch Expressions (Java 14+, enhanced in Java 17+):

A more concise way to write switch statements, allowing them to return a value.

public class SwitchExpressionExample {
    public static void main(String[] args) {
        int dayOfWeek = 3;

        String dayName = switch (dayOfWeek) {
            case 1 -> "Monday";
            case 2 -> "Tuesday";
            case 3 -> "Wednesday";
            case 4 -> "Thursday";
            case 5 -> "Friday";
            case 6 -> "Saturday";
            case 7 -> "Sunday";
            default -> "Invalid day";
        };

        System.out.println("The day is: " + dayName);
    }
}

Explanation:

switch (dayOfWeek) { ... }: Defines the switch expression.
case 1 -> "Monday";: Uses the -> operator to specify the value to return for each case.
default -> "Invalid day";: The default case.

Running Java Code:

Save the code in a .java file (e.g., MyProgram.java). Open your terminal or command prompt, navigate to the directory where you saved it.

Compile:

javac MyProgram.java

Run:

java MyProgram

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