If you recently started learning Java and you want to improve your grasp of the basics, try the exercises below. They cover only the absolute basics and are suitable for beginners.

1. Create a “Hello World” Java Program

Create a basic Java programme that simply outputs the text “Hello World”.

Hint: you’ll need to create a class and give it a ‘main’ method. Call the class “Application” (or anything else appropriate).

Show answer …. »

public class Application {

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

}

An Interactive Program

Create a program that asks the user to enter an integer. If the integer is less than 10, print the message “This number is too small”. If the integer is greater than or equal to 10, print “This number is big enough”.

Hint: use the Scanner class with an if … else statement.

Show answer …. »

import java.util.Scanner;

public class Application {

	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);

		System.out.println("Enter a number > ");

		int number = scanner.nextInt();

		if(number < 10) {
			System.out.println("This number is too small.");
		}
		else {
			System.out.println("This number is big enough.");
		}
	}

}

Arrays

Create a program that creates an array of five hard-coded floating-point values, then prints out just the second value.

Show answer …. »

import java.util.Scanner;

public class Application {

	public static void main(String[] args) {

		double[] values = { 0.1, 5.2, 5.3, 6.3, 83.3 };

		System.out.println(values[1]);
	}

}

Arrays and Loops

Modify the above program so that it uses a for loop to display all the values in the array, all on the same line, each value formatted to two decimal places and followed by a space.

Hint: use System.out.printf.

Show answer …. »

import java.util.Scanner;

public class Application {

	public static void main(String[] args) {

		double[] values = { 0.1, 5.2, 5.3, 6.3, 83.3 };

		for(int i=0; i < values.length; i++) {
			System.out.printf("%.2f ", values[i]);
		}
	}

}

Two Dimensional Arrays

A bit trickier, this one. Write an application that creates a two-dimensional array of Strings, with two rows and three columns. Print the value in the second row and third column.

Hint: you can create an array of strings just like you created an array of floating point values above, but with text in quotes (e.g. “I’m a string”) instead of floating-point numbers.

To define a 2D array, remember that the symbol [] means a 1D array (compare the answer to the exercise above), while [][] means a 2D array.

Also, remember that when you set the values of the array, each of the values is itself an array!

Finally, if this has you smashing plates and climbing the wall, check the answer, then hide it again and see if you can remember how it works.

Show answer …. »

public class Application {

	public static void main(String[] args) {

		String texts[][] = {
				{"dog", "cat", "bat" },
				{"fox", "box", "hat" }
		};

		System.out.println(texts[1][2]);
	}

}

Looping Through 2D Arrays

Create an application that uses two nested for loops to loop through the 2D array defined above and print the values.

Hint: Here’s the outer loop:

for(int row = 0; row < 2; row++) {
}

The inner loop should use a loop variable called “col” (or whatever you like) and should loop through the columns.

Hard code the number of columns and rows, as above.

Once again, this is one of the trickiest beginner’s tasks in Java. If you can’t work it out, check the answer, then hide it again and try to write it from memory.

Show answer …. »

public class Application {

	public static void main(String[] args) {

		String texts[][] = {
				{"dog", "cat", "bat" },
				{"fox", "box", "hat" }
		};

		for(int row = 0; row < 2; row++) {

			for(int col = 0; col < 3; col++) {
				System.out.print(texts[row][col]); // print the value
				System.out.print(" "); // print space
			}

			System.out.println(); // print a newline.
		}
	}

}

Create Classes and Objects

First, create a main program as in the first exercise.

Next, define a new class in its own file. Call the class Car. Give it a single method called “start”. Make the method simply print “Car started!”.

In your main program, create a new Car object and call its start() method.

Your final program should simply therefore display the text “Car started!”.

Show the car class … »

public class Car {

	public void start() {
		System.out.println("Car started!");
	}

}

Show the main application …. »

public class Application {

	public static void main(String[] args) {
		Car car = new Car();
		car.start();
	}

}

Constructors

Modify the above Car class so that it has an instance variable called name of type String. Add a constructor that accepts a string parameter and sets the car’s name using this parameter. Add a getName() method that returns the car’s name.

Finally, modify the main application so that it sets the car’s name via the constructor, then prints the cars name (retrieving it using getName()).

Show the car class … »

public class Car {

	// Note: the 'private' keyword here isn't
	// strictly necessary.
	private String name;

	public Car(String newName) {
		name = newName;
	}

	public String getName() {
		return name;
	}

	public void start() {
		System.out.println("Car started!");
	}

}

Show the main program class … »

public class Application {

	public static void main(String[] args) {

		Car car = new Car("My Car");

		System.out.println(car.getName());
		car.start();

	}

}

While Loops

Write an application that asks the user to enter the number ’5′ and loops over and over until ’5′ is entered.

When 5 is finally entered, print “Got it!”.

Hint: the application structure will look like this:

declare a variable and set it equal to 0 (for example)
create a new Scanner object so that we can use it to get input later on.
begin a while loop which loops until the variable is set equal to 5.
ask the user to enter ’5′
store the user input in your variable
end the while loop
print “Got it!”

The program will crash if you enter something that isn’t a number, but don’t worry about that.

Show answer …. »

import java.util.Scanner;

public class Application {

	public static void main(String[] args) {

		int value = 0;
		Scanner scanner = new Scanner(System.in);

		while(value != 5) {
			System.out.println("Enter the number 5: ");
			value = scanner.nextInt();
		}

		System.out.println("Got it!");

	}

}

Improved Use of Scanner

The above program crashes if a user enters something other than a number. The problem is that we use the nextInt() method of Scanner, assuming blindly that we will get an integer.

Modify the program so that no input can crash it.

Hint: you will need the hasNextInt() method of Scanner. You will also need the nextLine() method so that you can get a line from the user if they don’t enter an integer.

You’ll need to use an if…else statement that detects if the user enters an int, and simply gets a line (rather than an int) if he doesn’t. Don’t be afraid to consult the API docs for the Scanner class; type “java api scanner” into Google (without the quotes).

Show answer …. »

import java.util.Scanner;

public class Application {

	public static void main(String[] args) {

		int value = 0;
		Scanner scanner = new Scanner(System.in);

		while(value != 5) {
			System.out.println("Enter the number 5: ");

			if(scanner.hasNextInt()) {
				value = scanner.nextInt();
			}
			else {
				scanner.nextLine();
			}
		}

		System.out.println("Got it!");

	}

}

Switch Statements

Write a program that asks the user to enter an integer. If the user enters ’1′, print “Got 1″. If the user enters ’66′, print “Got 66″. If the user enters something other than these two numbers, print “Got something else”.

The program should use a switch statement.

Hint: you may need to look up switch statements on Google. Use the default clause to implement the case where the user doesn’t enter ’1′ or ’66′.

Show answer …. »

import java.util.Scanner;

public class Application {

	public static void main(String[] args) {

		Scanner scanner = new Scanner(System.in);

		System.out.println("Enter an integer: ");
		int value = scanner.nextInt();

		switch(value) {
		case 1:
			System.out.println("Got 1");
			break;
		case 66:
			System.out.println("Got 66");
			break;

		default:
			System.out.println("Got something else.");
		}

	}

}

Do…While Loops

A while loop checks its condition before the first iteration of the loop. A do…while loop checks the condition at the end of the loop. This means there’s always at least one iteration of the loop.

Write a program that asks the user to enter an integer, then gets the input from the user. The program should repeatedly ask the user to enter an integer until the user enters an integer greater than 10; then it should print “Integer greater than 10 detected!” and should end.

The program must contain only two print statements!

Hint: use a do…while loop to enclose the ‘prompt’ (i.e. the text that asks the user to enter the integer) and the bit that gets the user input.

Show answer …. »

import java.util.Scanner;

public class Application {

	public static void main(String[] args) {

		Scanner scanner = new Scanner(System.in);

		int value = 0;

		do
		{
			System.out.println("Enter an integer: ");

			value = scanner.nextInt();
		}
		while(value <= 10);

		System.out.println("Integer greater than 10 detected!");

Success?

Once you can do this lot from memory, you've mastered the absolute basics of Java. It gets easier from hereonin .... !!!