Java Basics In One Page

I realized I cannot really cover every part of Java by writing blog posts. I mean I can but there’s no point reinventing the wheel when so many tutorials are already available. What I can do though is write a short guide which I may use in the future to revisit Java basics. So here we go!

#1. Get the tools

To start with Java, you’ll need JDK installed. Get it from here. Once you install JDK in your computer. Check whether the following commands are valid in your terminal.

javac
java

They should be valid and shouldn’t return an error! Once you got that, just install a good text editor like Sublime Text (I recommend Sublime).

#2. Print Hello World

Next, just copy paste the following code in a new file inside Sublime text and save it as hello.java on your desktop!

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

Now open your terminal, navigate to desktop (usually cd desktop) and then run the following commands:

javac hello.java
java hello

First command should simply compile the code and create an object file like hello.class in the same directory. The next command will actually execute the class file which was created. This will print Hello World in the terminal.

#3. Variables And Basic Maths

Now create integer variables and so some basic maths using Arithmetic operators:

class hello {
	public static void main (String args[]) {
		int x = 5;
		int y = 10;
		System.out.println(x+y);
	}
}

This should create two variables (imagine boxes) containing 5 and 10. And then we simply output them on screen using the arithmetic operator plus (+). Try other operators for subtraction, multiplication, division and modulus (%). Modulus returns the remainder from the division.

#4. Strings

Like integers, you can also create string variables:

class hello {
	public static void main (String args[]) {
		String xyz = "Omkar";
		System.out.println(xyz);
	}
}

This will simply print Omkar on screen!

#5. Concatenation

You can print them both together using the concatenation operator (+ operator).

class hello {
	public static void main (String args[]) {
		String xyz = "My number is: ";
		int num = 5;
		System.out.println(xyz + num);
	}
}

This will simply print “My number is: 5” on screen by concatenating the two variables before printing them.

#6. If Statement

Decision making is an important part in coding. You can do this using If statement. It’ll check whether a condition is true and then execute a certain piece of code:

class hello {
	public static void main (String args[]) {
		int x = 5;
 
		if (x > 10) {
			System.out.println("Batman");
		}
	}
}

This will check if 5 is greater than 10 and return true or false. Since this condition returns false, Batman will not be printed.

#7. If Else

What if we want to print something when condition turns out to be false? We use Else block in that case:

class hello {
	public static void main (String args[]) {
		int x = 5;
 
		if (x > 10) {
			System.out.println("Batman");
		}
		else {
			System.out.println("Superman");
		}
	}
}

#8. Else If

We can have multiple condition checks using else if:

class hello {
	public static void main (String args[]) {
		int x = 5;
 
		if (x > 10) {
			System.out.println("Batman");
		}
		else if (x == 5) {
			System.out.println("I'm a Number");
		}
		else {
			System.out.println("Superman");
		}
	}
}

This should print “I’m a Number” as x equals to 5.

#9. Switch

Switch is another decision making statement. Here’s how you can use it to choose or switch the flow using different conditions:

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

		switch(x) {
			case 1: System.out.println("1");
					break;
			case 2: System.out.println("2");
					break;
			case 3: System.out.println("3");
					break;
			default: System.out.println("nothing");
		}
	}
}

This should print 3 on screen as x is 3. If nothing returns true, default case is executed.

#10. Array

My previous post was about arrays but I’ll go ahead and add the code here again to keep everything on a single page –

class hello {
	public static void main (String args[]) {
		int[] orray = {1,2,3,4,5,6};

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

Note: there are multiple ways to create an array. I’ve just shared one of them in the example above!

#11. Loops

I’ve written a separate post about loops already! You can check it out by clicking here. I had missed the for each loop in that post, so let’s cover that here:

class hello {
	public static void main (String args[]) {
		int[] orray = {1,2,3,4,5,6};

		for(int i:orray){  
     		System.out.println(i);  
   		}  
	}
}

For each loop basically does the same thing as a for loop but is very useful for traversing an array!

#12. End

That’s all in this post. There’s so much more which I can cover but I’ll stop here for now. Maybe I’ll add more to this post in the future but I want to keep this simple so I’ll just leave it for now 🙂