System.out.println("Hello world!");
//this prints out Hello world!
Let’s expand this further into…
public class helloworld
{
public static void main(String[] args)
{
System.out.println("Hello world!");
}
}
Alright, now that’s a huge program for a mere ‘hello world’! Why is that so?
Java is verbose when compared to Python, C, JS, etc.
Compare our previous code with…
print("Hello world!")
and
#include <stdio.h>
int main (void)
{
printf("Hello world!");
}
Short and simple do not belong in the books of this language. This makes it a slightly difficult choice for any beginner programmer, but an excellent choice for enterprises [they NEED that verbosity as it makes clear a lot of stuff!].
Let’s go back to our code:
public class helloworld
{
public static void main(String[] args)
{
System.out.println("Hello world!");
}
}
Just like C and C++, you use a main function. But, unlike them, you also have to define a class, whose name is the same as the file name. Note that our file name, in this example, would be
helloworld.java.
helloworld contains a main function [or method,s which we’ll be using from now onwards] within which a print statement lies.
When we execute the program, we find another file next to our .java file with .class extension. Here lies the helloworld class; you may see that the code is the same in both of them.
The print statement contains the parameter [anything that goes in the round brackets] ‘Hello world!’ and returns it as output when we print it.
Remember, System.out.println() is for printing something, then skipping a line after it. Use System.out.print() to print without newline.
Java uses a tool Scanner to read input from the system [as in, reading input typed from keyboard and such]. To use it, we’ve to import it [one of this language’s quirks is that we import A LOT].
import java.util.Scanner;
public class testing
{
public static void main(String[] args)
{
Scanner scanner = new Scanner(System.in);
}
}
We’ve imported the scanner utility [the scanner tool] into our program. Now we’ve to actually use it in the program.
We create an object named scanner of Scanner class in the main method. Every time we import an utility in Java, we’ve to create an object for its class [Scanner utility is of Scanner class, etc.].
This object produces values scanned from an input stream [user-input], whose source is from our keyboard [i.e. standard input].
Now let’s make some changes to our code…
import java.util.Scanner;
public class testing
{
public static void main(String[] args)
{
Scanner scanner = new Scanner(System.in);
System.out.println("Enter a number: ");
int num = scanner.nextInt();
System.out.println("Your number is: " + num);
}
}
We ask the user for input. Then, we create an object [basically a variable here] of type Integer and assign the scanner to it. After that, we print out our message and concatenate [join] our objects to it.
scanner.nextInt() is the longer version of Integer.valueOf(scanner.nextLine()). The scanner produces a String value, which we don’t need here. Instead, we convert the input from scanner into integer either directly [nextInt()] or indirectly [Integer.valueOf(scanner.nextLine())].
scanner.nextLine() to the String object.