Note that the line of code System.out.println("Hello, World."); ends with a semicolon, like the very first line. This means that this is another statement. We are commanding the computer to print out a message that says “Hello, World.” Also, note how everything is indented – the class is on the outside, the main method is indented one tab space, and the contents of the main method are indented two tab spaces. You don’t technically have to indent like this for the code to work, but it is a highly recommended practice as it makes the hierarchy of code groups easy to determine.

So what’s going on with the code? The first thing to look at is the System.out object. We’re already using objects! As was explained before – objects are essentially just things in Java that can do more things for us. In this case, the System.out object represents the standard output stream – where consoles applications send their text. So we are accessing the output stream and calling its println method. The println method is basically a function of the output stream which allows us to print a line of text to the console. println stands for print line. It takes a string, or sequence of text characters, encapsulated by quotation marks as its argument. An argument is a piece of information we pass on to a method.

So in summation, in this statement we are calling the println method of the standard output stream and passing it an argument with the message “Hello, World.” as a string of text.

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Println With No Argument

What happens if you use the println method, but don’t give it a String as an argument? What if we just leave the argument blank?

System.out.println();

If we use the println method with no argument it will simply print out a blank line, as is shown below:

System.out.println("Hello,"); //First line
System.out.println(); //Create an empty line
System.out.println("World."); //Third line

Hello
      
World

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The Debugger & Troubleshooting Issues

Common Issues

Here are some of the most common errors you might run across:

• Misspelled Words
  • If you type a method name wrong, like System.out.prntln() instead of System.out.println() your program will not work.
• Forgetting Semicolon
  • All statements must end with semicolons. If you forget one or more your program will not work.
• Forgetting Keywords
  • Keywords like class, public, static, etc. are essential to the function of a program. If you forget to type the static keyword in the main method your program will not run.
• Failing to use curly braces properly
  • All classes and methods must begin and end with an opening { and a closing } curly brace. Forgetting one or accidentally adding one where it does not belong will make your program fail to run.
• Logic Errors
  • These are harder to solve because they might not be caught by the debugger. Things like using the wrong formula to calculate something, or not doing things in the proper order could be considered logic errors. Your program may behave unexpectedly because you did not account for something properly in the logic when you designed it.
  • Even experienced programmers have logic errors sometimes. It can take a lot of hunting in large projects to figure out exactly what’s going wrong.

If you have tried all of the above steps and you still cannot get your program to run, try copy pasting the entire program. Make sure your class file is named Main in the project explorer, just like the example image. The package declaration on line 1 is not really necessary in this example, so you may remove that. If nothing works, you may try to download the entire project and import it into IntelliJ. Finally, there may be a more technical problem with your installation of the JDK or JRE. Visit Oracle and try to download and reinstall the JDK. You may sign into Kevin's Guides and post a comment using the form at the bottom of this page for any additional issues.

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The API

So how did I know to use System.out.println() in order to print this message to the console in the first place? Well I memorized it. In time, you will memorize a lot of commonly used methods from the API. However, if you need to look something up everything is explained in the Java API Specification. This documentation provides details about what each pre-built object in the API is used for and what each of their methods do. If you forget a particular method name or don’t know how to do something – Google is your friend. Understanding how to look at the API and read documentation to figure things out is an essential skill for programmers.

In addition to println, there are several other methods we can use to print text to the output stream. They are the printf() and print() methods. Let’s take a look at the differences between them.

First off, println prints out a line of text. The sample code below contains two println statements.

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

The output of that code in the terminal is as follows:

Hello,
World.

Note how the words “Hello” and “World” are on separate lines. That’s because the println method always starts printing on a new line.

Now let’s compare this to the print method, which does not add a new line.

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

The output of the code using print instead of println is “Hello, World.” all on one line as shown below:

Hello, World.

Finally, we have the printf method – which stands for printing a formatted string of text. We’ll get more into the details of this later, but for now understand that it behaves similarly to the print method.

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

Hello, World.

Note that if we use the print or printf methods with no arguments, the program will crash. We can only use the println() method with no arguments to print a line of code, as there's no point in printing nothing.