Compile and Run

There are two steps to running a Java program on the terminal:

1. Compile the program if you made any changes to the code since the last time you compiled it. This uses the command javac Filename.
2. Run the compiled program with the command java ClassName.

For practice, try using the following commands to compile and run the hello world program from the previous section: javac HelloWorld.java, then java HelloWorld.

A few notes about compiling and running Java programs this way:

• You need to be in the folder containing the source code (the .java file(s)).
• The javac command uses the program's file name, which ends in .java.
• The javac command will generate a .class file for each .java file it compiles. These .class files are your compiled program.
• The java command uses the class name, which does not end in .java.
• You can include additional arguments after the class name when using the java command. These arguments will end up in your main method's args parameter.

Example of Program Arguments

Try running the following program with different sets of command line arguments:

class ArgsProgram
{
    public static void main(String[] args)
    {
        System.out.println("Printing all program args:");
        for (int i = 0; i < args.length; ++i)
        {
            System.out.printf("\targs[%d] = %s\n", i, args[i]);
        }
    }
}

Try using each of the commands below (don't include the $, it just marks the start of a terminal prompt) and predicting what the output will be before you run the command. Are any of the results surprising? What can you learn about command line arguments from the results?

$ java ArgsProgram a b c
$ java ArgsProgram a,b,c
$ java ArgsProgram a, b, c
$ java ArgsProgram "a b c"
$ java ArgsProgram a\ b c
$ java ArgsProgram Hello, World
$ java ArgsProgram "Hello, World"

Compiler Errors and Runtime Errors

Our jshell scripts were interpreted by the jshell program. This means it looked at one line or code block at a time and tried to run that piece of code before looking at the next piece. If we typed some invalid code, then we would see an error when it tried to run that code and it would continue trying to run the rest of our script.

A Java program is compiled. This means the compiler checks the code to make sure it follows Java's syntax rules, then it converts the code into Java bytecode, which can be executed by the Java Virtual Machine. Certain types of errors will prevent our code from compiling, which means we can't even try to run the program. These are called compiler errors or syntax errors. If our code doesn't follow Java's syntax (grammar) rules, then it isn't valid Java code and will be rejected by the compiler.

Syntax errors can be annoying, especially as a beginner who makes this type of error frequently. However, these is the best kind of error to have in your program. A syntax error will be caught immediately when you compile your program, and some editors will warn you about syntax errors as you are editing your code, before you even try to compile. Your editor or compiler can tell you exactly where the syntax error is located, and the error message will often be helpful when you try to figure out the problem.

Runtime errors, on the other hand, cannot be caught by the compiler. A runtime error occurs while your program is running when the program tries to do something that it isn't allowed to do (such as index an array with a negative number or divide an integer by zero). A runtime error will immediately crash your program and show an error message. This type of error can be harder to diagnose, because it may only happen with certain inputs, and you won't get a warning from your editor or compiler about it. You'll usually have to spend a little bit of time investigating a runtime error before you'll know how to fix it.

Semantic Errors

It's also worth mentioning a third type of error, called a logic error or a semantic error. This type of error is usually the most difficult to fix, because it often won't crash your program and provide an error message to show you what went wrong. A logic error occurs when you make a mistake in your program's logic that causes it to run but produce incorrect output or otherwise behave incorrectly. Sometimes this can lead to an infinite loop, or maybe your program will print the wrong answer to a question five percent of the time. You can catch logic errors by manually testing your program or by writing automated tests, like the test scripts included with some of the projects.