Creating and running your program

You will need a text editor and additional tools to write and run Java application programs outside of this course. If your platform is Windows or UNIX, you can download the free Java SE Development Kit (JDK) as described in the next appendix. If you are a macOS user, Java is included with your operating system.

You might prefer to use an integrated development environment (IDE), which provides an editor, compiler, Java Virtual Machine, and other helpful tools for writing Java programs. The next appendix also provides links to these resources.

Writing the program

You can use any simple, plain-text editor or one supplied with an IDE to write a Java program. An IDE’s editor typically uses colors to highlight various components of the program. As you type your program, the characters are stored in the computer’s primary memory.

You risk losing your work if you do not save it in a file on your computer. Get in the habit of saving your work often; do not wait until you have finished typing the entire program. Note that Java expects the name of the file to be, where name is the name of the program.

Compiling the program

Once you have your program saved in a file, you will compile it. With an IDE, clicking a button will accomplish this step. With the JDK, however, you would type the command:


After compilation, the compiler saves the bytecode in a file named name.class.

Executing the program

You now can run the program. In an IDE, clicking a button will execute the bytecode. Frequently, one button both compiles and executes a program, but if compilation is unsuccessful, execution does not occur. Using the JDK, you run a program by typing the command:

java name

Notice that this command uses the name of the program, not the file named name.class.

Debugging your program

The lesson Assertions introduced assertions as a way to check a program’s logic during its development. Overview of Debugging Tools mentioned various debugging tools, such as breakpoints and single-stepping that a typical IDE provides. Here you will see how to accomplish these techniques using command-line tools in the Java Development Kit (JDK).

Enabling assertions

When you execute a Java program using the command:

java name

Any assert statements in the program are ignored by default. To enable them during execution, you must add -enableassertions to the previous command, as follows:

java -enableassertions name

The next time that you execute the program, assertions will be ignored, unless you explicitly enable them again.

Enabling command-line debugging tools

Before you can use the command-line debugging tools, you first must tell the compiler to generate some debugging information. You do this by providing the -g option to the javac command when you compile your program, as follows:

javac -g

Then, instead of using the command java to run the program, you use jdb.

For example, to compile and debug the program MyProgram, you would use the following sequence of commands:

javac -g
jdb MyProgram

The response will be

Initializing jdb ...

At the prompt >, you can tell the debugger to set breakpoints, trace execution, or exit the debugger. We describe each of these requests in the next sections.

Setting and using breakpoints

Setting breakpoints

To request a breakpoint in MyProgram at line 15, you type

stop at MyProgram:15

After the prompt >. The system response is

Deferring breakpoint MyProgram:1
It will be set after the class is loaded.

Stepping through execution

After requesting as many breakpoints as you like, you begin program execution by typing:


After the > prompt. When the breakpoint at line 15 is reached, you will receive a message and the statement at that line will be displayed. This statement will execute next when you use the command:


Each time you type step, the statement at the current line executes and the debugger advances to the next line.

Looking at the contents of variables

You can look at the current value of any variable by using the print command. For example, to see the current value of index, you would type:

print index

after the prompt >.

Continuing execution

You can continue single-stepping with the step command or you can resume execution until the next breakpoint by using the command


Removing a breakpoint

To remove a breakpoint, you use the clear command. For example, to clear the breakpoint we set previously at line 15, you would type

clear MyProgram:15

Tracing methods

After setting a breakpoint and beginning execution, you can trace when methods are entered and exited by giving the command:

trace methods

Now each time you type:


execution continues and then stops either at the beginning of a method or at its end. Tracing methods in this manner is just like setting breakpoints at the beginning and end of each method.

Ending debugging

To end a debugging session, you type the command:


📝 Note: jdb commands

The debugger jdb offers more commands than the ones we have introduced here. To see a list of these commands while you are within the debugger, you type the command


If you have not begun a debugging session, you would first type the command jdb, wait for the system response, and then type help. The result would appear, as follows:

Initializing jdb ...

📝 Note: Debugging in an IDE

Debugging actions like those we have presented here are available in a typical IDE, although the particulars of how to take those actions differ. If you are using an IDE, you should learn its debugging tools, as they will be more convenient and easier to use than the command-line tools. The approach to debugging, however, is the same regardless of the form the tools take.

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