Develop Reference

How I run individual java file in IDEA?

I am a java beginner, the first java IDE I downloaded was Visual Studio Code, it was very easy to use and everything is auto configured. But it kind overheats my laptop all the time, so I want to try IDEA, so far it's a very good experience, except when I open a java file and tried to run it in IDEA, it always pops out this run configuration window and I don't understand how to configure it. In visual Studio Code I can open any java file any time and run without any issues, but now I have to go through creating projects every time. Is there any solution for this?

From how the file icon looks:
your file is not recognized as the part of the sources of your project. Check the project settings to ensure that source directories are correctly set.
I'd also recommend you to look up and follow the conventions for the directory structure of java projects.
Once you've fixed the problem with sources, you'll see "run" icon next to your class, main method, or when you're right clicking the file.

Command-line
To run a single file, there is no need for an IDE.
In Java 11 and later, the java tool at the command-line can both compile and execute a single-file Java class. See JEP 330: Launch Single-File Source-Code Programs.
If your class named HelloWorld were in a file named HelloWorld.java, on a console type:
java HelloWorld.java
To be clear: The java command-line tool really only executes Java apps, while the javac command-line tool compiles Java source code. As a convenience, the java tool was enhanced to effectively call javac on your behalf for a single-file.
JShell
If you just want to run a few lines of Java, try JShell, the REPL tool bundled with Java 9 and later.
See:
Java Shell User’s Guide by Oracle
JEP 222: jshell: The Java Shell (Read-Eval-Print Loop)
Search to learn more and find tutorials.
BlueJ
Using an IDE such as IntelliJ, NetBeans, or Eclipse can be a daunting task for the new student of Java. Those IDEs are heavy-duty tools designed for professional programmers.
I recommend using an IDE designed for beginners. BlueJ comes to mind, designed specifically for educational purposes. BlueJ makes getting started with Java easier.
If you insist on using IntelliJ, read on.
If using IntelliJ, define a project
IntelliJ is not designed to work with single files. IntelliJ expects you to work within a project.
I strongly recommend learning the basics of Maven to create and drive your new project. By defining your project in Maven, the configuration is independent of any one IDE. You can move your project between major IDEs such as IntelliJ, NetBeans, and Eclipse.
Maven is also very useful for downloading needed libraries ("dependencies") that you may want to leverage in your work. And Maven is good at packaging your Java app as a JAR (or WAR or EAR).
In IntelliJ, choose "New Project". In the New Project window, click the Maven item on left. Check the Create from archetype box. Scroll the list to find item for org.apache.maven.archetypes:maven-archetype-quickstart. Under that, choose the "RELEASE" item. Click Next button.
In Name field, enter something like MyFirstProject. Click Next button.
On the Maven settings page, just click Finish.
Wait a moment for IntelliJ to download some stuff and configure your project. Eventually you should see a BUILD SUCCESS message in the Run pane.
You will also see a pom.xml file displayed. The POM contains your settings for Maven to run your project, in XML format.
Change the <maven.compiler.source> and <maven.compiler.target> elements to the version of Java you are using. The current version is Java 17.
After editing the pom.xml, look for a little floating windoid with a tiny Maven icon. Click the icon to have Maven process your changed POM. Wait a moment.
In the Project pane, navigate to the App file. There you see code to print “Hello World!”. Let's run that code now. Click the green triangle button on the left, in the gutter, next to the main method line. A pop-up menu appears offering a Run item. Choose that item to run the app immediately.
Down in the Run pane, you should see the results, the Hello World! text.
At this point you can add your single file to the org.example package seen in the Project pane.
By the way, you can change that package name by context-clicking and choosing Refactor > Rename….
Later, learn to use the Run/debug configurations feature of IntelliJ.
Know that you need not create a new project for each time you want to do a little experiment. Create one project for such experiments. Keep adding new .java class files for each experiment. Delete old class files you no longer need.
Eventually, I suggest updating the versions of various items in your POM. The QuickStart archetype is not configured for the latest versions (for reasons I cannot fathom).
And when you learn about unit testing, replace JUnit 4 in the POM with JUnit Jupiter (Aggregator) to use JUnit 5. One of the benefits of using Maven is that you can easily switch out dependencies such as going from JUnit 4 to JUnit 5.

The IDE needs to know what's called the entry point of the program, i.e. where to start running your code. That's what the "Edit Configuration" window is wanting you to do.
If your file "Lab3.java" is in a package, make sure to fully specify that in the field you have in red. Otherwise without knowing how your project is structured (as the other answer alludes to), it's difficult to pinpoint what we're missing here.

When you create your IntelliJ project, add a directory /src right at the root of your project. Right click on that folder and tell IntelliJ that you wish to mark it as a source root. The directory should turn blue in color.
Put your packages under /src. IntelliJ will know that those are Java files.
When you want to run a class with a main method, choose Run->Edit Configurations. Tell IntelliJ that you want to add an Application. It should prompt you with the classes that have main methods in them. You'll have no trouble running them.

Use Maven or Graddle. Make sure the project is configured with the build tool enabled and integrated, it will do basic things automatically. If you are not sure, please create a new project and add your files in. Steps:
Open the IDE
New Project
Choose from the left side bar "Maven" or "Graddle"
Give it a name and the location in your machine.
Click Finish
Now you have the project ready. You need the appropriate method to run in java. A main class. In IntelliJ you can just type "main" and the auto-complete will add it for you, make sure you inside the curly brackets of the class {}. More info about the main class. You seem to have this nailed down.
Lastly make sure you have a JDK installed in the IDE. I am pretty sure this is your issue here, make sure to use one of the option IntelliJ provides. A full guide from the developers is here and should satisfy your needs. I would suggest OpenJDK for a beginner, because that served me well at the beginning, at the end of the day its your choice.

How to make Netbeans use specific JVM parameters when running tests?

I'm using Netbeans with Maven and TestNG. When running tests with the maven surefire plugin, I can setup some configuration parameters, in particular the logging level used for my tests (trace):
-Dorg.slf4j.simpleLogger.defaultLogLevel=trace
-Dorg.slf4j.simpleLogger.logFile=System.out
However, when running a specific test file (CTRL + F6) or test method ("run focused test method"), Netbeans does not use surefire (which is good) and therefore ignores those parameters.
Is there a way to change the JVM parameters used by Netbeans when it runs tests that way?
This is somewhat similar to this other post but my question is specific to Netbeans.

From the documentation of Netbeans 7.2 (see Netbeans 7.2 changes, section Maven) :
... Now Test File always runs Maven by default, just like Test Project ...
What version of Netbeans are you using? Probably you should just upgrade to 7.2.

In the Project Properties, you can create profiles under Run. In theses profiles you can customize VM Options. You can add your parameters here, create a Test config and Run config.

Set fork property in the surefire plugin configuration in Maven. This will start a new JVM. Now, the second part is how to read the JVM parameters that you want into the new JVM. Depending on what you want to do, you might need to be read them from the environment.

In my case i went to project/properties, then "Actions" category. There you'll find "Test file" and "Debug test" actions. Select them and place whatever properties you need in "Set properties" box.
Not sure if this is only applicable for Maven projects...

Automate Runs in Eclipse

I have a main class that takes a series of arguments and I have 10 run configurations. Is there a way to have eclipse run them one after another?

The other answers are probably better solutions to your problem. However, if you install the Eclipse CDT into your Eclipse installation (using update manager or market place client), then you get an additional launch configuration type called Launch group.
Those launch groups allow creating a list of other launch configurations to be run one after the other. Make sure to set the Post build action in the dialog to "Wait until terminates" for each included launch configuration.

#Steven: To do it quick, you can write a JUnit Test case that just calls the intended classes in desired order and execute it. Eclipse already has the necessary jar for JUnit, so you are ready to go. Definitely, writing ANT/MAVEN script is a good practice.

As #gotuskar suggested, write test cases for your class. If you can afford running your ten configurations each time you build your project put them in its src/test/java directory, otherwise create a sibling project to your original one, make it depend on it, and put your tests there.

Creating a program that will run before and after compilation in Eclipse

I am currently working on a small program that should comment out some code used for testing.
I want it to auto run before the compiler while compiling the release version and another program that will comment the code back in after compilation was over.
The program works the only thing I am missing is to add it to the build process.
Thanks to all helpers!

In Eclipse, right-click a project, choose Properties → Builders and click New. You can add an Ant script or a command line that Eclipse will trigger when building the project. You can also control the order of builders in the same dialog.
However, I agree with JB Nizet – there are many advantages to having Ant or Maven build your project.

Don't use Eclipse to build the release version of your app. Use Ant, Maven, or any other build tool that is much more flexible than Eclipse, doesn't need a GUI, can be scripted and used by a continuous integration server.
All of these tools should easily be used to include your pre-compilation and post-compilation tasks in the build process.
That said, you could just use a public static final boolean constant FOR_TEST, and include all your testing code in
if (TestUtil.FOR_TEST) {
}
You would then have just a single place to change in the code to have all the test code removed from the compiled version. No need for a complex Java program to do that.
Or you could let all the testing code in the released version, and activate it when testing using a system property, for example. This wouldn't even need any precompilation/postcompilation phase, and would probably have a negligible cost.

How do I run JUnit from NetBeans?

I've been trying to understand how to start writing and running JUnit tests.
When I'm reading this article:
http://junit.sourceforge.net/doc/testinfected/testing.htm
I get the the middle of the page and they write, "JUnit comes with a graphical interface to run tests. Type the name of your test class in the field at the top of the window. Press the Run button."
I don't know how to launch this program. I don't even know which package it is in, or how you run a library class from an IDE.
Being stuck, I tried this NetBeans tutorial:
http://www.netbeans.org/kb/docs/java/junit-intro.html
It seemed to be going OK, but then I noticed that the menu options for this tutorial for testing a Java Class Library are different from those for a regular Java application, or for a Java Web App. So the instructions in this tutorial don't apply generally.
I'm using NetBeans 6.7, and I've imported JUnit 4.5 into the libraries folder. What would be the normal way to run JUnit, after having written the tests?
The JUnit FAQ describes the process from the Console, and I'm willing to do that if that is what is typical, but given all that I can do inside netbeans, it seems hard to believe that there isn't an easier way.
Thanks much.
EDIT: If I right-click on the project and select "Test" the output is:
init:
deps-jar:
compile:
compile-test:
test-report:
test:
BUILD SUCCESSFUL (total time: 0 seconds)
This doesn't strike me as the desired output of a test, especially since this doesn't change whether the test condition is true or not.
Any ideas?

One way is to right click on your project in the Projects pane and select "Tests". That will run the JUnit tests. You can also right click on the test file and select "Run Test" and that single file will be ran. The keyboard shortcuts depends on how you have your keymapping set, but you'll see them in the context menus.
You can also have NetBeans autogenerate tests for you by right clicking your source file and then "Tools > Create JUnit Tests".

Re-importing does not appear to be necessary. I had the same issue (imported project, right clicking did not bring up any JUnit test options). I took these steps, which resolved it, using NetBeans 6.8:
Add a folder called "tests" to your project.
Right-click your project and select Properties.
Select Sources.
Under Test Package Folders, click the Add Folder button, and select the "tests" folder.
Right clicking a file + Tools > Create JUnit Tests.
Once a test is created, right-clicking a file + Test File runs the test.

All the above answers are correct, but if you are using in mac little change needed.
Step 1 Write your junit class.
Step 2 Right click on the class ->Tools-> Create/Updates Tests-> Select framework as Junit-> click ok.
Step 3 Right click on the file again ->Test File.
Now it will run as junit, will show the Test Result.

Even though I've accepted an answer, I thought I should mention my difficulty, as someone else may encounter it.
When importing a project from existing sources into NetBeans, if you do not specify a folder for test packages, then NetBeans will not offer the JUnit options on the tools menu.
The only solution I found was to re-import the project. While primitive, it worked.

I had the same issue after imported a eclipse project into NetBeans.
To resolve it, I followed the above steps outlined by alangalloway, but instead of
creating a new folder, I just pointed to the imported test folder.
Maybe in future release, NetBeans can automatically recognize imported test cases.
Thanks.

Had a similar issue. In Netbeans 7.0.1, what worked for me was to locate the project.xml file (i.e. {project}/nbproject/project.xml) and change:
<test-roots/>
to:
<test-roots>
<root id="src.dir"/>
</test-roots>
(in my case the test files are in the same dir as the source dir)