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.

Using Git with Eclipse causing lots of errors

I made a HelloWorld app in Eclipse that had no errors and runs fine. It was located in
Android/workspace/HelloWorld
Then I created a Git repo elsewhere in, say
Git/MyApp
I want to have MyApp contain the HelloWorld project from eclipse, and when I make edits to HelloWorld, it will show up in git status and I can commit and push those changes.
I tried copying the HelloWorld directory from the workspace into MyApp, then pushing my changes to my repo, which was fine.
Then I deleted the original HelloWorld in the workspace, went to Eclipse, hit import existing project, and selected the HelloWorld from the MyApp repo. But this resulted in a ton of compilation errors. How can I do what I'm trying to do without all these errors?
I've attached a link to a screenshot of the errors I get upon compilation.

I have Understood your problem,your project have a relationship with appcompact.Whenever you import your project you have to check below scenario,
Right click your project ---->select properties--->In side bar select "Android" Tab--->now you can see Project build target and Library Box--->in Library box click add and choose appcompact project and then click apply.
After done above one clean and build your project.Then it will work fine.

try Clean operation on your project and then build your project. replay whether it solves the issue or not.

As a matter of fact it looks to me like your eclipse doesnt recognize your app-compat anymore.
This happend to me once too, after I closed and reopend the appcompat-project.
Try to close all your projects -> open the appcompat first -> restart eclipse and maybe do a clean on your project
Hope this helps.

Java project from existing ant build file doesn't appear as in version control

I have a repository in which I have a simple ant project (only the src directory and the build.xml file). I did a checkout of this project in a local copy. Then I opened Eclipse, created a "Java Project from existing Ant build file" and it worked fine. I can build, clean, run, generate javadocs, etc. However, my problem is that the source files in the packages, as well as the project itself, doesn't appear to be under version control in Eclipse. I can work with it fine from Cygwin, but not from the GUI.
The odd thing is that I have several regular Java projects, on the same repository which are synced fine by Eclipse, so it's not a version issue.
When I created the projects that work, I had to go to Team->Share Project... and Eclipse told me that they were already under version control and updated its configuration. However, in this current case, this isn't working. Any suggestion please?
EDIT:
I'm using svn 1.8.
UPDATE: I tried creating a regular Java project, and this time the version control looks fine, so it appears it's an issue related only to Java Project from Existing Ant build file.

The problem is that Eclipse doesn't know your project is using Subversion as a version control tool. You did a checkout outside of Eclipse, then you set up Eclipse as if this was a non-version controlled project.
To get Eclipse to recognize the project as under Subversion, you need to let Eclipse know. This means you should have said "File->New->Project, and select SVN->Checkout Project from SVN in the new project wizard. Then, Eclipse would have checked the project out in Subversion for you, and Eclipse would understand this project is in Subversion.
Unfortunately, I am not quite familiar enough with Eclipse to know how to tell Eclipse that the current project is really under version control. The best I could come up with was:
Right-click on the project and select Team->Share
Say this is a Subversion project you want to share.
Select the Repository to use
Select the folder where you want this project, and select the current folder.
You get the following warning:
Warning: The specified folder already exists in the repository. If you continue, that folder will be checked out to the existing location. Do you want to continue?
Your project will now be "checked out" over your current project. Everything should be okay because the files you have either match what is in Subversion or are modified versions of what is in Subversion.
You will be then asked to use the Synchronized view. Check "Yes", and then switch back to the Java view. Eclipse will now know your project is under Subversion and everything should work fine.
Note the use of Shoulds and Coulds. This is my way of saying that the advise given is done on an "as is" basis and I am not responsible for any damage done by following this advice, so please don't sue me.
It's basically your standard open source license.

Can you force eclipse to run/debug another project when a particular one is ran/debugged?

I'm working with Ligdx, an API for cross plattaform game development that by default generates at least 2 projects:
The core project: This is the one you are supposed to work on, where all the classes and logic should be placed.
The desktop project: This one is automatically generated from the core project.
The core doesn't have any class to start it because it's supposed to be started from the desktop (or other platforms) project.
It is also always selected -because its what you are working on- so pressing the "run" or "debug" buttons will attempt to run the core project and not the desktop one and immediately fail thus forcing you to select the desktop project before hitting run/debug. This can get extremely annoying, especially when debugging.
So is there a way to force Eclipse to always run the desktop project?

Rekaszeru's answer to this question shows how to set up Eclipse in order to always launch the previously launched application.
You simply have to go to preferences / run-debug / launching and check the "Always launch previously launched application" box as he shown in the picture he used to illustrate his answer:
This will not force one of the projects to always be launched but it would pretty much solve the problem unless you are working on multiple projects at the same time or you alternate between Web/Android/Desktop.

If Your asking how to start a Core project from already existing project present in the eclipse then all you need to do is Right Click on desktop folder of the project and go over Run As and select Java Application
But if that is not what you have posted for I can only understand that you need to understand how a LibGDX project is deployed.... From Desktop project "In xyzMain.java" file you will have a method call to the core project saying:
new LwjglApplication(new Sample(), cfg);
This is where you'r Sample Class from the core project is called over.
But from the title I get a diff feeling that you wish to add another project to your already existing project...
You can do that by changing the build Path of the project....by including Other proj to the one you wish to compile and run.
Hope this helped you out..

I´m not aware of doing exactly what you have described, but I´d suggest using the Launch Configurations menus and dialgos to do what you want in a similar quick way.
Note that you can configure your launch configurations doing: Run -> Run Configurations... (also Run -> Debug configurations...)
Option 1).
Once you have run your Desktop Project, simply use the Run Drop-down button, which contain your launch configurations:
Option 2).
Save a launch configuration file in your Core project (or wherever you want) using the last "common" tab of the launch configuration dialog:
Then you can simply right-click on your that file to launch your Desktop Project:
P.S: I´d also suggest reading a little bit about run configurations in lars tutorial

Using multiple projects in Eclipse with Play framework 2.1.x

I have an existing project in Eclipse (let's call it "NotPlayProj") which has a lot of java code still under development. I made a new eclipse project using play 2.1.0 (let's call it "PlayProj"). My goal is to use code from NotPlayProj in PlayProj and have both Eclipse and the Play compiler notice changes in either project.
If I go into the properties for PlayProj and add NotPlayProj via the Project tab, then method completion and inclusion works within eclipse, but the Play compile doesn't see the result. I've looked at Play modules and those don't seem to do what I want.
Is there any way to do this, ideally without modifying the NotPlayProj?
Edit ---
I've looked at http://www.playframework.com/documentation/2.0/SBTDependencies which shows how to export a jar from NotPlayProj into the PlayProj/lib directory, but this requires a manual export for each time NotPlayProject changes. I suspect that the Managed dependency section is supposed to cover this, but I've never used SBT before and am therefore probably missing something basic.

What you need is continuous integration.
Have a look at Jenkins: http://jenkins-ci.org/
You should setup a Continuous integration server and customize the builds you need.
Example:
You have your PlayProj running in some server, it needs to be able to use some of the latest classes from the other project called NoPlayProj.
Rebuild is a must, things such as downtime zero are difficult to achieve(At least I don't think this is what you are asking for either).
The steps you need to automate with Jenkins are:
1 - Build and deploy the latest version of NoPlayProj which is located in some repositorium
2 - Build and deploy the latest version of PlayProj which is located in some repositorium and also is contains your last commit where you updated the dependency that exist with NoPlayProj
A not very complex build and deployment instructions can be configured in Jenkins. This should speed you up a bit.
Also another suggestion would be to mavenize both projects if possible, this will help you manage the dependencies easier.
Just to clarify one thing, you said: My goal is to use code from NotPlayProj in PlayProj and have both Eclipse and the Play compiler notice changes in either project.
Well the order in which you execute the builds will be dependent in what you want to do as long as you update the dependency before you commit the code.
One last thing, if you don't want to deploy you don't have to do so you can create the Jenkins jobs, in such ways that you only build. With Jenkins you can do a lot of stuff, also you could execute some help scripts of your own that can provide you additional functionality.
I hope this was useful.

To let Eclipse see changes in NotPlayProj when working with PlayProj, it's enough to change configuration of PlayProj. Properties-> Java build path -> Projects -> Add NotPlayProj as dependency.
There is no straightforward way to let Play compiler handle dependencies, until you package it as jar. Consider configuration of simple ant task (External tools configuration -> Ant build ), which will copy your jar file. Task can be triggered by pressing the key or button.
With managed dependencies, every time you made change in NotPlayProj, you have to manually rebuild it. To let Ivy/Maven put dependency in your local repository. After that Play will take latest snapshot from your local repository.
Both approaches requires some efforts. Maybe you can take a look at Python scripts, which run Play, maybe it's enough to extend classpath with NotPlayProj when executing play start

Though I've never used the play framework, I would think that there is a format that both the play framework and eclipse understand and that is Maven. Look at http://www.playframework.com/modules/maven-head/home