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Eclipse Oxygen - The project was not built since its build path is incomplete

I just installed Eclipse Oxygen and tried to open an existing project into the workbench but I get this error:
The project was not built since its build path is incomplete. Cannot
find the class file for java.lang.Object. Fix the build path then try
building this project
I tried right clicking on the project - went to Properties - Java Build Path - Libraries - Add Library - JRE System Library and selected Workbench default JRE (jre1.8.0_60).
I then cleaned and rebuilt the project but it then causes more errors to come up like:
ActionBar cannot be resolved to a type
Activity cannot be resolved to a variable
AdapterView cannot be resolved to a type
ArrayAdapter cannot be resolved to a type

Here is what made the error disappear for me:
Close eclipse, open up a terminal window and run:
$ mvn clean eclipse:clean eclipse:eclipse

Are you using Maven? If so,
Right-click on the project, Build Path and go to Configure Build Path
Click the libraries tab. If Maven dependencies are not in the list, you need to add it.
Close the dialog.
To add it:
Right-click on the project, Maven → Disable Maven Nature
Right-click on the project, Configure → Convert to Maven Project.
And then clean
Edit 1:
If that doesn't resolve the issue try right-clicking on your project and select properties. Select Java Build Path → Library tab. Look for a JVM. If it's not there, click to add Library and add the default JVM. If VM is there, click edit and select the default JVM. Hopefully, that works.
Edit 2:
You can also try going into the folder where you have all your projects and delete the .metadata for eclipse (be aware that you'll have to re-import all the projects afterwards! Also all the environment settings you've set would also have to be redone). After it was deleted just import the project again, and hopefully, it works.

In Eclipse, Right click Project -> Maven -> Update Project. It fixed errors in my project.

I'm also using Eclipse Oxygen, migrated from Mars. I faced the same error. I deleted .metadata, .recommenders folders and added projects from archive and issue is solved. I also use Android Studio but i like Eclipse much more.

At my system the Java Runtime JAR file jrt-fs.jar was not found because it was in the wrong directory. This file should be located in the "lib" subfolder. If you installed Java at "C:\Temp\java" the file should be here at C:\Temp\java\lib\jrt-fs.jar .
These are the steps to make jrt-fs.jar available to Eclipse:
Window / Preferences -> the "Preferences" Window opens
Select in the left menu: Java / Installed JREs
Press the [Add...] Button -> A window opens
Select "Standard VM" - The window "JRE Definition" opens.
Enter your path information:
JRE home = "C:\Temp\java"
JRE name = "MyJRE"
Select [Add External JARs...] to link the jrt-fs.jar
JRE system libaries: "C:\Temp\java\lib\jrt-fs.jar"
It worked for me with OpenJDK12 and Eclipse 2019-09 (4.13.0).
Eclipse reported me this error:
The project was not built since its build path is incomplete. Cannot find the class file for java.lang.Object. Fix the build path then try building this project

I had faced this problem. I added the C:\Users\Office 2\android-sdks\platforms\android-19\android.jar to the Java Build Path's Library. And the project started to run again.
The problem occurred due to my carelessness. Yesterday night I was updating the Android SDK, and at the same time I tried to run the project, while the SDK Manager was uncompressing the system image. The Project couldn't start and the error occurred.
I tried all the methods given by others. But nothing worked. It solved when I added the android.jar to the library.
I guessed it that android library is missing. not the Java, when I saw the errors only at the android methods and imports. But when I read the posts, I got confused and wasted some time trying wrong way.

I had faced the same issue and tried different solutions, I have reinstalled my JDK 11 and restarted the machine, after that the issue gets resolved.

I got this out of the blue in a workspace that was working properly before. This problem seems to indicate that the project configuration somehow got corrupted.
Restarting Eclipse didn't help, but in the same vein as the answers dealing with Maven, regenerating the Eclipse project config with:
./gradle eclipse
or
./gradlew eclipse
if you use the wrapper, solved this problem for me.

I have similar issue when importing Spring Boot, jdk 11 (using
inside docker) project to Eclipse 2019 (2021), in Ubuntu, instead when
importing to Inteelij IDEA Community or NetBeans IDEs they do
not produce the same set of after-import errors as in Eclipse.
The Spring Boot is multimodule maven projects and two child
modules give such 2 "problems" in appropriate problems tab:
The project was not built since its build path is incomplete.
Cannot find the class file for java.lang.Object. Fix the build
path then try building this project
Next 2 errors are produced in two classes of aforementioned
maven modules:
The type java.lang.Object cannot be resolved. It is
indirectly referenced from required .class files
Manipulation with adding JDK 11 to submodules buildpath
just leads to the thousands of problems. Also it is
got when I use Eclipse-File-Maven-Update.
Moreover when I try to edit most of existing classes or add new ones
I got red underline that tells
Implicit super constructor Object() is undefined for default
constructor. Must define an explicit constructor
. So almost no opportunity to use Eclipse
even at prebuild stage as docker-compose should run build and
run. Indeed I noted that such issue is produced just when project
also uses modules (module-info files introduced in java9).
When I import project on jdk 8 (even not installed on
system), or even on jdk11 without module-info files all
is fine in Eclipse after import.
Another difference is that of when I use problem old no module
project the Build Path is composed with jre environment 8 and
maven dependency items, but when using
"module-info" project the Build path contains -module path,
and -classpath. So I even do not know where to add jre 11,
indeed adding does not resolve issue, just make more errors.
So what is the resolution of the case??? What settings to do
in Eclipse?

Spring boot project can not recognize after Windows blue screen Crash

I had a windows crash with blue screen as often happens. in this time my Intellij-Idea Data is gone and reset to default value and also my project data. so Intellij-Idea do not recognize my project as Spring-boot and I can not Run it.
I reimporetd the project, it is not working. clean, clear cache, Invadidate and restart and result was same. I check the project structure and see every dependency in module is gone although all dependency is still in my pom.xml ! so how I can fix it?

One thing you can try is to click the Reimport all maven projects button in the maven projects view , the circled arrows.
As a last possibility you can try - after closing IntelliJ - to delete from your project directory the *.iml file defining the IntelliJ project and the .idea folder. As this folder name starts with a dot, it's a hidden directory in some OS, so the best thing is to do this from a shell.
After that start IntelliJ and do a File/Open... and select the pom (or the directory containing the pom) to recreate the project.
Hope that helps.

I finally solve this problem by re Installing JDK. I find out that JDK is damaged by Windows blue screen cause the Intellij Idea even do not recognize Java classes of project like String.
after install last version of JDK every things worked fine.

Could not find or load main class in STS/Eclipse

I have a main method in a package in one of my projects. Say, the package is com.ant.car. I am trying to run and/or debug this main method, and I keep getting the error Could not find or load main class com.ant.car.
I've searched this problem, and it seems like I can't figure out what is wrong.
1) I've checked run configurations. In Run->Run Configurations, I've checked that the Main class is com.ant.car.
2) I've checked build path. If I right click on the project, I select Build->Build Path, and under the Libraries tab I make sure there are no missing folders with red Xs next to them.
Not really sure what else to do. Any suggestions?

Project -> Clean
this is working

Try updating project by right clicking on it. Maven -> Update Project

Today I ran into the same problem and I tried a lot of answers. Nothing helped. Cleaning the project, build automatically is already checked, deleting .metadata, etc.
Eventually I tried this and it worked perfectly:
menu Project -> Properties
Java Build Path, tab Libraries
Remove the JRE System Library from there, press Add Library, take the JRE System Library and press Next. Workspace default JRE and press Finish.

What worked for me:
Menu Project -> Properties
In Java Build Path, tab Libraries
Delete all libraries with a red [x] next to them.
In my case, problem happened when I switched from Kepler to STS IDE.

The solution to this was the following:
Close Eclipse/STS
Use a file explorer on your operating system to navigate to your workspace (In my case, I'm on Windows so I used Windows Explorer)
Delete the .metadata directory (or to be safe, copy the directory somewhere else to be safe, then delete it)
Restart Eclipse/STS
Is there a more improved answer than this? I don't want to look like I'm trying to boost my own reputation points, so if someone can provide a better answer then please do so.

Sometimes the problem can be caused by some newly added dependencies. Try removing the recently added jars. That's how I fixed my problem.

Project -> Clean... -> check project not working -> Clean
I already had build automatically set, but forcing STS to rebuild it fixed it.

This issue occurs when the main .class file moved or not found because you changed the directory for committed/shared the project into the git or another repository.
To Resolve this issue -->
Remove existing run configuration and new one.
Find the parent pom.xml or project pom.xml and open cmd/command prompt and run the below commands,
mvn clean install package
mvn eclipse eclipse

It worked for me:
1. Delete metadata from work-space directory.
2. Import the project again, but selected copy to work-space option.
I think the cause for the error was Non-English characters in the original saved directory.

You just need to delete Run Configuration file and restart STS and run command -mvn clean install and try to restart the service

The Error "Could not find or load main class com.ant.car" occur when your class is not in build path. please make sure that .class file is generated and it's in build path ..

I faced the same issue..
just do follow these steps:
STS/Eclipse --> Project --> Enable "Build Automatically"
then refresh your project, it will resolve your issue.
Still not refreshed your projects automatically, just restart your STS and check.
Hope it will help you.

I think .class files are deleted/missing from JavaProject/bin folder.
To resolve this issue ->
1) Just cut paste and save the code contents of all the files that you are using then .class files will be regenerated.
2) Then run the code and you can see it works fine if there is no syntactical errors.

1 ) Clean the Project
2 ) Enable build automatically Option
3 ) Update the maven project by use the short cut Alt + F5

Deleting target folder manually and mvn clean install worked for me

A quick and easy fix is to directly run your SpringBootApplication class (i.e. Right click, Run As -> Spring Boot App). This runs the app and creates a run configuration automatically.

I spent several hours on this issue, finally it is fixed by doing this:
Properties -> Java Compiler: uncheck the checkbox "use '--release' option"

This worked for me to solve the error. (I got this error after removing AWS ToolKit)
Close the Eclipse/ STS Eclipse.
Go to the WorkSpace folder.
Delete the .metadata folder.
Open the eclipse.
Run maven install on pom.
Run the project with your Run configuration.

Remove project from STS/Eclipse
Close or Refresh the Eclipse/ STS Eclipse.
Run maven install on pom.
Run the project with your Run configuration.
Tried above mentioned steps to resolved issue.

You can try one of the two solutions below:
Solution 1: Right click the project -> Gradle -> Refresh Gradle Project.
Solution 2: Add gradle or maven in your environment variable. My problem was that I had not added gradle in my environment variable. Before, make sure Gradle or maven is installed in your OS.

I tried all the answers but finally what worked for me was deleting the project from eclipse workspace and importing it again.

Intellij IDEA Project view doesn't show structure as it did before.

My directory structure in the Project seems to have disappeared. The first picture is the "bad". I recently upgraded to 12.1 and the vim emulator at about the same time. I don't know if I missed a dialog that reset something? I can't figure out how to get it back like the second picture.
I've tried to re-import the project but it has not helped. I keep the project on Dropbox and have another PC where I have not reloaded project. Is there a way to get my view from that machine?
The picture is the "good" and how I want it to look.

I've come across this problem a few times myself. What happened is that intellij lost the module in my project settings. Here's my steps to fix it:
File -> Project Structure -> Click on modules (notice empty list) ->
Click on '+' -> Click on import module -> Find existing myProj.iml

I also have quite a few IntelliJ projects and also git folders of text files. I found IntelliJ does not always reconstruct the .idea files correctly. However the following methods work for me.
Open IntelliJ in the top folder where there is no .idea folder. Often IntelliJ will just construct a new .idea folder and works.
Copy an .idea folder from a similar project and modify to fit. The xml files in the .idea are reasonable to modify.

Open IntelliJ in the root project level works for me.
I have a mixed project folder tree like the following:
git_root
- proj_android
- proj_ios
- proj_react
- lib
- src
My problem is that when I open IntelliJ on my proj_react folder, I could not see my src folder or lib folder. (My proj_react is one level down from my git_root.)
Then I open IntelliJ on git_root, I could see everything. The problem is solved.

If you are working on a software development project where you are using Maven, Gradle, Flash Builder etc., the below mentioned steps should restore the project structure:
Ctrl+Alt+Shift+S (or) File>>Project Structure
Select 'Modules' in 'Project Settings'
Alt+Insert (or) Select '+' to add a new module
Select 'Import Module'
Select the folder you want to import the module from(Generally the parent folder)
Select 'Import module from external model' and the corresponding external model
This resets your project structure.

You can sometimes rescue projects like these by going and resetting their content root in the module settings. Or even recreate the module as another answer suggests.
This is something that would happen to me if I opened an IntelliJ project in PhPStorm, then tried to open it in IntelliJ again.
However as was mentioned the project format changed recently, so you should use the same version of intelliJ to work on your project.

If you have a content root for a project module that intersects with another content root, then this problem will occur. For example:
Project A (content root)
- source for A
- module B (content root)
- source for B
If you have a project that has a source root for "Project A" and you add a source root for "module B" you will end up with missing structure, such as your first picture. This happens because the content root for module B is within project A structure.
This worked at one time, as I have the setup above with multiple modules. When I upgraded to 12.1.6 this stopped working. If you have overlapping source roots you will see this problem.
I'm using 12.1.6 on windows 7.

On Mac:
Preferences
search for Project Structure or go directly to:
Project: Your_Project_Name / Project Structure
Mark all as Sources (mine was marked as "Excluded")

What did you update from? If you did a few major versions update, intellij really changed how their projects are laid out. You probably need to edit the projuct structure and add a new java module.

This issue is 8 years old and the problem still exists in IntelliJ. For projects with many modules, it can be a real pain reconstructing them after each time this occurs. I suggest that once you reconstruct the modules, back up the .idea folder so you can easily restore it next time

In the bottom left corner of IntellIJ you'll find an icon, using which you can fetch project structure, etc..,

This is the quick fix that i have done:
Close the IDE and open the project folder in file manager
Delete the .idea folder and reopen the project in IDE.
To view the hidden folder and files:
Open the folder and press cntrl+H (in linux), And in windows Go to folder options and check view hidden files option.

How to avoid "Refreshing external Folders" in Android development in Eclipse

I am having this annoying problem which takes 10-20 seconds and sometimes more every time my android project is refreshed or rebuilt.
I searched for android forums and also in stackoverflow and but solutions does not make sense. Please give me the solution or link to it.
My android project depends on two external java libs and android libs. Tell me if you need more info.
Earlier I asked this question and it solved the problem by removing 'source attachment'. But I believe it is not a good solution because I need source to be attached with jar. It helps while debugging, code browsing. If source is not attached then, your breakpoint stops there and you can't go in readable source.
Better Solution Needed. Is there a launcher option? which disables when app launched. or build options?

Instead of attaching a source folder to the .jar file, you should zip the source folder and attach the zipped file to the .jar.
See this answer for more info https://stackoverflow.com/a/9699242/637754

The reason for this is that you have attached source with your dependent jars. Just the remove the source from the class path. This how it can be done.
Right click on the project -> Select Build Path -> Configure Build Path.. -> Select Libraries
Select the Source attachment and Remove button will highlight. Press Remove and OK.
I think that should fix your issue.

this helped me so you can try too:
Go to your Workspace root/.metadata/.plugins/org.eclipse.jdt.core/ and remove .org.eclipse.jdt.core.external.folders folder
it contains .project file with links to external folders which takes too long to refresh

You can disable "Refresh on access" option in Window->Preferences->General->Workspace - it have no effect as I think, because even it's turned on you must manually refresh projects if you modify them outside of Eclipse

If anything else fails (my case), install a fresh copy of Eclipse Juno
Go to Help-Install Software, and add this repo:
https://dl-ssl.google.com/android/eclipse/
Check all options and install.
For migration reasons I was trying to install Eclipse on MAC OS Yosemite, since Android Studio is now the official IDE, it is hard to find Eclipse setups with the ADT plugin included. I tried several versions of Eclipse, and JUNO did the work.