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

Question

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

Answer 1

Here is what made the error disappear for me:

Close eclipse, open up a terminal window and run:

$ mvn clean eclipse:clean eclipse:eclipse

Answer 2

Are you using Maven? If so,

1. Right-click on the project, Build Path and go to Configure Build Path
2. Click the libraries tab. If Maven dependencies are not in the list, you need to add it.
3. 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.