Netbeans IDE 11 cannot access java.lang Fatal Error: Unable to find package java.lang in classpath or bootclasspath

I also had the same issue.
Solved using manually setting the default jdk.

1. open the netbeans.conf from <install_dir>/netbeans/etc
2. set the JDK home path for netbeans_jdkhome property

I am using Ubuntu 19.10

After exiting netbeans edit the config file netbeans.conf using

nano ~/netbeans-11.2/netbeans/etc/netbeans.conf

In the line netbeans_jdkhome edit the path like

netbeans_jdkhome="/usr/lib/jvm/java-11-openjdk-amd64"

(Found at askubuntu.com)

After a complete uninstall of my distros Netbeans version, I resorted to installing Netbeans 11 LTS version from the https://netbeans.apache.org/download/nb110/nb110.html into /usr/share/netbeans. This seems to have resolved the issues in the IDE. The program also seems to compile and run faster now.

I was having very similar problems with Netbeans IDE from the Ubunutu/Mint repositories which was still on version 10 the open JDK was version 11. I could not get the IDE to display without errors – but the program would compile and run from the command line fine.

If you’re using Maven for the project and OpenJDK the reason could be the way that you define the source and target options in the maven-compiler-plugin. I had a little project build with JDK 1.8 and when I migrated it the maven compiler plugin show me that error.
The solution that worked for me was change the format of the java version on the source and target parameters in maven-compiler-plugin definition:

Before:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.8.1</version>
    <configuration>
        <source>1.7</source>
        <target>1.7</target>
        <compilerArguments>
            <bootclasspath>${sun.boot.class.path}${path.separator}${java.home}/lib/jfxrt.jar</bootclasspath>
        </compilerArguments>
        <showDeprecation>true</showDeprecation>
    </configuration>
</plugin>

After:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.8.1</version>
    <configuration>
        <source>7</source>
        <target>7</target>
        <compilerArguments>
            <bootclasspath>${sun.boot.class.path}${path.separator}${java.home}/lib/jfxrt.jar</bootclasspath>
        </compilerArguments>
        <showDeprecation>true</showDeprecation>
    </configuration>
</plugin>