Compilation issues from the command line

Question

I am trying to run everything from the linux command line to which I am relatively new too.

I am trying to compile a class which uses some external libraries and when i do javac classname.java i get warnings....

It is recommended that the compiler be upgraded.
warning: /opt/pi4j/lib/pi4j-core.jar(com/pi4j/io/gpio/GpioPinOutput.class): major version 51 is newer than 50, the highest major version supported by this compiler.

Even though it is just a warning - I have tried to then run java on the class file which results in

Exception in thread "main" java.lang.UnsupportedClassVersionError: com/pi4j/io/gpio/GpioFactory : Unsupported major.minor version 51.0
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:634)
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:277)
    at java.net.URLClassLoader.access$000(URLClassLoader.java:73)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:212)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
    at ControlGpioExample.main(ControlGpioExample.java:9)

I searched stackoverflow and found a post suggesting to use cross compilation - which I have tried via

javac -target 1.6 ControlGpioExample.java

How do I know which target to use? Do i just keep going through them all until it works?!

Thanks

a downvote for a serious question? I tried myself, I posted what I did, all after searching stackoverflow — Biscuit128, Oct 29 '13 at 20:34
You didn't search enough if you didn't find the duplicate :) — , Oct 29 '13 at 20:36

Jon Skeet · Answer 1 · 2013-10-29T21:10:01.560

2

It's not the code you're compiling that's the problem - it's the code you're compiling against, in pi4j-core.jar, which appears to have been built with Java 7.

I strongly suggest you upgrade the JDK, basically. Either that, or:

Find a version of pi4j which was built with Java 6
Rebuild it yourself

edited Oct 29 '13 at 21:10

answered Oct 29 '13 at 20:30

Jon Skeet

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Did you mean JDK? JRE doesn't come with `javac`. :) – Vivin Paliath Oct 29 '13 at 20:34
@VivinPaliath: Both, really :) – Jon Skeet Oct 29 '13 at 21:09

score 0 · Answer 2 · answered Oct 29 '13 at 20:32

You're using Java 6 (version 50) to compile code that uses a library that has been compiled with Java 7 (version 51). Either upgrade your compiler or find a version of pi4j which was compiled with Java 6, and you should be good to go. Upgrading to JDK7 would be the best option.

As a reference, here are the version numbers and what they correspond to:

J2SE 7 = 51
J2SE 6.0 = 50
J2SE 5.0 = 49
JDK 1.4 = 48
JDK 1.3 = 47
JDK 1.2 = 46
JDK 1.1 = 45

score 0 · Answer 3 · edited Jun 20 '20 at 09:12

Please have a look at the pi4j (pi4j-native) build requirements. I took them from here

gcc 4.6.3 or newer (sudo apt-get install gcc)
git-core (sudo apt-get install git-core)
A working JDK (OpenJDK or Oracle JDK) (sudo apt-get install openjdk-7-jdk)
A valid defined JAVA_HOME environment variable
wiringPi source and platform compiled binary (...)

So, as the other answers already suggest, it would be best to upgrade to JDK 7.

Basically, pi4j may use some features available in JDK7 that your present JDK6-based compiler is not able to handle.

score 0 · Answer 4 · answered Oct 29 '13 at 20:38

0

This issue will happen when you compile the Java code with higher version JDK and try to run the same with lower version of JDK. Do you know which version of Java are you using?

answered Oct 29 '13 at 20:38

SarZ

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Compilation issues from the command line

4 Answers4