Getting a java.lang.NoClassDefFoundError error even when .class file exists

Question

This is my file structure:

T1 -> T1sample -> src -> t1sample -> T1.java
T1 -> T1sample -> src -> t1sample -> T1.class

I created these folders and files myself (except for T1.class, I got that by running 'javac T1.java' from the shell).

This is T1.java:

package t1sample;

public class T1 {
    public static void main(String []args) {
        System.out.println('Test');
    }
}

Now when I try to run java T1 it returns an error saying:

java.lang.NoClassDefFoundError:  T1 (wrong name: t1sample/T1)

Any idea why?

@EddieB I get a `Error: Could not find or load main class t1sample.T1` error. — SilentDev, Sep 29 '16 at 01:41
@EddieB Yup, I ran `javac T1.java` which created the `T1.class` file, and then ran `java T1` and `java t1sample.T1`. — SilentDev, Sep 29 '16 at 01:43
@EddieB When you tried, did you create the folders and files with shell (i.e. `mkdir`)? Not using Eclipse? Because I didn't use Eclipse. — SilentDev, Sep 29 '16 at 01:45
Run `java t1sample.T1` command from the `T1sample/src` directory ... — Edward J Beckett, Sep 29 '16 at 01:49
@EddieB Ah okay, that works. Any idea what `java T1` was not working from inside `t1sample`? — SilentDev, Sep 29 '16 at 01:57
Yes... you have a `package` declaration :) which means that `t1sample` is a package so you have to run the class from the `root` ( src ) directory ( see my answer ) — Edward J Beckett, Sep 29 '16 at 01:58
re "ah okay...any idea" -- the short answer is that it's simply not how it works. Although there is a relation to package and subdirectory when dealing with "loose" .class files (vs. classes packaged in a JAR or other archive), there is no concept of finding a class 'relative' to the current directory (i.e. there is no ./T1 -- it's name is t1sample.T1 and java will try to find it as ./t1sample/T1.class) — jamey graham, Sep 29 '16 at 02:13

Edward J Beckett · Accepted Answer · 2016-09-29T02:08:33.857

Just for clarification the reason your getting a NoClassDefFoundError is because you declared t1sample as a package. Therefore T1 has to be run from the src directory, not from t1sample.

First create the directory structure... (Using gvim as an editor)

~/Desktop $ mkdir -p T1sample/src/t1sample && cd T1sample/src/t1sample && gvim T1.java

Create the source...

package t1sample;

public class T1 {

    public static void main(String... args) {
        System.out.println("Test"); 
    }
}

Compile ...

~/Desktop $  cd T1sample/src/t1sample/
~/Desktop/T1sample/src/t1sample $  javac T1.java

Confirm...

~/Desktop/T1sample/src/t1sample $  ls -la
total 2
-rw-r--r-- 1 Edward None 411 Sep 28 21:46 T1.class
-rw-r--r-- 1 Edward None 137 Sep 28 21:45 T1.java

Run it from the src directory...

~/Desktop/T1sample/src/t1sample $  cd ..
~/Desktop/T1sample/src $  java t1sample.T1
Test

Getting a java.lang.NoClassDefFoundError error even when .class file exists

1 Answers1