I've been working on this for about an hour and thumbing through Q&As on stackoverflow but I haven't found a proposed solution to my problem. I'm sorry if this is a duplicate, but I couldn't find any duplicate question with an answer that solved my specific problem.
I am trying to write and compile a java program from terminal for the first time (up until this point I have been using Eclipse for java and VIM for everything else, but I feel its time to switch entirely to VIM). Here is my current HelloWorld code:
package main;
public class HelloWorld {
public static void main(String args[]) {
System.out.println("Hello World!");
}
}
I compile and run using the following commands (specifying the classpath to ensure that isn't the problem):
javac -cp "./" HelloWorld.java
java -cp "./" HelloWorld
This gives me the following error message:
Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld (wrong name: main/HelloWorld)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:791)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:449)
at java.net.URLClassLoader.access$100(URLClassLoader.java:71)
at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
at java.lang.ClassLoader.loadClass(ClassLoader.java:423)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:480)
I know it is seeing the file HelloWorld.class and trying to access the class HelloWorld because if I change the run command to:
java -cp "./" Foo
I get an entirely different error message:
Error: Could not find or load main class Foo
I have tried several dozen pages worth of troubleshooting and come up short, including the following:
Exception in thread "main" java.lang.NoSuchMethodError: main
http://introcs.cs.princeton.edu/java/15inout/mac-cmd.html
java -version yields:
java version "1.7.0_07"
Java(TM) SE Runtime Environment (build 1.7.0_07-b10)
Java HotSpot(TM) Client VM (build 23.3-b01, mixed mode)
My operating system is LinuxMint and uname -a yields:
Linux will-Latitude-D620 2.6.38-8-generic #42-Ubuntu SMP Mon Apr 11 03:31:50 UTC 2011 i686 i686 i386 GNU/Linux
package main;
This means that your class resides in the main package, and its canonical name is main.HelloWorld.
Java requires that package names should also be mirrored in the directory structure. This means that:
Your HelloWorld.java file should be in a directory named main
You should execute javac and java from the directory containing main, not from main itself
The classpath should contain the directory where the main directory is, not main itself
java expects the canonical name of the class to execute, so main.HelloWorld
So, to recap:
You should have something like myproject/main/HelloWorld.java
From myproject, run javac main/HelloWorld.java
From myproject, run java -cp ./ main.HelloWorld
You've put your class in a package named "main", but you're trying to treat it like it isn't in a package. Since you put package main; at the top of your source file, you need to put HelloWorld.java in ./main, then run javac ./main/HelloWorld.java, followed by java -cp . main.HelloWorld.
These commands will get you the working example you're trying to build:
mkdir main
echo 'package main; public class HelloWorld { public static void main(String... args) { System.out.println("Hello World"); } }' > main/HelloWorld.java
javac main/HelloWorld.java
java -cp . main.HelloWorld
As a beginner you might encounter a very similar scenario where the error output is the same. You try to compile and run your simple program(without having any package set) and you do this:
javac HelloWorld.java
java HelloWorld.class
This will give you the same java.lang.NoClassDefFoundError since java thinks HelloWorld is your package and class your class name. To solve it just use
javac HelloWorld.java
java HelloWorld
See the Java page - Lesson: Common Problems (and Their Solutions)
Problem:
Basically, the Exception in thread "main" java.lang.NoClassDefFoundError:
means, that the class which you are trying to run was not found in the classpath.
Solution: you need to add the class or .jar file which contains this class into the java classpath. When you are running a java class from the command line, you need to add the dot (.)
java YourSingleClass -cp .
into the classpath which tells the JVM to search for classes in actual directory.
If you are running a class from a .jar file, you need to add this jar file into the classpath:
java org.somepackage.SomeClass -cp myJarWithSomeClass.jar
Related
My java swing code compiled successfully but its not executing from cmd
package swing_1;
public class JavaCalculator implements ActionListener{
the above class contains the main method
when i'm compiling there's no error showing javac JavaCalculator.java
but when I trying to execute javac JavaCalculator.java
its showing
Error: Could not find or load main class JavaCalculator
Caused by: java.lang.NoClassDefFoundError: swing_1/JavaCalculator (wrong name: JavaCalculator)
Its seems you are missing a manifest. Try calling it this way:
java -cp MyJar.jar com.mycomp.myproj.dir2.MainClass2
where the latter part is the location of your Class that contains the main method
That is because you should compile in the directory where the subdirectory swing_1 resides:
javac swing_1/JavaCalculator.java
java -cp . swing_1/JavaCalculator.class
Start with an IDE like NetBeans or IntelliJ (community) or eclipse.
System details:
Ubuntu 17.10
openjdk version "1.8.0_151"
OpenJDK Runtime Environment (build 1.8.0_151-8u151-b12-0ubuntu0.17.10.2-b12)
I can't get my java program to run. I don't know why it won't find the class. It compiles with the -classpath flag, but doesn't find the class when running.
$ ls -ltra
total 668
-rw-rw-r-- 1 bvpx bvpx 653275 Jan 19 14:45 javax.mail.jar
drwxr-xr-x 3 bvpx bvpx 4096 Jan 19 14:59 ..
-rw-r--r-- 1 bvpx bvpx 960 Jan 19 15:07 Example.java
drwxr-xr-x 2 bvpx bvpx 4096 Jan 19 15:07 .
Compiling without -classpath does not work (I thought -classpath defaulted to .?)
$ javac Example.java
Example.java:2: error: package javax.mail does not exist
Specifying the -classpath helps, the program now compiles and produces Example.class:
$ javac -classpath javax.mail.jar Example.java
$
Here's the source code:
import java.util.*;
import javax.mail.*;
import javax.mail.internet.*;
public class Example {
static final int PORT = 587;
/* ... */
public static void main(String[] args) throws Exception {
/* ... */
Transport transport = session.getTransport();
try
{
System.out.println("Sending...");
transport.connect(HOST, SMTP_USERNAME, SMTP_PASSWORD);
transport.sendMessage(msg, msg.getAllRecipients());
System.out.println("Email sent!");
}
catch (Exception ex) {
System.out.println("Error message: " + ex.getMessage());
}
}
}
Running the program produces this error:
$ java -Xdiag -classpath javax.mail.jar Example
Error: Could not find or load main class Example
java.lang.ClassNotFoundException: Example
at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:335)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:495)
Running java without -classpath causes the JNI to not find javax/mail even though it's in the directory.
$ java -Xdiag Example
Error: A JNI error has occurred, please check your installation and try again
Exception in thread "main" java.lang.NoClassDefFoundError: javax/mail/Address
at java.lang.Class.getDeclaredMethods0(Native Method)
Why can't java find the Example class?
You seem to be missing some fundamental concepts here.
The classpath gives a list of directories and JAR files to search for needed classes. When trying to load a class foo.bar.MyClass that is not part of the standard library, the default classloader will look for it in each classpath element in turn, in order, until it finds the class or runs out of elements.
Note well, however, that it searches by fully-qualified name. For classpath entries that are directories, that means that it looks for foo/bar/MyClass.class relative to the directory. For classpath entries that are JAR files, it looks for foo/bar/MyClass.class relative to the root of the JAR. Classes that belong to the unnamed default package are a little special, or so it may seem, because their class files (e.g. InDefaultPackage.class) are expected to be located directly in the root of the designated JAR or directly in the specified directory.
Compiling without -classpath does not work (I thought -classpath
defaulted to .?)
$ javac Example.java
Example.java:2: error: package javax.mail does not exist
The classpath does default to .. This is the name of a directory, so when searching it for classes in, say, the javax.mail package, it looks for a subdirectory javax/mail, and if that is found, it examines the class files within. Note that it does not descend into JAR files it discovers in the directory tree. It looks only in those JARs explicitly named in the classpath.
The error message is telling you that javac didn't find any classes at all from the javax.mail package. You could have solved it either by specifying the JAR in the compilation classpath (as ultimately you did) or by unpacking the JAR in the current directory.
Specifying the -classpath helps, the program now compiles and produces
Example.class:
$ javac -classpath javax.mail.jar Example.java
$
Note that the compiler will store the classfile in a directory structure corresponding to its package, just where the java command will look for it.
Running the program produces this error:
$ java -Xdiag -classpath javax.mail.jar Example
Error: Could not find or load main class Example
java.lang.ClassNotFoundException: Example
at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:335)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:495)
You clarified in your answer that you solved this problem by removing a package statement from Example.java. That's ok, but it doesn't really explain the problem, which is that java expects you to give it the fully-qualified name of the class. That includes the package name if the class is in a named package. Thus, if Example.java contained this package statement:
package com.my;
then the class name you would need to specify to java would be com.my.Example. You specified just Example, which designates a class named "Example" in the default package, and your solution to the class not found problem was to move your class into the default package.
Note also that it is conventional and helpful to lay out your Java source files, too, in a directory structure matching their package structure. Thus, the source file for class com.my.Example would conventionally be located in com/my/Example.java. The Java compiler will rely on this scheme to locate sources for classes that it does not find.
Running java without -classpath causes the JNI to not find
javax/mail even though it's in the directory.
$ java -Xdiag Example
Error: A JNI error has occurred, please check your installation and try again
Exception in thread "main" java.lang.NoClassDefFoundError: javax/mail/Address
at java.lang.Class.getDeclaredMethods0(Native Method)
No, javax/mail/Address was not in the directory. It was in a JAR file in the directory. That's not at all the same thing, and the difference is significant.
I had to set -classpath to include the current directory. According to the documentation classpath is delimited by :. The correct classpath string was:
javax.mail.jar:.
Below is a working example.
$ javac -classpath javax.mail.jar:. Example.java
$ java -classpath javax.mail.jar:. Example
Sending...
Email sent!
Another thing to note was that there was originally a package definition at the top of Example.java. I had to remove it.
I am a beginner in java and taking the course Algorithm, which is provided by Princeton. In the course, professor asked us to download algs4.jar to a folder and add algs4.jar to the classpath.[1]
I followed it step by step, and try to program a HelloWorld like
import edu.princeton.cs.algs4.StdOut;
public class HelloWorld {
public static void main(String args[]) {
StdOut.print("Hello World!");
}
}
However when I compile the file, console reminds me that
NPP_EXEC: "java_Compile_Run"
NPP_SAVE: G:\java\helloworld\HelloWorld.java
javac -encoding UTF-8 "G:\java\helloworld\HelloWorld.java"
Process started >>>
<<< Process finished. (Exit code 0)
==========编译成功后开始运行==========
java -cp "G:\java\helloworld" "HelloWorld"
Process started >>>
Exception in thread "main" java.lang.NoClassDefFoundError:
edu/princeton/cs/algs4/StdOut
at HelloWorld.main(HelloWorld.java:5)
Caused by: java.lang.ClassNotFoundException: edu.princeton.cs.algs4.StdOut
at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 1 more
<<< Process finished. (Exit code 1)
================ READY ================
I have checked my classpath, and my programming file. What does this error mean? And how can I fix it?
Any advice is helpful. Thank you.
[1] http://algs4.cs.princeton.edu/code/
If you're referring to a jar file that should be on the classpath, you must name it explicitly. E.g.
java -cp "G:/java/helloworld;G:/whereever/algs4j.jar" HelloWorld
Do they really provide another name for System.out? In this case you can also safely ignore that jar by using System.out instead of StdOut
According to the instructions of this course you should use javac-algs4 to compile and java-algs4 to execute if you imported their library.
If you want to know the differences between these two commands and original commands javac and java, you could
use type command to find where is this command is
use cat or vim to see what's the content of this command
You can see that java-algs4 added -cp parameter to original java command just as Olaf Kock said.
You'll have to probably use -cp flag to set the class path to include the package.
While using the -cp flag, don't forget to include the current working directory using .
So, something like javac -cp thejar.jar:. should work in linux or javac -cp thejar.jar;. should work for windows
For Mac, using M1, cd in your folder:
javac -classpath ".:./algs4.jar" HelloWorld.java
java -classpath ".:./algs4.jar" HelloWorld
I just included a classpath argument in both javac and java commands like so:
javac -classpath ".;drive\path\to\algs4.jar" Hello.java
and
java -classpath ".;drive\path\to\algs4.jar" Hello
Also if you are manually adding the CLASSPATH environment variable, then remember to close and restart the cmd console.
You imported class StdOut in your java code (import edu.princeton.cs.algs4.StdOut;), you have to tell java how stdOut implement
Accroding to the link you provided (http://algs4.cs.princeton.edu/code/). You have to follow the "Installing the textbook libraries." section to install this lib first.
This question already has answers here:
Why can't I run my java Hello World program if it is inside a package?
(3 answers)
Closed 9 years ago.
I'm currently learning Java and the use of my command prompt as a compiler. But every time I execute the java command followed by my test class "Hello" I get the below error messages:
Exception in thread "main" java.lang.NoClassDefFoundError: Hello (wrong name: hello/Hello)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:792)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:449)
at java.net.URLClassLoader.access$100(URLClassLoader.java:71)
at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:482)
Now I've checked my "CLASSPATH" environment variable and it is correct as follows: .;C:\Program Files\Java\jdk1.7.0_25\bin; I've even tried removing the .; from the beginning of the CLASSPATH but it didn't do anything different. Now my javac command works just fine by creating a .class version of my .java class. But I just can't get it to actually execute the java command.
The name of my class is Hello so I typed javac Hello.java to compile my file as a class file and it worked. But when I enter: java Hello is when I get the above error messages. I've tested the program on my NetBeans IDE that I made it in, and it works perfectly fine with no errors.
What could possibly be going on that would prevent me from executing my java command to run a .class file?
The name of my class is Hello so I typed javac Hello.java to compile
my file as a class file and it worked. But when I enter: java Hello
The most likely problem is that while running your java program you are not putting the complete class name along with the package structure. It should be run as mentioned here:
java packagenhierarchy.Hello
Assuming your package name is com.my.hello and your main class name is Hello then it should be run from the directory containing the top level package as:
java com.my.hello.Hello
UPDATE: As per your comments and knowing the working directory, here is what you should run:
java -cp C:\hello\src\hello hello.Hello
You need to understand how java tool works which is little different than how javac works. In order to run a program with java command-line command:
The class that has the main(String[] args) method should be in the classpath.
Instead of type java Hello you should use the fully qualified name, such as:
java com.mypackage.Hello
assuming that you set the classpath with the variable CLASSPATH. Otherwise, it should be like this:
java -cp C:\projects\myprojct\bin com.mypackage.Hello
assuming that bin is the root directory that has the following hierarchy:
bin -
|
com -
|
mypackage -
|
Hello.class
Note that if you don't use neither CLASSPATH nor -cp nor -classpath, then the current directory is by default is in the classpath. In other words, the following should work:
cd C:\projects\myprojct\bin
java com.mypackage.Hello
Maybe your current directory is not in the Class path. Try
java -cp pathToYourHelloCompiledFile Hello
You must have a Hello.class file in your above folder whose path you provide as the class path.
I have been mostly using eclipse so far. Now I'm trying to run java from terminal but I have a problem with packages.
This is my Main.java file:
package main;
class Main {
public static void main(String[] args) {
System.out.println("it's working");
}
}
I compile this using javac Main.java and then run with java Main which gives me:
java Main
Exception in thread "main" java.lang.NoClassDefFoundError: Main
Caused by: java.lang.ClassNotFoundException: Main
at java.net.URLClassLoader$1.run(URLClassLoader.java:217)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294)
at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
Could not find the main class: Main. Program will exit.
When I remove package Main everything works fine. What am I missing?
java -version gives:
java version "1.6.0_24"
OpenJDK Runtime Environment (IcedTea6 1.11.4) (6b24-1.11.4-1ubuntu0.12.04.1)
OpenJDK 64-Bit Server VM (build 20.0-b12, mixed mode)
You need to run the java command up one directory level and give it in the fully qualified package name, eg: java main.Main
See How the Java Launcher Finds User Classes to learn how this works.
You can use this command:
java main.Main
Make sure the main (lowercase) package directory is on the classpath.
It is possible that your classpath is not set correctly.
Since you gave your .java file a package it is unnamed no longer.
An example:
java -cp ./package1/ main.Main //from the current directory and
//if main package is contained in package1
You need to fully qualify the class name.
For future reference if you want to run from the command line you must stop the indirection (for lack of a better term) at the package level.
Say your class was in the package package1.package2.Main.java
I would run it like so java -cp /blah/blah package1.package2.Main
Compile
Windows:
javac main\Main.java
Mac:
javac main/Main.java
Run
java main.Main
If you add package Main, then you must put your source file in folder Main/Main.java. After that you can compile. When you run the program, go to Main folder using "cd", then write java -cp Main.Main
See my question similiar to yours noclassdeffounderror
try this...
In window , you just compile the code as
javac - d . Main.java
then a package(folder) with the name you specified in your class is created (in your code, package with name "main" is created) in the same path where your program reside...
Then you just run the program as
java main.Main
or
java main/Main