I know this question has been asked before, but I’ve read through most of them and still can’t figure out my problem.
I’ve compiled the below code into "HelloWorld.class" located in the directory shown below.
package helloworld;
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World");
}
}
But I cannot get the .class file to run using the Java command on Window Command Prompt:
I believe my PATH and CLASSPATH are set correctly (as shown below). But I still can’t find the class. Any ideas why?
The name of the class is not HelloWorld. It goes like:
java helloworld.HelloWorld
and you have to call that from the directory above helloworld.
In other words: the package name is part of the class name. When you invoke java; you have to provide that full name; and the classpath needs to point to the directory that contains that package.
Related
I have read the previously posted questions. Some are vague and none solved my problem so I am forced to ask again.
I have two simple classes,
package One;
import One.Inner.MyFrame;
public class test
{
public static void main(String args[])
{
MyFrame f= new MyFrame();
}
}
And the other class is,
package One.Inner;
import java.awt.*;
import javax.swing.*;
public class MyFrame extends JFrame
{
public MyFrame()
{
setPreferredSize(new Dimension(400,560));
setVisible(true);
}
}
I am at base folder "basic" in Windows cmd. I compile using
basic> javac *.java -d .
A folder and subfolder is created.
cd One
basic\One> java test
This generates a big set of errors. Many answers directed to specify the full path which didn't work.
My classes are in One so specifying One using -cp didn't work either.
You'd run it as:
java One.Test
... but from the root directory (basic), not from the One directory. You always specify the fully-qualified class name.
Oh, and package names in Java should be lower-case, so it should be one and one.inner, not One and One.Inner. Just a convention, but one which pretty much everyone follows.
If the directory is:
basic\One
Run java from the base directory of the package:
basic>java One.test or basic>One.test <optional arguments>
(ideally the package would be lowercase and the class upper case):
basic>java one.Test
If you get 'does not exist' messages, then the java command cannot find classes you referenced in your class. You can point to them with the -cp option ('.' means 'here', and you can add as many places as you like divided by ';' on Windows and ':' on Linux).
basic>java -cp . one.Test
or
basic>java -cp .;..\..\someJar.jar;c:\someDirectory\classesDirectory one.Test
The following line of Haralan Dobrev code solves the problem.
java -cp ../ one.Test
while creating a class with a package if you want to run it from cmd you must created a directory with same name of package put the .class in it and then you can easily run it for example you created a class with name "one" and this class in the package with name pack ,you must run these commands
1 javac one.java
after compilation created a directory with the name pack ,after that run this command
2 java pack.one
Note:
all this must be done in the current working directory and the name "one" i chose it here as file name and main class name
we all know the first name used in the first command is file name and second one is main class name
This is because if you are declaring package in your java file, then JAVA compiler believe you are having same folder architecture in your system.
In your case Java compiler looking for One as a package and then test.class., or to be very specific just look inside your .class file you can see what path it looking for. Please have a look for below Image (I my case I use Hello and Tester)
As you can see path in image is Hello/Tester(my case example), so architecture should be like Hello->Tester.
And if you are not having same architecture and want to create same while compiling, then use javacp command.
I compiled classes using eclipse, but when I try to run them in the command line, it returns "Cannot find or load main class".Command line.
My class is:
package Chapter10;
public class Hfpd10
{
public static void main(String[] args)
{
System.out.println("it works");
}
}
I am running the command from within the C:\Users\John\Documents\Java\EclipseWorkspace\HeadFirstDesignPatterns\bin\Chapter10 folder, where the class files are. The command is:
java Hfpd10
It runs in eclipse and I used the exact same path.
Eclipse path
The only question I could find like this went unanswered. cannot run java file in command line that created by eclipse
Change to the parent directory and run the following command:
java Chapter10.Hfdp10
This required is because your class is in a package and you need to reference it with its full package name.
I'm a little lost here. We were given a jar file which contained 3 different .class files in it. I successfully built the path to the jar file, but I don't know how to use them. The class files obviously contain methods that can be used, but I've never worked with a jar file before so I don't know how to use them. I'm not sure if I have to import them somehow, how to instantiate them or anything. I have searched for tutorials to no avail.
All I'm looking for is some guidance so I can get moving on this project. If it is a case where I have to import them somehow how would I do it? For example, I have WikiEdits.class contained in WikiEdits.jar. The name of my main class is P2. Can someone show me a brief example of how this works?
Add the jar to your classpath, if you are using an IDE.
Then, a java class that uses it would look like something like this:
package p2;
import blah.WikiEdits; //references a class in the jar
public final class P2 { //(this is a strange name for a class, by the way)
public static void main(String... args){
//builds a new object of the given class
WikiEdits thing = new WikiEdits();
}
}
If you are using the command line, these examples may help:
http://www.javapractices.com/topic/TopicAction.do?Id=243
you need to add WikiEdits.jat to your path project, then import and instanciate the class.
import WikiEdits
P2 p = new P2();
p.somemethod();
Static class:
WikiEdit.someMethod();
In your java class add the relevant imports from the jar. And then from command line, you can compile and run your class using the classes from jar by definining the right classpath:
Compilation
on windows:
javac -cp .;pathtoyourjar YourClass.java
on linux:
javac -cp .:pathtoyourjar YourClass.java
Execution
on windows:
java -cp .;pathtoyourjar YourClass
on linux:
java -cp .:pathtoyourjar YourClass
If you are using Eclipse then follow this link to know the steps to add jar to your project:
http://www.cs.duke.edu/courses/cps004g/fall05/assign/final/addlibrary.html
I wrote and tested a small Java program using Eclipse. I'm now trying to deploy it on a Windows 7 box and Java cannot find the class. I copied the class file to C:\dxtester\classes. I'm trying to run it from the dxtester directory with: C:\dxtester>java -classpath classes;. dxtester
This produces this exception which I think I understand. Java examined the class file and is prompting me to provide the fully qualified name.
Exception in thread "main" java.lang.NoClassDefFoundError: dxtester (wrong name:
dxtester/dxtester)
If I use the FQN I get
C:\dxtester>java -classpath classes;. dxtester.dxtester
Error: Could not find or load main class dxtester.dxtester
The application is a simple test driver where everything is done in main().
package dxtester;
public class dxtester {
public static void main(String[] args) {
This seemed like an extremely simple thing to do but I'm completely baffled. What am I missing?
Your current directory is dxtester;
in this directory you have dxtester.class (I presume);
your classpath is the current directory.
This setup is wrong: your classpath must be the base directory such that package names correspond to its subdirectories. In your case you should cd to C:\ and repeat the command; ideally, however, you will have your package structure in a dedicated directory instead of the root.
I should also mention that class names should be in CamelCase.
package pack;
public class sample{
public static void main(String input[])
{
NumberFormat numberFormat = new DecimalFormat("#,##0.00##");
System.out.println(numberFormat.format(44533125.00));
}
}
the code is working fine in the current dir.. (c:/myprogram/).
after that i copy the sample.class file and paste it in other dir(d:/myprogram).
i got error while running, like
Exception in thread "main" java.lang.NoClassDefFoundError: sample (wrong name: pack/sample)
In java .class file can run anywhere right? but why i am not able to run?
You should have the class file within the package - so it should be in a directory called pack. Then with the parent directory in the classpath, you'd run
java pack.sample
(You should also change the class name to Sample to follow conventions, btw - and run pack.Sample.)
If you're building with javac, specify the "-d" option to tell it the base directory, and it will create the appropriate package structure if necessary. For example:
javac -d classes Sample.java
or
javac -d classes src/pack/Sample.java
will (in both cases) create
classes/pack/Sample.class
You could then run
java -cp classes pack.Sample
IntelliJ and maybe other IDE's do not refactor your Run/Debug configuration. You must manually change your package name preceding the name of your Main class. For instance, change 'sample.Main' to 'com.company.package.ui.Main' so it will launch correctly next time you try to run it.
The IDE might have already marked the Run/Debug button with a red cross because it couldn't find the main class. It also gives a warning when you open the Run/Debug configuration.
If you are not using a single java/class file you can also remove the package statement.