I am new to Java and I got this book to help me start.
I have successfully compiled Hello.java using "javac Hello.java".
Now it says to type in "java Hello" and I am getting "Could not load of find main class Hello". I have tried to find out how to fix it before but all the answers are complicated and confusing. If someone could explain how to fix this, that would be awesome.
The problem appears to be of CLASSPATH.
Solution 1
Add the path of your directory where you have compiled your class to the CLASSPATH variable in Environment Variables.
Solution 2
Every time you run the program add the current folder and libraries you are referencing in the classpath using -classpath. eg:
java -classpath .;lib/referenced-libs.jar my.package.MainClass
Make sure the main method with exactly this syntax is present in the Hello.java file:
public static void main(String[] args) {
// your code will go here...
}
you should check if the hello.class file is present in the folder. java runs these .class files. These files are created on successful compilation.
Related
I know this question has been asked loads of times before, but I'm a rookie programmer and despite trying many of the solutions on this site I still can't fix this issue. I'll be really thankful if you can take the time to figure out what I've done wrong.
Operating system: Windows 8
Java version: 1.8.0 update 25
The command prompt I'm using is the one that comes with Windows. (I'm presuming there are other types so I'm just making it clearer.) The code's a really basic one.
package com.thefinshark.intro;
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome.");
}
}
So, first I changed the directory to C:\javawork, where Welcome.java is saved. I set the path to C:\Program Files\Java\jdk1.8.0_25\bin, then compiled the code. The compilation seemed fine, I found the Welcome.class file in the C:\javawork as well. The execution, however, kept returning "Could not find or load main class Welcome". I've tried C:\javawork>java Welcome and C:\javawork>java com.thefinshark.intro.Welcome, and loads of other variations. I've also changed the classpath to C:\ and C:\javawork but it still dosen't work. Someone answering a similar question suggested adding dt.jar and tools.jar to the classpath but no dice.
It'll be great if someone could help, and I'll be happy to help pass on the information to the others who have problems like this as well. (As I'm typing this I'm looking at a whole long list of similar questions.)
The directory structure must match the package name of your source file. So, if your class is in the package com.thefinshark.intro, then your source file must be in a directory com\thefinshark\intro.
So, for example, you should save your source file as C:\javawork\com\thefinshark\intro\Welcome.java, and then compile and run it from the directory C:\javawork:
C:\javawork> javac com\thefinshark\intro\Welcome.java
C:\javawork> java com.thefinshark.intro.Welcome
Note: The javac command expects a filename of the source file you are compiling (com\thefinshark\intro\Welcome.java), and the java command expects a fully-qualified class name (com.thefinshark.intro.Welcome).
See Lesson: Packages for more details on how to work with packages.
I am trying to compile and run some java files I have made in Eclipse. The full path to the .java file is C:\Users\MYNAME\Documents\Java\Introduction\src\tests\Test.java. tests is the package I created in Eclipse and src is a folder that Eclipse made under Introduction (which is the project name).
In my environment variables, I have the following relevant variable:
JAVA_HOME C:\Program Files (x86)\Java\jdk1.7.0_40\bin
Under system variables I have the following:
CLASSPATH %JAVA_HOME%
I go to my cmd and cd into the tests directory (cd C:\Users\MYNAME\Documents\Java\Introduction\src\tests). Then I compile using javac Test.java. This seems to work as I then have a Test.class file under the same directory. Now I want to run the file, I type java Test and I get the error, "could not find or load main class". I've tried a variety of things including appending .class and .java to the end but I keep getting the error. I looked at some answers and docs and I managed to get it to work if I cd into:
cd C:\Users\MYNAME\Documents\Java\Introduction\src (i.e, get out of the package)
and then run:
java -cp . tests.Test
So that seems to temporarily set the class path to the current directory, and run Test from the package tests. However, I want to simply be able to type java Test. I know it's possible as I used to be able to do it, but now for some reason I cannot (I must have changed something along the way...).
Any help is appreciated.
However, I want to simply be able to type java Test
That will only work if Test is in the default package - it's as simple as that. You need to pass the java executable the fully-qualified name of the class you want to launch. There's no way round that.
Of course, you could create your own launcher which looks in the current directory for class files, finds out the fully-qualified name of the classes within those files, and launches java providing the full name and probably specifying an appropriate classpath... but that seems like a lot of hassle compared with just including the package name in the command.
You could be making the same mistake I made. So, try the following.
Here is my code for your reference.
class A{
public static void main(String args[]) {
System.out.println("Hello world");
}
}
Once you saved this as "C:\JavaStudy\ClassA.java", try the following.
c:\JavaStudy>javac ClassA.java
c:\JavaStudy>java A.class
Error: Could not find or load main class A.class
c:\JavaStudy>java A
Hello world
c:\JavaStudy>
Note: You don't need to use " java.exe -cp . " if you have class file in the same directory from where you are executing.
I looked around, but none of the solutions that worked for others worked for me.
I installed java 1.8.0
My path variable is C:\Program Files\Java\jdk1.8.0_05\bin
I try to run the following program hello.java:
package hello;
public class hello{
public static void main(String[] args){
System.out.println("Hello");
}
}
The program compiles fine when I run javac hello.java
But when I use java hello , java -cp . hello, or java -classpath . hello it returns the error 'Could not find main class hello'.
I know this is a very basic problem, but I really can't figure it out.
Thanks in advance
In very similar answers that I've provided, provided that you're compiling in the current directory, then you need to ensure that your compiled class has made its way to a folder called hello/.
If it has, then you can run this:
java -cp /path/to/hello hello.hello
The above adds the hello/ folder to the classpath, and then you can run the main class using its fully qualified name.
Here you define package so you cannot run directly your compiled class because JVM is not able to find your class so you have to write the path of your hello directory in run command.
for example:
java -cp c/workspace/hello hello
I try your code. And see what the problem is. If you put the "hello.java" in a folder named "JavaTrials" and compiled it there as "javac hello.java", it compiles and produces "hello.class" right there. This command does not produce a folder named "hello" for the package.
Your compiled code "hello.class" should be in a folder named "hello" which is the name of the package. And then you have to run the command "java hello.hello" not from inside the folder "hello" but from the containing folder.
The better way is to put your code in a folder named "hello" before compiling it. This folder represents the package. Then compile it from the outside of the "hello" folder with command "javac hello/hello.java". And then you can run it by "java hello.hello"
As a side note, in java coding tradition class names begins with upper case. Se better use "Hello" instead of "hello".
remove first line "package hello;"
I have Windows 7, installed jdk1.7.0 and its supporting jre7.
My problem is compilation part works perfectly, but while running the Java program I get this error saying:
"Could not find or load main class"
I am storing all my programs in javalab folder. I have set the path to it. Procedure looks like this:
C:\Users\user>cd\
C:\>cd javalab
C:\javalab>autoexec.bat
C:\javalab>set path=C:\Program Files\Java\jdk1.7.0\bin
C:\javalab>javac p1.java
C:\javalab>java p1
Error: Could not find or load main class p1
C:\javalab>
I was having a similar issue with my very first java program.
I was issuing this command
java HelloWorld.class
Which resulted in the same error.
Turns out you need to exclude the .class
java HelloWorld
Try:
java -cp . p1
This worked for me when I had the same problem, using Fedora (linux)
Simple way to compile and execute java file.(HelloWorld.java doesn't includes any package)
set path="C:\Program Files (x86)\Java\jdk1.7.0_45\bin"
javac "HelloWorld.java"
java -cp . HelloWorld
pause
javac should know where to search for classes. Try this:
javac -cp . p1.java
You shouldn't need to specify classpath. Are you sure the file p1.java exists?
I had almost the same problem, but with the following variation:
I've imported a ready-to-use maven project into Eclipse IDE from PC1 (project was working there perfectly) to another PC2
when was trying to run the project on PC 2 got the same error "Could not find or load main class"
I've checked PATH variable (it had many values in my case) and added JAVA_HOME variable (in my case it was JAVA_HOME = C:\Program Files\Java\jdk1.7.0_03)
After restarting Ecplise it still didn't work
I've tried to run simple HelloWorld.java on PC2 (in another project) - it worked
So I've added HelloWorld class to the imported recently project, executed it there and - huh - my main class in that project started to run normally also.
That's quite odd behavour, I cannot completely understand it.
Hope It'll help somebody. too.
i guess that you have a different class name in p1.java
Check you class name first. It should be p1 as per your batch file instruction. And then check you package of that class, if it is inside any package, specify when you run.
If package is x.y
java x.y.p1
Here is my working env path variables after much troubleshooting
CLASSPATH
.;C:\Program Files (x86)\Java\jre7\lib\ext\QTJava.zip;C:\Program Files (x86)\Java\jdk1.6.0_27\bin
PATH <---sometimes this PATH fills up with too many paths and you can't add a path(which was my case!)
bunchofpaths;C:\Program Files (x86)\Java\jdk1.6.0_27\bin
Additionally, when you try to use the cmd to execute the file...make sure your in the local directory as the file your trying to execute (which you did.)
Just a little checklist for people that have this problem still.
I've had similar problems. If you work with Eclipse, you need to go to the folder where you have your src/ folder... If you used a package - then you use
javac -cp . packageName/className
which means if you've had a package named def and main class with name TextFrame.java you'd write
javac -cp . def/TextFrame
omitting the trailing .java extension, and then you run it with the
java def/TextFrame
and if you have have arguments, then you need to supply it with arguments corresponding to your program.
I hope this helps a bit.
First, put your file *.class (e.g Hello.class) into 1 folder (e.g C:\java). Then you try command and type cd /d C:\java. Now you can type "java Hello" !
You might have the CLASSPATH environment variable already added!!
Use following to avoid further usage of -cp . in java -cp . CLASSFILE
Add . to CLASSPATH in system properties->environment variables or by cmd
set CLASSPATH=%CLASSPATH%;.;
I faced a similar problem in Eclipse. Whenever I clicked on the Run button it gave me the message, "Error: Could not find or load main class". But when I right click on the java file in the project explorer and Run As Java configuration, it works perfectly.
I think this is because it tries by default to run it in some other configuration which causes problems.
Hope this answer helps some.
If you have a single .java file to compile using command-line , then remove
topmost package parts from the code, the compile again, it will work.
This worked for me.
Sometimes what might be causing the issue has nothing to do with the main class. I had to find this out the hard way, it was a referenced library that I moved and it gave me the:
Could not find or load main class xxx Linux
I just delete that reference and added again and it worked fine again.
i had
':'
in my project name e.g 'HKUSTx:part-2'
renaming it 'HKUSTx-part-2' worked for me
You can use NetBeans IDE which is free to download and use "Open Source". You can even do other programming language in this IDE. The latest of which it supports HTML5. This makes your programming easier. If you're not familiar with it choose a book that is NetBeans integrated like Sams Teach Yourself Java in 24 Hours
I'm learning java and I would like to know how to compile a java program from other directory.
For example, my compiler is in my drive c:\ and I want to compile my java program from drive e:\
How should I do that?
I'm getting this error, what does it mean?
The current directory should be in the default CLASSPATH, but maybe it's not. Try java -cp . Assignment
It's been a while since I've done java, but it seems like the compiling isn't your problem. Since javac returns properly, it seems to be a problem with Assignment.java. Does your Assignment class have a main method?
Well the easy way is to set ur classpath variable. Since from screen shot it seems ur using windows i suggest u right click the my computer nd select properties. Go to advance setting and click environment variable tab.
Then a new window pops up which has System Variable at bottom. Select new and create a variable
JAVA_HOME = path where u installed java eg for me its c:\java
Now once u add this search an existing variable path and choose edit. Now at the end append the following ;%JAVA_HOME%\bin
Now ur done u cn run java program frm any location on ya comp....
You are using package in your code...so it shows NoClassDefFoundError when you run
you should create folder which contain your package name...compile that java file...and you can run that file from previous directory of that java file directory...
For example your code is
package test;
class Assignment{
public static void main(String args[]){
System.out.println("Hai");
}
}
it saved on this path "E:\java\test"
compile this file and you can run this file from this path "E:\java"
command to run this file
java test.Assignment
E:\java> java test.Assignment
Is there a package declaration at the top of your Assignment.java? If so, remove it and recompile for a quick fix.
To work with Java packages, you'll need a directory structure that matches the package declarations.
For example, say this is your Assignment.java:
package myjava;
public class Assignment {
public static void main(String[] args) {
....
....
}
You run this command to compile:
E:\java>javac -d . Assignment.java
And you get myjava\Assignment.class if all went well. The -d . option means "place generated class files in the current directory". javac creates the package hierarchy as directories for you.
Now that your directories match your packages, this should work:
E:\java>java myjava.Assignment