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;"
Related
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 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.
I tried to google this, went to oracle.com and read all the questions on this forum related to this. I wrote a simple "Helloworld" program
package helloworld;
public class Helloworld {
public static void main(String[] args) {
System.out.println("Hello World!");
}
}
NetBeans compiles the source code into a .class file. I move that file to C:\MyJava
I try to run it by C:\MyJava> java -cp . Helloworld and all possible variations of such. I keep getting the NoClassDefFoundError: Helloworld (wrong name: helloworld/Helloworld).
To make sure. There's a question almost exactly like this (difference in "wrong name"). The solution to that question does not work in my case.
You get the "wrong name" error because your class is in the package helloworld. Java expects you to provide the fully-qualified class name on the command line:
C:\MyJava> java -cp . helloworld.Helloworld
The directory structure must match the package structure. This means that you should have a directory C:\MyJava\helloworld that contains the class file Helloworld.class.
You need to tell it the package name (which is helloworld):
C:\MyJava> java -cp . helloworld.Helloworld
Below post is similar to your problem. I hope it guides you;
How do I run .class files on windows from command line?
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
I'm new to working with Java from the command line and I don't get it. I read previous CLASSPATH questions but still didn't get my problem to work.
I have the following class in C:\Temp\a\b\c
package a.b.c;
public class Hello
{
public static void main(String args[])
{
System.out.println("Hello World!");
}
}
The package name is intentional.
I compiled it fine and I put the Hello.class file inside C:\Temp\a\target
Now in the command line I go to C:\Temp\ and execute the following:
java -cp .\a\target a.b.c.Hello
It complains that it cannot find the class a.b.c.Hello
Thanks in advance!!
and I put the Hello.class file inside C:\Temp\a\target
This is wrong. It should be placed in the same folder as the .java file. The source code itself is declared to be in the package a.b.c; so, the .class file should really be kept in \a\b\c folder.
Then, to execute it just do:
C:\Temp>java -cp . a.b.c.Hello
Avoid "putting" the classfiles anywhere. The following should work:
javac -d c:\temp c:\temp\a\b\c\Hello.java
# creates Hello.class in c:\temp\a\b\c
java -cp c:\temp a.b.c.Hello
To expand on BalusC's point: the classpath defines a "root". When java is looking for your classes, it will start at each root (or jar) in your class path and drill down through the directories to match the package strucutre. You still need to have you class in a directory structure that matches its package name. In your case, to execute
java -cp .\a\target a.b.c.Hello
you would move the file to
.\a\target\a\b\c\Hello.class
Years ago, I too found this baffling.
Java will try to search for a directory structure a\b\c from starting in target and as you notice, it wont work.
Move the whole directory into target and you'll be fine, it should look like:
C:\Temp\a\target\a\b\c\Hello.class
You may compile it with the -d option which tall the compiler where to put the class file.
Many project structures are like this.
C:\whatever\projectname\src
C:\whatever\projectname\classes
C:\whatever\projectname\bin
C:\whatever\projectname\lib
C:\whatever\projectname\doc
That way you can always step on your project directory and type:
javac -d classes src\*.java
Which will compile all the sources in the src directory and will place them in the classes directory.
Then execute your program:
java -cp classes a.b.c.Hello
You may optionally place required jars in lib
This works pretty fine for small programs ( < 10 src files and 2 - 3 jar libraries ) If it grows beyond that, you could probably use an IDE or ant
The good thing about following this project structure is that some IDES ( as IntellJ idea ) just pick them very easily when you create a new project. You select "Create project from existing sources" and then you can continue from there.
I like compiling and editing at the command line a lot!!