'Error:
Could not find or load main class HelloWorld' error occur when you try to run
the main class HelloWorld, but Java runtime unable to find it.
Let’s try
to produce the error and see the possible options to resolve it.
Step 1:
Create a folder
hierarchy com/sample/app and define HelloWorld.java file in it.
HelloWorld.java
package com.sample.app;
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World");
}
}
Folder
hierarchy looks like below.
$tree
.
|____com
| |____sample
| | |____app
| | | |____HelloWorld.java
Step 2:
Navigate to the
folder, where package com is create and compile HelloWorld.java file by executing below command.
javac com/sample/app/HelloWorld.java
bash-3.2$ tree
.
|____com
| |____sample
| | |____app
| | | |____HelloWorld.java
bash-3.2$
bash-3.2$ javac com/sample/app/HelloWorld.java
bash-3.2$
bash-3.2$ tree
.
|____com
| |____sample
| | |____app
| | | |____HelloWorld.class
| | | |____HelloWorld.java
Step 3:
Run HelloWorld class
file, by executing the command 'java com/sample/app/HelloWorld'.
bash-3.2$ java com/sample/app/HelloWorld
Hello World
Since, Java look into the current directory for the main class, program ran successfully.
How about running HelloWorld class from different directory?
Let's navigate to the root folder and try to run HelloWorld class.
bash-3.2$ cd /
bash-3.2$
bash-3.2$ java com/sample/app/HelloWorld
Error: Could not find or load main class com.sample.app.HelloWorld
We got
this error, since Java class loader unable to find the main
class HelloWorld in the current directory.
How to
resolve this error?
Approach
1: Set the classpath
using CLASSPATH variable. You can read this post for more details.
export
CLASSPATH={path_to_the_parent_of_package_folder}
If you are
using Windows system, use below statement
SET
CLASSPATH={path_to_the_parent_of_package_folder}
In my case, I defined com/sample/app package hierarchy in /Users/krishna/Documents/technical-documents/examples folder.
export CLASSPATH=/Users/krishna/Documents/technical-documents/examples
Once you set the classpath, you can run the HelloWorld class from anywhere.
$export CLASSPATH=/Users/krishna/Documents/technical-documents/examples
$
$cd /
$
$java com/sample/app/HelloWorld
Hello World
You can
even run using . syntax.
$java com.sample.app.HelloWorld
Hello World
Approach
2: Set the class path
using -cp or -classpath option.
$java -cp /Users/krishna/Documents/technical-documents/examples com.sample.app.HelloWorld
Hello World
$
$java -classpath /Users/krishna/Documents/technical-documents/examples com.sample.app.HelloWorld
Hello World
Note
In case if
you are using maven as a build tool for your project, it generate the class
files under project_directory\target\classes folder.
You may like
Interview Questions
Different ways to iterate over a map using for-each loop
How for-each or enhanced for loop works in Java?
How to solve UnsupportedClassVersionError in Java?
How to find the Java major and minor versions from a .class file
Can I run a class file that is compiled in lower environment?
