i got a run-time error on executing a file 'complement.java' that read
D:\java\files\java complement
Error:Could not find or load main class complement
the class structure follows as below
import java.io.*;
class billion
{
.
.
.
//class definition
}
what could be the cause for the prompt mentioned above?
First of all, this question can be easily answered by 5 seconds of quick googling, supposing you dont know java.
Anyway, the error only shows when your main class is missing a main method, which start the whole program. The main method is
public static void main(String[] args) {}
Add this to your main class, and start it again.
File name and class name need to be same. And the class you mention in java command should have a main method.
public static void main(String[] args) {}
Related
I have run into an error. I think my code is right. here it is... please tell me what I have done wrong!!!
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, World");
}
}
The error I keep getting is...
Error: Could not find or load main class javatest.java
The file name of the Java Class file must be the same one as your Class.
Class: public class HelloWorld {}
File: HelloWorld.java
The code seems right. Perhaps the issue in how you run it.
The convention is to keep the file name and the public class name as the same.
When compiling, use javac <filename>.java. Then to run simply use java <classname>. So, you would use java HelloWorld, HelloWorld being the class that contains the main method.
I have a simple groovy file as follows:
class test2 {
public static void main(String[] args) {
println("In groovy!!");
}
}
My gradle task is compiling this into a test2 class file
How do I run this file from prompt ?
java test2 (from the location of the test2.class file)
causes a : Error: Could not find or load main class test2.class
I assuming I need to add asm and groovy to the class path. However:
java -cp "groovy-all-2.3.6.jar;asm-all-3.3.1.jar" test2
also doesn't work (files are in correct locations).
I know this may be a bit late for the OP but nevertheless:
Given that your groovy main exists, the error message:
Error: Could not find or load main class YOUR_MAINCLASS_HERE
from the java command while executing a groovy main (of a compiled groovy file which produced classes) means basically that your groovy jar is not in the classpath.
Longer Answer:
Lets see why that is for a simple hello world example. I have a file called main.groovy with the following content:
class Main {
static void main(String[] args){
println('hello world')
}
}
Put this somewhere in your filesystem. Open a command prompt in the same directory and ensure that groovy and java is accessable though the PATH.
In the command prompt, compile the file with groovyc, so just type in:
groovyc main.groovy
This will produce a file called Main.class (with uppercase M because of the class name).
Ok now we have the appropriate test setup. If you now try to run the file just with java command:
java Main
you will get the error message:
Error: Could not find or load main class Main
This is a bit unexpected, because one can think that we just can invoke the main in our Main.class without linking the groovy library, so we would expect an exception like ClassNotFoundException.
In contrast, try again with groovy in your classpath. I will refer to the directory of your groovy installation as GROOVY_HOME. To run the hello world Main class finally, we can type in:
java -cp ".:/$GROOVY_HOME/lib/*" Main
which produces the expected output on unix-like systems (on windows you need to replace the colon with a semicolon and the variable access would be like %GROOVY_HOME%).
The reason is quite simple: Groovy produces for groovy main methods not the same signature as required by the java specification. Therefore, you can only invoke a groovy main with groovy on the CLASSPATH - what makes totally sense!
You can check this for yourself. Try now the command:
javap Main.class
This will give you a quick analysis of the bytecode and the present interfaces of the class "Main.class". All along you will see something similar to this output:
Compiled from "main.groovy"
public class Main implements groovy.lang.GroovyObject {
public static transient boolean __$stMC;
public Main();
public static void main(java.lang.String...);
protected groovy.lang.MetaClass $getStaticMetaClass();
public groovy.lang.MetaClass getMetaClass();
public void setMetaClass(groovy.lang.MetaClass);
public java.lang.Object invokeMethod(java.lang.String, java.lang.Object);
public java.lang.Object getProperty(java.lang.String);
public void setProperty(java.lang.String, java.lang.Object);
}
Of interest is line 5:
public static void main(java.lang.String...);
This seems quite similar to a normal java main, but with one difference: groovyc used an java.lang.String ellipsis (as stated by the three dots) and not and java.lang.String[].
So this could be the reason. I'm not so sure, because normally java will give you an appropriate error output if it can find the class but not the method signature. For example, try:
java java.lang.Integer
which has clearly not a main method. Java sees that correctly:
Error: Main method not found in class java.lang.Integer, please define the main method as:
public static void main(String[] args)
or a JavaFX application class must extend javafx.application.Application
I'm also not sure, what groovy does during class loading for understanding this kind of main signature (or lets says this kind of bytecode), but if you compare that with a normal java hello world javap output, you get
public class JMain {
public JMain();
public static void main(java.lang.String[]);
}
which has a different and the normal java main signature.
Maybe someone from the pivotal groovy team can clarify.
I hope this will give you a hint.
The test2.class needs to be on your CLASSPATH. For example, if it is at /Users/you/classes/test2.class then /Users/you/classes/ needs to be on your CLASSPATH.
Since you are building with Gradle, you could also just let Gradle sort all of that out for you using JavaExec. See http://www.gradle.org/docs/current/dsl/org.gradle.api.tasks.JavaExec.html for more info. A simple example in your build.gradle might be something like this:
task myTask(type: JavaExec, dependsOn: 'classes') {
main = 'test2'
classpath = sourceSets.main.runtimeClasspath
}
I hope that helps.
I have looked through many answers to similar questions. But couldn't narrow down to a solution.
Following is the code: (Simplifying names for readability)
First class:
package p1;
public class C1 {
public static void test() {
System.out.println("Boom!");
}
}
Second class:
package p2;
import p1;
public class C2 {
public static void main(String[] params) {
C1.test();
}
}
Clean-Build doesn't give any error. (No compilation error)
But at runtime I'm getting following error:
Exception in thread "main" java.lang.NoSuchMethodError: C1.test()V
at C2.main(C2.java:6)
Java Result: 1
P.S. I'm using Netbeans.
This means that you are running your class C2 with an old version of class C1 in the classpath (a version that did not yet have the test() method).
Make sure you don't have old versions of C1.class somewhere. Remove all your *.class files and recompile everything, and then try to run it again.
Addition: As Kevin Bowersox noted in a comment, your main method must look like this:
public static void main(String[] args)
It must take a String[] as an argument.
It will properly compile and run only if main function will have String tab as args.
But also check versions of class C1 and C2, try rebuild your project to recompile that classes.
public static void main(String args[]) {
C1.test();
}
i think you should import it as
import p1.*;
Than you will get access to all classes and member functions in it.
Netbeans sometimes likes to get stuck after some changes and clean build doesn't work then.
Try editing each file that has been recently modified and saving it again (e.g. put a whitespace in a random place). After that, clean and build the project again.
If my memory refreshes and as Jesper pointed out, I also encountered that same issue NoSuchMethodFoundException under that same scenario (having still old class references that have not been cleaned).
I just copied your code snippets with 2 different packages directly in to my netbean, compiled and runned C2. It did print the BOOM! message.
In my case using :
public static void main(String args[]){
}
does not make a difference when I compiled and runned the code.
public static void main(String params[]){
}
It makes sense since the main class should have the correct method signature of main.
Here args or params, should not make a huge difference, I believe; as what we have inside the method is simply a reference for the inner body of the method that it uses.
Still definitely it is good practice to follow the standard signature for main.
I would recommend to clean the project and copy the contents from scratch in a new project and build it again, sometimes netbeans can go crazy.
I have the following code as part of an assignment
class Base {
public static void main(String[] args){
System.out.println("Hello World");
}
}
public class Factorial extends Base{
}
My task is run the code and then explain the output.The name of the file is Factorial.java. The code runs without problem and Hello World is printed which to me is surprising. Before typing the code, I was thinking that it wont compile because the parent class, which is being extended should be in another file but now I am not so sure. Would appreciate soome clarification.
Java allows you to define multiple classes within a single .java file with the condition that you can have at most one public class and if you do then the name of that public class must match the name of the .java file. In your case, the class declared public is Factorial and hence your file name has to be Factorial.java.
The inheritance is working as usual here and the public static void main() is inherited by Factorial which is why you see your output on executing java Factorial.
You can have more than one class in the same file, but only one public , as Base isn't a public class, but it's not a recommended practice.
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Closed 10 years ago.
Possible Duplicate:
Exception in thread “main” java.lang.NoSuchMethodError: main
I got the above message. The code is as follows:
class Test
{
public static void main(String ar[])
{
printf("hai");
}
}
How is this problem caused and how can I fix it?
The class which you're trying to execute doesn't have a main method.
Since your main method looks syntactically fine, this can have two causes:
You're executing the wrong class.
The actual class file doesn't contain this code.
The solution is obvious:
Make sure that your command is pointing the correct class file, you might have multiple class files with the same name and be sitting in the wrong directory.
Make sure that you've compiled the correct source file into the correct class file before, you might have edited one and other and forgot to recompile.
In addition to the problem that's causing the current exception (see BalusC's answer), the proper "Hello World" in Java is:
class Test
{
public static void main(String[] args) {
System.out.println("hai");
}
}
See: java.lang.System
I see your problem, the signature is not correct.
It should be public static void main(String[] args)
It could also be a class path issue which causes Eclipse to get confused and not able to find your class when it tries to run it. I would look at the Java Build Path in the Project Properties to make sure there are no errors.