I'm having problems when trying to compile the following code:
I first tried compiling with this code:
class HelloWorldApp {
public static void main(String[] args) {
System.out.println("Hello World!"); // Display the string.
}
}
it works as it should. Now, if I try to replace the class name for any other name, it won't work anymore, as it seems to always look after HelloWorldApp. I made sure the file is being saved and so, I even reopened Eclipse. Still the same error. Maybe this is a common problem, with a small work-around?
Thanks
edit: I see what you guys mean, but why does it work when I have the file name as "Main.java" and a class name of "HelloWorldApp" ?
You need to rename your .java file to match the class name.
Eclipse will rename your .java file to match your class name automatically if you use its refactoring support. Right click on the class name, hover over Refactor, and select the Rename option. Now when you rename your class, Eclipse will automatically rename your .java file to match.
This is how Java works. Class must have the same name as the file. So the filename for the class MyApp, must be MyApp.java
Rename main as HelloWorldApp so that your app becomes HelloWorldApp.java.
When changing the name of your class use the REFACTOR option - If you try to edit the name manually yourself the Eclipse Project loses track of your objects.
Related
I have been working on an assignment for my class in programming. I am working with NetBeans. I finished my project and it worked fine. I am getting a message that says "No main class found" when I try to run it. Here is some of the code with the main:
package luisrp3;
import java.io.FileNotFoundException;
import java.io.PrintStream;
public class LuisRp3 {
public static void main(String[] args) throws FileNotFoundException {
java.io.File newFile = new java.io.File("LuisRamosp4.txt");
if (newFile.exists()) {
newFile.delete();
}
System.setOut(new PrintStream(newFile));
Guitar guitar = new Guitar();
I posted this before but had a couple issues. i have fixed the others and now have just this one remaining. Any advice will be greatly appreciated.
Right click on your Project in the project explorer
Click on properties
Click on Run
Make sure your Main Class is the one you want to be the entry point. (Make sure to use the fully qualified name i.e. mypackage.MyClass)
Click OK.
Run Project :)
If you just want to run the file, right click on the class from the package explorer, and click Run File, or (Alt + R, F), or (Shift + F6)
Also, for others out there with a slightly different problem where Netbeans will not find the class when you want when doing a browse from "main classes dialog window".
It could be that your main method does have the proper signature. In my case I forgot the args.
example:
public static void main(String[] args)
The modifiers public and static can be written in either order (public static or static public), but the convention is to use public static as shown above.
Args: You can name the argument anything you want, but most programmers choose "args" or "argv".
Read more here:
http://docs.oracle.com/javase/tutorial/getStarted/application/
When creating a new project - Maven - Java application in Netbeans
the IDE is not recognizing the Main class on 1st class entry. (in Step 8 below we see no classes).
When first a generic class is created and then the Main class is created Netbeans is registering the Main class and the app could be run and debugged.
Steps that worked for me:
Create new project - Maven - Java application
(project created: mytest; package created: com.me.test)
Right-click package: com.me.test
New > Java Class > Named it 'Whatever' you want
Right-click package: com.me.test
New > Java Main Class > named it: 'Main' (must be 'Main')
Right click on Project mytest
Click on Properties
Click on Run > next to 'Main Class' text box: > Browse
You should see: com.me.test.Main
Select it and click "Select Main Class"
Hope this works for others as well.
The connections I made in preparing this for posting really cleared it up for me, once and for all. It's not completely obvious what goes in the Main Class: box until you see the connections. (Note that the class containing the main method need not necessarily be named Main but the main method can have no other name.)
I had the same problem in Eclipse, so maybe what I did to resolve it can help you.
In the project properties I had to set the launch configurations to the file that contains the main-method (I don't know why it wasn't set to the right file automatically).
In project properties, under the run tab, specify your main class.
Moreover, To avoid this issue, you need to check "Create main class" during creating new project. Specifying main class in properties should always work, but if in some rare case it doesn't work, then the issue could be resolved by re-creating the project and not forgetting to check "Create main class" if it is unchecked.
If the advice to add the closing braces work, I suggest adding indentation to your code so every closing brace is on a spaced separately, i.e.:
public class LuisRp3 {
public static void main(String[] args) throws FileNotFoundException {
// stuff
}
}
This just helps with readability.
If, on the other hand, you just forgot to copy the closing braces in your code, or the above suggestion doesn't work: open up the configuration and see if you can manually set the main class. I'm afraid I haven't used NetBeans much, so I can't help you with where that option is. My best guess is under "Run Configuration", or something like that.
Edit: See peeskillet's answer if adding closing braces doesn't work.
There could be a couple of things going wrong in this situation (assuming that you had code after your example and didn't just leave your code unbracketed).
First off, if you are running your entire project and not just the current file, make sure your project is the main project and the main class of the project is set to the correct file.
Otherwise, I have seen classmates with their code being fine but they still had this same problem. Sometimes, in Netbeans, a simple fix is to:
Copy your current code (or back it up in a different location)
Delete your current file
Create a new main class in your project (you can name it the old one)
Paste your code back in
If this doesn't work then try to clear the Netbeans cache, and if all else fails, then just do a clean un-installation and re-installation of Netbeans.
In the toolbar search for press the arrow and select Customize...
It will open project properties.In the categories select RUN.
Look for Main Class.
Clear all the Main Class character and type your class name.
Click on OK.
And run again.
The problem is solved.
If that is all your code, you forgot to close the main method.
Everything else looks good to me.
public class LuisRp3 {
public static void main(String[] args) throws FileNotFoundException {
java.io.File newFile = new java.io.File("LuisRamosp4.txt");
if (newFile.exists()) {
newFile.delete();
}
System.setOut(new PrintStream(newFile));
Guitar guitar = new Guitar();
}}
Try that.
You need to add }} to the end of your code.
You need to rename your main class to Main, it cannot be anything else.
It does not matter how many files as packages and classes you create, you must name your main class Main.
That's all.
import java.util.Scanner;
public class FarenheitToCelsius{
public static void main(String[]args){
Scanner input= new Scanner(System.in);
System.out.println("Enter Degree in Farenheit:");
double Farenheit=input.nextDouble();
//convert farenheit to celsius
double celsuis=(5.0/9)*(farenheit 32);
system.out.println("Farenheit"+farenheit+"is"+celsius+"in celsius")
{
I also experienced Netbeans complaining to me about "No main classes found". The issue was on a project I knew worked in the past, but failed when I tried it on another pc.
My specific failure reasons probably differ from the OP, but I'll still share what I learnt on the debugging journey, in-case these insights help anybody figure out their own unique issues relating to this topic.
What I learnt is that upon starting NetBeans, it should perform a step called "Scanning projects..."
Prior to this phase, you should notice that any .java file you have with a main() method within it will show up in the 'Projects' pane with its icon looking like this (no arrow):
After this scanning phase finishes, if a main() method was discovered within the file, that file's icon will change to this (with arrow):
So on my system, it appeared this "Scanning projects..." step was failing, and instead would be stuck on an "Opening Projects" step.
I also noticed a little red icon in the bottom-right corner which hinted at the issue ailing me:
Unexpected Exception
java.lang.ExceptionInInitializerError
Clicking on that link showed me more details of the error:
java.security.NoSuchAlgorithmException: MD5 MessageDigest not available
at sun.security.jca.GetInstance.getInstance(GetInstance.java:159)
at java.security.Security.getImpl(Security.java:695)
at java.security.MessageDigest.getInstance(MessageDigest.java:167)
at org.apache.lucene.store.FSDirectory.<clinit>(FSDirectory.java:113)
Caused: java.lang.RuntimeException
at org.apache.lucene.store.FSDirectory.<clinit>(FSDirectory.java:115)
Caused: java.lang.ExceptionInInitializerError
at org.netbeans.modules.parsing.lucene.LuceneIndex$DirCache.createFSDirectory(LuceneIndex.java:839)
That mention of "java.security" reminded me that I had fiddled with this machine's "java.security" file (to be specific, I was performing Salvador Valencia's steps from this thread, but did it incorrectly and broke "java.security" in the process :))
Once I repaired the damage I caused to my "java.security" file, NetBeans' "Scanning projects..." step started to work again, the little green arrows appeared on my files once more and I no longer got that "No main classes found" issue.
Had the same problem after opening a project that I had downloaded in NetBeans.
What worked for me is to right-click on the project in the Projects pane, then selecting Clean and Build from the drop-down menu.
After doing that I ran the project and it worked.
Make sure the access modifier is public and not private. I keep having this problem and always that's my issue.
public static void main(String[] args)
i have a ready to use .jar file and want to know if its possible to extract and rename the packages?
so when usually i start the .jar file with:
java -cp myFile.jar com.codehelper.demo.Main
i want to rename the "codehelper" in it to something different that i can run it by
java -cp myFile.jar com.NEW_NAME.demo.Main
i tried to decompile all files, add it to the folderstructure with renamed "codehelper" path and compile it again but it didnt work. i also renamed all the package includes in each file like
import com.codehelper...
so is my goal unreachable or can i do this? and if someone can explain me how to do, it will be very nice.
thank you and sory for my poor english
edit: it seems the only file i cant compile is a file containing this switch case.
private int priotiryLevel(DiscoveryInfoBehave info)
{
int ret = 0;
switch (1.$SwitchMap$com$peerialism$natcracker$common$GatewayDevice$GatewayType[info.getNatDevice().getGatewayType().ordinal()])
{
case 1:
ret = 0;
break;
case 2:
ret = 4;
break;
case 3:
ret = 5;
break;
}
return ret;
}
i tried also to rename the specific word inside this switch case but no effort.
Write a new wrapper class:
package com.NEW_NAME.demo;
public class Main
{
public static void main(String[] args)
{
com.codehelper.demo.Main.main(args);
}
}
compile it and add it to the jar. You can now invoke it as:
java -cp myFile.jar com.NEW_NAME.demo.Main
and it will silently dispatch to the real implementation.
This is not possible generally. If you rename some files, Java wont be able to find them (public class name must be same as file name). You can rename file with main class and call it as #Joe suggested in his answer. But if you rename somthing else, it will stop working. There could be cals to those classes in code. Same goes for the "codehelper" name. You can not remove it from code. Even if you remove it from one file, anyone will still be able to see this somewhere else in the code.
Rename directory is the same as rename file. You destroy namespace (package) and code will no longer work, because inside classes, this package is used. Plus there is no need to have import in the code, since you can call directly com.package-name.class from the executive code. By renaming package, you will destroy this and program will crash. It may run for a while, but once the program reach to this call, it will crash.
So this
import com.codehelper...
is not mandatory in the code, even if the code is using the package. You can write directly
com.codehelper.*** xy = new com.codehelper.***();
Even if you rename everything in the code, you still dont have guaranted functionality. Code may be using reflection and create class instances from sting code. For example see this:
Java how to instantiate a class from string
Under the line comment:
Doing some decompile -> compile work, is seems like code stealing, if you are not willing to pay licence and you want to hide it.
Plus doing something like this, it is ALWAYS a bad practice. I dont see any real use for this.
I can run programs which do not have a package without any hitch. If I try and add a package then java simply cannot find them. I have set the class path and I have tried running - java packagename.ProgramName.
I have found a number of similar threads on here and have spent four hours going through all of them and trying everything and nothing works for me.
Yet as soon as I edit the .java file and recompile without a package heading - it immediately works perfectly. Why? And how can I fix it? I would like to be able to have my classes organised in packages!
This is the code I am using (I normally use eclipse and just wrote this to try out cmd out of curiosity).
package hello;
public class HelloWorldApp{
public static void helloWorld(){
System.out.println("Hello world");
}
}
and
package hello;
public class HelloBackApp{
public static void helloBack(){
System.out.println("Hello back");
}
public static void main(String[] args){
HelloWorldApp.helloWorld();
helloBack();
}
}
As I say if I delete both the package heading java HelloBackApp runs just fine.
And my path to my program is
c:\Users\sam\javastuff\hello
I have of course tried java hello.HelloBackApp from both the javastuff dir and the hello dir. No joy
It works immediately if I delete both the package headings and type java HelloBackApp from the hello directory.
try as follows,
create folder structure as your package and place java file in that folder
For ex, my java file is under
c:\code\com\test\Test.java and package is "package com.test".
I compiled and run code from
c:\code>
c:\code> javac com\test\Test.java
c:\code> java com.test.Test
Ok after much research I realised what my problem was and have fully resolved it. I think I see why I have been unable to find an "answer" to this question in forums. It is not a simple quick fix - my whole understanding of how to correctly get the class path set up and get a proper compile done was very poor. It becomes a whole new subject if you switch from compiling/running on an IDE to doing so from the command line. I think it is an excellent thing for new programmers to do though as I believe the improved understanding of CLASSPATH is going to be something that will stand us all in good stead for the future.
I found all the answers to my questions here : http://www.ibm.com/developerworks/java/library/j-classpath-windows/
and recommend anyone having similar problems I was having to read through this excellent document. Best wishes to all the other guys struggling with this out there! :)
I can not get some simple Java code to run in Eclipse. My code is as follows:
class DrinkAgeTest{
static int age =14;
public static void main(String[] args){
if (age >=21){
System.out.println("you may drink");
}
else {
System.out.println("you may not drink,come back in " + (21-age));
}
}
}
My class created in the source folder is named MathLearning. When I try to run my code I get the following error message:
Error: Could not find or load main class MathLearning
Is it because my class DrinkAgetest is different from the class file name, which is named MathLearning?If so how do I fix it. I couldn't find a rename button on the src right click menu.
which class have main that is must be public..
public class DrinkAgeTest{
}
How many source files do you have?
What is in them?
If it's just one - say "A.java" make sure it defines "public class A" in itself.
As said by others already, the class name must match the file name. Then go to
a command prompt and do "javac A.java". This will produce file A.class.
Then again from a command prompt do "java A". This will run your A.class file.
"javac" is the Java compiler
"java" is the JVM
Renaming the class file is not the way to do.
The class name and the name of the file containing the class have to be the same.
If you are using eclipse, it should fix it for you automatically.
To rename your class:
Right click on the class DrinkAgetest in your src folder, and click Refactor. Then click Rename...
If renaming the class doesn't fix the problem (it shouldn't fix it), then try what subash said:
change class DrinkAgeTest to public class DrinkAgeTest.
There could be other problems, possibly your compiler is looking for the wrong class altogether because you deleted it or something.
I am trying to run this project called "hello user". I am new to Java, so wrote a simple program that takes your name, and displays "Hello ". while Running it, I get the following error:
run:
Error: Could not find or load main class hello.world.HelloWorld
Java Result: 1
BUILD SUCCESSFUL (total time: 0 seconds)
But when I run file HelloWorld.java, it does it fine
I am doing this on Netbeans IDE 7.2
Rather than the coding error, it could be related to IDE. Since the "Run File" runs okay, but 'Run Project" does not, I believe you have something to set up in IDE itself. Right click the project, and select "Set is as Main", now run the project. I am just giving it a guess, may not help you. But it worth a shot.If it does not help, please paste your code too.
Your class needs a public static void main(String[] args) function. And moreover I suspect that the error could be in the package.
If you want your class in <main_package>.<sub_package>, The directory structure is
- main_package
- sub_package
-HelloWorld.java
And be sure to write your class like this.
package main_package.sub_package;
public class HelloWorld {
public static void main(String[] args){
System.out.println("Hello " + args[o]);
}
}
This is all due to the naming convention in Java
You need to run the .class file containing the public static void main(String[] args) method..
Here, your HelloWorld.java file might contain a class with main() method.. So, you can run it..
This is because, execution of any Java program starts with the invocation of main().. JVM needs an entry point to your code.. Which is main().. If it doesn't find one.. It will not run..
So, make sure, whatever class file you are running, it should have main() method..
UPDATE :- And for the starting point, may be you can skip using packages.. Just go with plain Java class without packages..
This message can also appear in Eclipse (Juno 4.2.2 in my case) and I have found two potential causes for it.
In my cases:
1. a DTD was in error. I deleted the file and that solved the issue*.
2. having cleaned the project, an external Jar that I had built externally had been deleted as could be seen from Properties -> Java Build Path -> Libraries.*
*Having solved either of the above issues, it was necessary to restart Eclipse
if you are using intellij idea then just rebuilding (clean and build) project might solve your problem . because intellij might be still trying to load the old classes which are not there or changed
Make sure you call looks like below:
public class HelloWorld {
public static void main(String[] args) {
System.out.println("hello user");
}
}
To run a Java class in stand alone mode, public static void main(String[] args) is the entry method, which is must.