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Java program runs yet compilation fails

I wrote a Java program whose filename was (intentionally) different from the class I wrote inside the file. The javac command failed as expected on both CMD and WSL. The java command however worked and ran my print statement. I wrote the code intentionally this way so there is no way it was a previously compiled version of the code. The following code was written in a file called "explainJava.java" (notice the filename is different from the class name).
public class explain{
public static void main(String[] args) {
System.out.println("Java is weird");
}
}

I've had to google this myself, but I think I've found an explanation in this article.
According to that source as of Java 11 java is capable of compiling a single source file into memory.
What I conclude from that: When the file is compiled into memory and not written to disk it obviously cannot have a file name. If there is no filename there is no such thing as a wrong filename, therefore the code executes.
Please also note that the restriction of having to name a file like the public class within that file is more of a design decision to make work for the compiler easier/ faster. It is not a physical restriction so to speak. Have a look at the following thread for more details.

If you put this code:
public class explain {
public static void main(String[] args) {
System.out.println("Java is weird");
}
}
into a file named explainJava.java, and then compile it with this:
javac explainJava.java
you will get an error that correctly informs you that your filename ("explainJava") and the class defined inside that file ("explain") do not match:
explainJava.java:1: error: class explain is public, should be declared in a file named explain.java
public class explain{
^
1 error
If you run this command:
$ java explainJava.java
Java is weird
you see expected output, because you're skipping the explicit compilation step (that is, you aren't running javac first) and instead relying on behavior introduced in Java 11 that allows you to compile+run in a single step. Here's an explanation: Does the 'java' command compile Java programs?
So the answer is to either:
rename your file to match the class, so change the filename to "explain.java", or
rename the class to match the file, change public class explain to be public class explainJava

How to package these two java classes

So right now I have two classes, one of which creates an object out of another class:
import java.io.*;
public class PostfixConverter {
public static void main(String args[]) throws IOException, OperatorException {
...
String postfixLine;
while ((postfixLine = br.readLine()) != null) {
// write some gaurd clauses for edge cases
if (postfixLine.equals("")) {
...
Cpu cpu = new Cpu();
and
public class Cpu {
Cpu() {
// this linkedListStack is for processing the postfix
...
}
Currently I'm running javac PostfixConverter.java to compile the class but it cannot find the Cpu symbol. What can I do so that the compiler can discover the missing symbol? Shouldn't everything by default be packaged in the default package and therefore find each other?

javac PostfixConverter.java
This command should work if both files are located on the current directory, as the default classpath (-cp option) is the current directory (.).

You should compile both files, so that the Cpu class file will be available to PostfixConverter:
javac Cpu.java PostfixConverter.java
Keep in mind that in general it is not desirable to build an application where everything sits in the default package. Consider creating an appropriate package here. Also, you may want to use either an IDE and/or Maven, which would make the build process easier for you.
As the other answer mentions, Java will also automatically build all Java source files in the current directory, which might explain why other source files are also getting built.

Uncompiled java class but still executes

I have following piece of code in one of my java program.
public class Solution {
public static void main(String[] args) {
System.out.print("Hello World");
}
public static void printOutput(String[] arr){
//Note: The semi colon is omitted intentionally.
System.out.print("Hello Incomplete World")
}
When I build it I get a compilation error but still it generates a .class file. when I run the .class file, it gives an output of "Hello World".
How is this possible ? I always believed a .class file having unresolved compilation problems will never be an executable one. Can anyone provide some information on that ?

What's probably happening here is that a successfully compiled class file already exists. When the Java compiler runs, it'll produce a .class file if it compiles a source file successfully, but it will not remove subsequent compilation fails.

You're assuming the compiler will clear out your old class files - it doesn't.
Check the modification date on your current .class file - you'll see that it's older than your compilation. That's because it was generated from working code, not from your current source file. If you delete this class file, then try to recompile, you'll see that a new class file is not created.

The class file you are running is from a previous compilation. Compile your program again and look at the time the class file was previously modified.

Getting "class does not have a static void main method accepting String[]" error even though main signature is correct

My DrJava was working fine, but now I keep getting the folowing error whenever I run anything:
Static Error: This class does not have a static void main method accepting String[].
So it will compile OK, but then it shoots out the error . This happens even though everything I test does indeed have a public static void main(String[] args) in it. It seems like a classpath/resources type of error. I appreciate any tips
EDIT: my class
public class Test{
public static void main(String[] args){
System.out.println(" hashmap ");
}
}

There's nothing wrong with the code, so the problem must be with the environment.
Check that you're actually executing that class. Find out where the class that's executed is specified and check it's correct
Check that you're compiling the class. Maybe the code you're looking at has not been compiled and you're trying to execute an old version that was compild before you coded a main()
Check your classpath. Is the compiled class accessible in the classpath of the java command

You don't need to reinstall java, nor is it a java version issue. It may be the way that your are running the program.
To check if it is a problem with your code, do the following:
Make a new folder and put Test.java in it.
Open up Command Line Or Terminal and change to that folder .
Type javac Test.java. Test.class should be in the folder now.
If you want, open up the class with a text editor. This is what I get:
˛∫æ2
<init>()VCodeLineNumberTablemain([Ljava/lang/String;)V
SourceFile Test.java hashmap Testjava/lang/Objectjava/lang/SystemoutLjava/io/PrintStream;java/io/PrintStreamprintln(Ljava/l ang/String;)V! *∑±
% ≤∂±
Back to the command line or terminal, type java Test.
If you get an error, which you shouldn't, I don't know what to say. It should produce the string " hashmap " on to the command line or terminal.
Why re-installing Dr. Java may not work is because you may be using the same working directory, causing same run settings to be used. Dr. Java may be running an external program, one without a main method.
I think that you should install the Eclipse IDE for Java. It is much easier to get around, it looks nicer, and it runs the file or project that you are looking at currently.

Sometimes this problem happens because may be mistake in saving file.you always your file using double quotes and with the .java extension which is main class means that class containing main method.
you should save your file by class name which is public .if there is two classes and both have main method then you should save your file by class name that is public and that class will be run.As like your compiler looking for main method in public static void main(String [] args) that is contract for jvm to run a programme
so it is not able to found that main method that is static and it looking for your Dr class.java
See this Example it have two main methods and practice these kinds of question.I also got this kind of problem in starting.
public class TestFirst
{
public static void main(String [] args){
System.out.println(" TestFirst ");
}
}
class Test{
public static void main(String [] args){
System.out.println(" hashmap ");
}
}
if you save pro-gramme by "TestFirst.java" then o/p will come TestFirst if you do some mistake in main method because we have saved our programme by TestFirst then you will get error like you got.
# 2nd mistake may be this
debian#debian:~/Geany_java$ javac Test1.java
debian#debian:~/Geany_java$ java Test1
Exception in thread "main" java.util.NoSuchElementException
at java.util.StringTokenizer.nextToken(StringTokenizer.java:349)
at Test1.main(Test1.java:11)
your classpath has not set properly See above Compiling successfully but running showing same kind of error you got.Which OS is using I can guide you properly.

Check that actually your file have the .java termination nor the .dj

There is nothing wrong with the code.
It is the executing environment which might have problem. Please share the details.

Check if program compiled correctly.
Check time-stamo of .class file.
Check permissions on folder/directory where class-files are getting generated.
Check if DrJAVA has appropriate permission on the directory.
Did you create a file, compiled it with out main?
Check class-path. Might be possible that previous class file is still being found by JDK in classpath.
Try compiling .java file from cmdLine instead of editor.

As others have mentioned, your code is fine. There must be a problem with your environment. I recently experienced a similar issue when investigating and answering this question.
Basically, in that question, the code Void.class instanceof Class resulted in a compiler error because a user-made Class.class existed in the classpath, so one Class (the Java built-in java.lang.Class) didn't match with the given Class (user-made).
Something similar may be at work here. It is possible that there is a user-made String.class in your classpath. Then in your main signature, String[] args would mean an array of your String, when Dr. Java must be looking for a main method taking an array of the Java built-in String, i.e. java.lang.String[]. If you have a custom String class in your classpath (or in your project?), then the Java compiler will choose it over the built-in String. If you were to compile and run your Test class from the command line, then you would get the runtime error: Exception in thread "main" java.lang.NoSuchMethodError: main.
Following #S0urceC0ded's suggestion, you may find this when looking at Test.class in a text editor:
main([LString;)V // A user-made String class
instead of what it's supposed to be:
main([Ljava/lang/String;)V // The built-in java.lang.String class
If so, remove your own String class (at least the .class file, but also the .java file so the .class file isn't re-created) from the classpath, and compile and run your Test class again.
Without a look at your environment, I can't tell for sure that this is the issue. But it can explain it.

If you are using Dr.Java as IDE, then you need to make sure that the main class containing 'public static void main' should be at the very top of your program. Otherwise Dr.Java throws this error during runtime.

how to make java use the newest version of a file

I have this problem that i have a program that writes and creates a .java file and puts it in my package folder, after this it takes the information from the .java file and uses it in it self. (it creates a new class with a method that i then import).
The problem is that if it wont work until i with eclipse update the "self created file". is there a way to make my main file update the "self created file".
Sorry if this is a duplicate. I just couldn't find it any where.
my code:
package dk.Nicolai.Bonde;
import java.io.*;
public class main {
public String outputString ="Math.sqrt(25)" ;
static String outputPath ="src/output.txt";
/**
* #param args
* #throws UnsupportedEncodingException
* #throws FileNotFoundException
*/
public static void main(String[] args) throws IOException{
new main().doit(args);
}
public void doit(String[] args) throws IOException{
PrintWriter writer = new PrintWriter("src/dk/Nicolai/Bonde/calculate.java", "UTF-8");
writer.println("package dk.Nicolai.Bonde;");
writer.println("public class calculate{");
writer.println("public void calc(){");
writer.println("System.out.println("+outputString+");");
writer.println("}");
writer.println("}");
writer.flush();
writer.close();
calculate calcObj = new calculate();
calcObj.calc();
}
}

Your main mistake is that you expected that it's during runtime automagically compiled into a .class file after save (which a sane IDE such as Eclipse is doing automatically for you behind the scenes everytime you press Ctrl+S). This is thus not true. During runtime, you need to compile it yourself by JavaCompiler and then load by URLClassLoader. A concrete example is given in this related question&answer: How do I programmatically compile and instantiate a Java class?
You'll in the concrete example also notice that you can't do just a new calculate(); thereafter. The classpath won't be auto-refreshed during runtime or so. You'd need to do a Class#forName(), passing the FQN and the URLClassLoader. E.g.
Calculate calculate = (Calculate) Class.forName("com.example.Calculate", true, classLoader);
Your other mistake is that you're relying on the disk file system's current working directory always being the Eclipse project's source root folder. This is not robust. This folder is not present at all when building and distributing the application. You should instead write the file to a fixed/absolute folder elsewhere outside the IDE project's structure. This is also covered in the aforelinked answer.

No, you cannot. You have to manually update resources in Eclipse. Although you can write a plugin for Eclipse which runs your file and update resources.
Eclipse uses directories and files to store its resources but is not direct representation of file system.

Your code could not work, because
calculate is required at compile time of main. You supply it at runtime.
calculate.java will not compiled, so even other techniques to dynamically load classes will not work
If you want to build classes at runtime, consider to use the reflexion API