Develop Reference

How to run multiple java class files from command prompt?

I have multiple java files in a single directory that are related to a single program (Main). I have been using the following in the command prompt,
javac -cp . game/textbased/*.java
java -cp . game/textbased/Main
All related files are in a package called game.textbased. The parent folder series is textbasedgame/src/game/textbased.
I am not sure what is going wrong, the java files seem to compile as nessessary, however, when I try to run I get the following:
Exception in thread "main" java.io.FileNotFoundException: items.dat (The system cannot find the file specified)
at java.base/java.io.FileInputStream.open0(Native Method)
at java.base/java.io.FileInputStream.open(FileInputStream.java:216)
at java.base/java.io.FileInputStream.<init>(FileInputStream.java:157)
at java.base/java.io.FileInputStream.<init>(FileInputStream.java:111)
at java.base/java.io.FileReader.<init>(FileReader.java:60)
at game.textbased.WonderLand.loadItems(WonderLand.java:310)
at game.textbased.WonderLand.initGame(WonderLand.java:63)
at game.textbased.WonderLand.<init>(WonderLand.java:24)
at game.textbased.Main.main(Main.java:6)
I am really not sure what is going wrong, and how to run the files from here. I have read other stack overflow inqueries of a similar nature, and generally they say to go from a higher directory but that has not been working for me. I tried going from outside src, or from within the textbased folder, but only going from within the src folder has given me this particular error.
Thanks!

Java runs in eclipse and will compile, but wont execute on cmd, but still runs in eclipse. How can I get it to execute in cmd?

So I have a basic hello world set up in eclipse and I can compile it using cmd easily (I have set all the necessary paths), however when I then try to use the java command to execute the hello world, it always returns the same error:
Error: Could not find or load main class helloWorld
Caused by: java.lang.NoClassDefFoundError: net/codejava/helloWorld (wrong name: helloWorld)
This is the code used:
package net.codejava;
public class helloWorld {
public static void main(String[] args) {
System.out.println("Hello World");
}
}
I am cd in the right directory (I think, I cd into the src directory and then into the package file stored in src) and am using Windows 10 with java 18.0.1 and JRE build 18.0.1+10-24
Any help would be greatly appreciated, as this is highly frustrating, when the code runs effortlessly on the eclipse console. Thanks.

Your file has a 'package' of net.codejava and a name of helloWorld, meaning, the full name of this class is net.codejava.helloWorld.
The java command, at least in the way you're using it, requires that you pass the full name, thus, you must run java net.codejava.helloWorld. Just java helloWorld simply isn't going to work.
But that's not all.
Java needs to then find the class file that contains the code for class net.codejava.helloWorld. It does this by first turning that full name into a path of sorts: net/codejava/helloWorld.class, and it will then scan each entry in the classpath for that. You can put directories and jar files on the classpath.
Thus, you have a directory on your system; let's call this directory X. X contains a directory named net, which contains a directory named codejava, which contains a file named helloWorld.class. If there is no such X (i.e. your class file is not in a dir named codejava for example), you're going to have to fix that by making these directories.
Then, X (and not the codejava dir!) needs to be on the classpath. Usually (it depends on how you configured things), 'the current dir' is by default on the classpath.
Given that your code is in, say, /home/PythonSux/workspace/learningjava/net/codejava/helloWorld.class, that means the dir that needs to be on the classpath is /home/PythonSux/workspace/learningjava. After all, if you, from there, look for net/codejava/helloWorld.class, you find the right file.
Therefore, either cd to that directory, or run java -cp /home/PythonSux/workspace/learningjava net.codejava.helloWorld
Note that this isn't usually how you actually run java apps. You either run them from your IDE, or you ask your build tool to run it, or you package your java app into a jar file and run that, etcetera.

Get project directory from java classes

I have a problem with System.getProperty("user.dir") giving different directory when run by IDE and when I manually compile & run it in cmd. My thing is this, I have project structure like this:
project
- exports
- src
- main
- java
- Main
- file1
- file2
One of the args in main method is the name of one of those 2 files, that I then access.
When I configure my run in IDE it works like a charm - the directory I get is C:\Users\**\**\**\project and it is able to read and write to the file.
But when I compile it in cmd javac Main.java and then run it, I get C:\Users\**\**\**\project\src\main\java and because of that, I am unable to access the file without having to modify the path.
My question is, is there like a golden way, that would work for both these cases, without me having to alter the returned path?
EDIT:
For clear understanding, I know what System.getProperty("user.dir") returns, but my question was, if it is possible to get the same result somehow with using Path or if I have to get the path and edit it, so that it will end in project directory?
in IDE I get: C:\Users\petri\Desktop\CZM\bicycle-statistics
in cmd: C:\Users\petri\Desktop\CZM\bicycle-statistics\src\main\java
I want to get the same path in cmd, that I got in IDE.
I tried using Paths.get("").toAbsolutePath(), but it is the same thing.
So, what I did is this:
Path path = Paths.get("").toAbsolutePath();
while (!path.endsWith("project")) {
path = path.getParent();
}
And it works, but I am trying to ask, if there is some more elegant way, because I will have to defend my solution in front of my supervisor.

Normally your IDE will build source files in src/main/java and write the class files out to some other directory, like target/classes.
If your IDE built the project that way, then you can run it from the command line by switching to your project directory (cd C:\Users\**\**\**\project using your example) and then running:
java -classpath target/classes Main
assuming that target/classes is where your IDE put the files. If you really do have the class files in the source directory, then use -classpath src/main/java.
If you always run the program from the project directory, then you can assume within the program that the current directory is the project directory. You don't even have to use user.dir then, just use relative path names for everything, e.g., path/to/whatever.dat will automatically resolve to C:\Users\**\**\**\project\path\to\whatever.dat.

One of the args in main method is the name of one of those 2 files
Then make sure you enter the name correctly.
E.g. if the current working directory is the project folder, then name file1 will refer to the file1 file. If the current working directory is the java folder, then the argument to the program needs to be ..\..\..\file1.
That is because you give relative file names, which means they are relative to the current working directory.
Alternatively, give a fully qualified name, then the argument will be the same, regardless of what the current working directory is:
C:\Users\**\**\**\project\file1

Problems running Java from Atom

I currently don't have a working way to edit and run Java on my computer, so I'm trying to get Atom working with Java (I realize it's not a Java IDE and I'm not trying to make it one, I just want to be able to do some light Java work on my laptop). I've installed the script and instant-build packages for Atom and wrote the following test code in a file called "main.java' in my project folder:
class Main{
public static void main(String[] args) {
System.out.println("please");
}
}
When I try to run the code with cmd+i (I'm on a 2012 MacBook Pro) and get the following error message:Error: Could not find or load main class main.
I'd be happy to provide any further information; thanks for helping!

The huge problem of learning Java is that you need to launch the projects in a very strict way, and setting the classpath is always problematic. The solution lays in the following (pretty enigmatic) line of the "Script" documentation:
Project directory should be the source directory; subfolders imply packaging.
So, instead of opening a plain file, open the project (folder) with .java classes or define inside the file, the package to which your .java belong.
It's due to the fact that JDE needs to create a virtual target in form of .classess and single .java file definitely can't be launched as standalone file. I suppose that "Script" is not able to locate the source folder when you try to execute seperate .java file.
Before launching your .java files always "Add Project Folder..."
Please remember that it's not possible to have several folders opened if they don't belong to the same project. Such situation cause problems of locating the right classpath and in the end javac prompts the error.

You have to name your file with the first letter in upper case Main.java, since it must match the name of your class

I replicated the issue quite easily. If I created a new directory in Atom itself and then tried to run the code it didn't work as your error message came up for me as well. Error: Could not find or load main class main.
I used an existing directory and then created a file inside that folder in Atom and ran the same code it worked. I then copied and pasted that same file into the directory of my choice and it worked.

How to get current path when program is loaded dynamically

I'm not able to give this question an apt title so apology for that.
I am making a modularised application. I load various jar files at runtime and invoke particular method of a particular class (of the jar file) at run time.
The jar file has some supported file. Now my jar file uses another application , lets say abc which is located in the same directory where i have kept the jar file. When i run the jar file then
new File(".").getAbsolutePath()
gives the correct path (this is where abc is located) and program runs fine. But when i load this jar file dynamically and invoke method using reflection above code gives the path of the parent program and abc is not found at that path.
Now my question is how do i find the path in which my jar file exists when i'm running my jar file's code using reflection.
Please let me know if you need more explanation.

Try something like this:
public static void main(String[] args) {
System.out.println(StringUtils.class.getResource("StringUtils.class"));
}
(Note: StringUtils is present on my classpath as a Maven dependency at the time) This gives me:
jar:file:/home/******/.m2/repository/org/apache/commons/commons-lang3/3.4/commons-lang3-3.4.jar!/org/apache/commons/lang3/StringUtils.class
Since the class is in a JAR file, it also gives me the location of the class file within the JAR.