Some background: I'm a student just learning Java, and usually the professor handles making sure our assignments have the right class path. However, the whole semester I've been plagued by the same problem, and I still don't understand what's going wrong.
As an example, I have two files, MyProgram.java located in the folder "MyProject" and MyProgramTest.java located in the folder "ClassProject", which also contains "MyProject". MyProgramTest creates a MyProgram object and lets you test its functionality.
MyProgramTest has the line import MyProject.MyProgram; The compiling instructions my instructor gives is to use javac MyProject/*.java while in "ClassProject" which works fine. Then, we are to use javac MyProgramTest.java in the same directory. However, the compiler claims:
import MyProject.MyProgram;
bad class file: .\MyProject\MyProgram.class
class file contains wrong class: MyProgram
Please remove or make sure it appears in the correct subdirectory of the classpath.
So I don't quite understand why this is happening. MyProgram is in the MyProject directory, and that directory is in the folder I'm in. Since the instructor uses this exact method to compile these programs, I keep getting screwed since mine never compile correctly. Any idea what I'm doing wrong, or how I can fix the file to compile this way without changing the structure of the directories?
Ensure this:
The MyProgram.java file should contain this line at the top of the file:
package MyProject;
Compile MyProgram.java from the ClassProject folder:
javac -d . MyProject/*.java
Then Compile MyProgramTest.java from the same folder:
javac MyProgramTest.java
This will create the class files correctly in the appropriate folder structure.
This should solve your problem.
Hope this helps!
Related
I have a pile of .java files. They all have the same class name public MyClass. They all have a main method. They all may or may not have a package declaration at top, and I do not know ahead of time.
I am trying to write a script to compile and run these java programs. This is easy for the files without the package declaration... I just do some cp operations to setup, javac MyClass.java and java MyClass, then rm to teardown. However, the files with the package declaration require special attention. I have a few options that occur to me, including deleting the package lines, or attempting to read the package lines so that I know what the resulting directory structure should be. Both of these require me to go parsing through the .java files, which makes me sad.
Is there a way to compile and run these files without having to parse the .java files? Something like:
javac --ignore_package_structure MyClass.java
would be ideal, but a quick look at the javac man pages suggests that such a thing doesn't exist.
If we can assume that each student submits a single source file named HelloWorld.java, then we can use the "Launch Single-File Source-Code Programs" feature added by JEP 330 in Java 11:
java HelloWorld.java
We don't run javac first, we don't get a .class file (no cleanup needed), and any package declaration is handled automatically.
Remember, the students are still allowed to use many classes, they just all have to be submitted to you in a single source file.
The name of the class doesn't even matter. The first class in the source file is executed.
There isn't any easy way to do this. You could use regex though, and replace all imports with this simple java regex:
"package \w+;"g
Simply stated, you create a Java program to replace all the package names.
How to replace files: Find and replace words/lines in a file
I'm a C++ developer - not a java developer, but have to get this code working...
I have 2 public classes that will be used by another product. I used the package directive in each of the java files.
package com.company.thing;
class MyClass ...
When I try to compile a test app that uses that I add
import com.company.thing.*;
The javac compiler fails with errors about com.company does not exist. (even if I compile it in the same directory as the class files I just made a package of)
I am sure I am doing something bone-headed and silly.
I've read the http://java.sun.com/docs/books/tutorial/java/package/usepkgs.html pages and tried to set up a directory structure like /com/company/thing etc, but either I have totally screwed it all up or am missing something else.
EDIT
thanks for the suggestions - I had tried the classpath previously. It does not help.
I tried compiling
javac -classpath <parent> client.java
and the result is:
package com.company does not exist
I have the code I want to import (the two java files) in \com\company\product. I compile those fine. (they contain MyClass) I even made a jar file for them. I copied the jar file up to the parent directory.
I then did (in the parent directory with the client java file)
javac -cp <jarfile> *.java
the result is:
cannot access MyClass
bad class file: MyClass.class(:MyClass.class)
class file contains wrong class: com.company.product.MyClass
Please remove or make sure it appears in the correct subdirectory of the classpath.
EDIT
I got the client code to compile and run if I used the fully qualified name for MyClass and compiled it in the parent directory. I am totally confused now.
compiled with no classpath set - just
javac *.java
in the parent directory - and it worked fine.
I can get a test app to compile, but that is not going to cut it when i have to integrate it into the production code. Still looking for help.
EDIT:
Finally - not sure why it didn't work before - but I cleaned up all the files all over the directory structure and now it works.
Thanks
Okay, just to clarify things that have already been posted.
You should have the directory com, containing the directory company, containing the directory example, containing the file MyClass.java.
From the folder containing com, run:
$ javac com\company\example\MyClass.java
Then:
$ java com.company.example.MyClass
Hello from MyClass!
These must both be done from the root of the source tree. Otherwise, javac and java won't be able to find any other packages (in fact, java wouldn't even be able to run MyClass).
A short example
I created the folders "testpackage" and "testpackage2". Inside testpackage, I created TestPackageClass.java containing the following code:
package testpackage;
import testpackage2.MyClass;
public class TestPackageClass {
public static void main(String[] args) {
System.out.println("Hello from testpackage.TestPackageClass!");
System.out.println("Now accessing " + MyClass.NAME);
}
}
Inside testpackage2, I created MyClass.java containing the following code:
package testpackage2;
public class MyClass {
public static String NAME = "testpackage2.MyClass";
}
From the directory containing the two new folders, I ran:
C:\examples>javac testpackage\*.java
C:\examples>javac testpackage2\*.java
Then:
C:\examples>java testpackage.TestPackageClass
Hello from testpackage.TestPackageClass!
Now accessing testpackage2.MyClass
Does that make things any clearer?
Yes, this is a classpath issue. You need to tell the compiler and runtime that the directory where your .class files live is part of the CLASSPATH. The directory that you need to add is the parent of the "com" directory at the start of your package structure.
You do this using the -classpath argument for both javac.exe and java.exe.
Should also ask how the 3rd party classes you're using are packaged. If they're in a JAR, and I'd recommend that you have them in one, you add the .jar file to the classpath:
java -classpath .;company.jar foo.bar.baz.YourClass
Google for "Java classpath". It'll find links like this.
One more thing: "import" isn't loading classes. All it does it save you typing. When you include an import statement, you don't have to use the fully-resolved class name in your code - you can type "Foo" instead of "com.company.thing.Foo". That's all it's doing.
It sounds like you are on the right track with your directory structure. When you compile the dependent code, specify the -classpath argument of javac. Use the parent directory of the com directory, where com, in turn, contains company/thing/YourClass.class
So, when you do this:
javac -classpath <parent> client.java
The <parent> should be referring to the parent of com. If you are in com, it would be ../.
You got a bunch of good answers, so I'll just throw out a suggestion. If you are going to be working on this project for more than 2 days, download eclipse or netbeans and build your project in there.
If you are not normally a java programmer, then the help it will give you will be invaluable.
It's not worth the 1/2 hour download/install if you are only spending 2 hours on it.
Both have hotkeys/menu items to "Fix imports", with this you should never have to worry about imports again.
The standard Java classloader is a stickler for directory structure. Each entry in the classpath is a directory or jar file (or zip file, really), which it then searches for the given class file. For example, if your classpath is ".;my.jar", it will search for com.example.Foo in the following locations:
./com/example/
my.jar:/com/example/
That is, it will look in the subdirectory that has the 'modified name' of the package, where '.' is replaced with the file separator.
Also, it is noteworthy that you cannot nest .jar files.
Just add classpath entry ( I mean your parent directory location) under System Variables and User Variables menu ...
Follow : Right Click My Computer>Properties>Advanced>Environment Variables
So I'm completely new to programming, and I've been writing some Java with the NetBeans IDE. My code runs fine within NetBeans, but I've tried to run it using the command line as well. However, if I run it from the command line, I have to delete the line:
package firstprogram;
which NetBeans automatically places at the top of each new file, or I get the error:
Error: Could not find or load main class FirstProgram
However, if I do delete the line, then the program no longer runs within NetBeans! It doesn't seem right that I have to choose whether to run a .java from within NetBeans or without.
The research I've done makes me think that this is something to do with directory structure? But everything I read on that goes straight over my head. NetBeans has a structure with "build", "dist", "nbproject", and "src", but when I use the command line I just place the .java file in an empty directory and javac from there.
Any explanation is appreciated! The books and tutorials I'm learning from either assume you're just using NetBeans or don't have the package line at all.
You can compile your class using javac command from anywhere, as long as you provide correct relative or absolute path. The problems come when you want to run your program using the java program.
You have to provide the correct path corresponding to your package declaration. For example, if I had 'MyClass' in package mypackage, first line would look like this:
package mypackage;
class source stored on disk:
c:/MyNetbeansProject/src/mypackage/MyClass.java
Compiled bytecode:
c:/MyNetbeansProject/build/classes/mypackage/MyClass.class
Now, if I would have opened a command prompt/terminal in folder c:/MyNetbeansProject/build/classes/, I could run the program using java mypackage/MyClass or java mypackage.MyClass.
However, if i would be somewhere else, I would have to say where the class files are located using the cp option: java -cp c:/MyNetbeansProject/build/classes mypackage/MyClass. The path in cp option can be relative or absolute, use "" when it contains spaces.
Package are directory architecture.
If your class is in the package com.acme.test, the class should be in the com/acme/test directory.
Instead of placing your class in an empty folder, place it in a folder named firstprogram and do javac firstprogram/youclass.java
The package (and folder) permit you to arrange your architecture with logical pattern.
More info here : http://www.tutorialspoint.com/java/java_packages.htm
So like OcterA said, you should keep organized, but with only one class this is not the issue. I believe that your problem is that you are not entering the correct command into the command line.
First cd to the correct directory and when you want to execute a file within a package in that directory you need to enter
java packageName.className
In this case
java firstprogram.FirstProgram
I hope this question is not repeated. But just can't find answer anywhere:
I have ONE folder containing two files one A.java another B.class.
Now in A.java I am trying to declare
public class A extends Applet{
...
B aB;
}
The compiler gives me:
B cannot be resolved to a type
I read a lot of posts that say if the files are in the same folder, I don't need to import. Could anyone help me to "resolve" this problem?
Thanks much appreciated!
-----------SOLVED! - SEE ANSWER BELOW------------------
The .class files need to reside in a directory referenced by the classpath variable. Usually you put your .java files in one directory (src), compile to another directory (bin) and have external .class files in a third directory (lib). The commands will look like this:
# compile
javac -sourcepath src -classpath lib -d bin
# run
java -classpath bin:lib A
Using an IDE like eclipse should help a lot here as it takes care of most of the details
The simple case that you've posted works for me. I'd check the following things:
Are you sure that B.class is present in the same folder as A.java?
Are you running javac from that folder?
Have you typed the class name B correctly everywhere in your program? This includes capitalization, as Java identifiers are case sensitive.
Are there any package declarations in your program? If there are, none of this is going to work, since you're implicitly using the default package by just throwing everything into a folder.
The compiler looks for *.class file in its class path. It will only look for *.java files in the same source directories. You need to set the class path to include the directory.
Or you could use an IDE which sets all this up for you and saves a lot time in the process.
I have an executable jar that has one class file with a wrong value. I requested that file from the author and corrected the value.
Now I want to compile it and add the compiled class file into the jar and run it.
Not surprisingly I get multiple "cannot find symbol" errors for references of custom object that were in the jar file.
I tried to compile the file by referencing the jar file in the classpath like so
C:/> javac file.java -classpath C:/folder/where/jar/is
but this doesnt seem to work... I get the same errors as if just doing
C:/> javac file.java
Is there a way to compile this individual class somehow referencing the other files in the jar?
Thanks.
Errors I am getting while following some of the suggestions below here:
javac -classpath C:/jar/location.jar File.java
File.java:226: cannot find symbol
symbol : class Stuff
location: class com.shared.stuffers
Stuff s1 = new Stuff();
^
Stuff class is found in the Jar, but can not be seen by the javac program... I feel like I am doing something wrong but not sure where? Thanks.
You will want to compile your file like so:
javac -classpath C:\folder\where\jar\is\the_jar_file.jar file.java
per the javac usage instructions:
C:\Console2>javac -help
Usage: javac <options> <source files>
Once you've compiled the new file (such as in Mr. Will's answer), you can add the new file to the jar using:
jar uf C:\folder\where\jar\is\the_jar_file.jar file.class
You probably have to specify the JAR file itself and not just the directory it resides in.
javac file.java -classpath C:\folder\where\jar\is\the_jar_file.jar
I'm just guessing, but did you check whether it doesn't need any external jar libraries you may have to include in your compilation command? Another thing you could do is to compile all of the classes by doing something like
javac *.java ...
As others have mentioned, once you have the .class file recompiled you need to replace the older version in the .jar
You'll likely need to have any compile time dependencies available to rebuild this class. If it's an open source project this could be an easy thing to come up with. If not, it's more difficult. If the author sent you the file he can probably help you with this as well. You might be able to get the author to produce a patched distribution for you as well. Odds are he/she already has the build environment set up and this should be relatively easy to do.
I'd try this approach (and it should work, unless the 'debugged' class doesn't introduce a new error):
create a new jar by taking the old
one and deleting the classfile that
you want to replace
compile the
corrected java file and make sure
that the modified jar is on the
classpath
add the newly compiled
classfile to the jar
This should work. If not - ask the author for a new complete library.