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
Related
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.
I ran into library loading problems after creating a jar from my code via maven. I use intelliJ idea on Ubuntu. I broke the problem down to this situation:
Calling the following code from within idea it prints the path correctly.
package com.myproject;
public class Starter {
public static void main(String[] args) {
File classpathRoot = new File(Starter.class.getResource("/").getPath());
System.out.println(classpathRoot.getPath());
}
}
Output is:
/home/ted/java/myproject/target/classes
When I called mvn install and try to run it from command line using the following command I'm getting a NullPointerException since class.getResource() returns null:
cd /home/ted/java/myproject/target/
java -cp myproject-0.1-SNAPSHOT.jar com.myproject.Starter
same for calling:
cd /home/ted/java/myproject/target/
java -Djava.library.path=. -cp myproject-0.1-SNAPSHOT.jar com.myproject.Starter
It doesn't matter if I use class.getClassLoader().getRessource("") instead. Same problem when accessing single files inside of the target directory instead via class.getClassLoader().getRessource("file.txt").
I want to use this way to load native files in the same directory (not from inside the jar). What's wrong with my approach?
The classpath loading mechanism in the JVM is highly extensible, so it's often hard to guarantee a single method that would work in all cases. e.g. What works in your IDE may not work when running in a container because your IDE and your container probably have highly specialized class loaders with different requirements.
You could take a two tiered approach. If the method above fails, you could get the classpath from the system properties, and scan it for the jar file you're interested in and then extract the directory from that entry.
e.g.
public static void main(String[] args) {
File f = findJarLocation("jaxb-impl.jar");
System.out.println(f);
}
public static File findJarLocation(String entryName) {
String pathSep = System.getProperty("path.separator");
String[] pathEntries = System.getProperty("java.class.path").split(pathSep);
for(String entry : pathEntries) {
File f = new File(entry);
if(f.getName().equals(entryName)) {
return f.getParentFile();
}
}
return null;
}
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.
I've got the problem that the following code snip returns null:
System.out.println(Logic.class.getResource("effects\\newball.wav"));
I have a source folder in my project called effects. in this folder there's the referred file. I think there's a syntax error... Because THE FILE IS THERE. I must refer in this way (means with getResource) to my file because I will export it as jar later.
Thank you
Your effect directory should be a direct child of the src dir. Also in which case, you need a / to start the string path. So you would need this
System.out.println(Logic.class.getResource("/effects/newball.wav"));
ProjectRoot
src
effect
newball.wav
What I normally do using an IDE is just create a new package and name it whatever I want the file to be - in your case "effect". It's easier that way.
UPDATE
"I did it exatly so, but it still returns null"
It works fine for me
package stackoverflow;
public class Test {
public static void main(String[] args) {
System.out.println(Test.class.getResource("/effects/stack_reverse.png"));
}
}
Output: file:/C:/Android/workspace/StackOverflow/bin/effects/stack_reverse.png
Resource paths should use forward slashes, regardless of the filesystem on the machine you are using: try "effects/newball.wav"
See http://docs.oracle.com/javase/7/docs/technotes/guides/lang/resources.html (Under "resources, Names, and Contexts -- "The name of a resource is independent of the Java implementation; in particular, the path separator is always a slash (/).")
I am new to java and to the eclipse IDE.
I am running Eclipse
Eclipse SDK
Version: 3.7.1
Build id: M20110909-1335
On a windows Vista machine.
I am trying to learn from the book Thinking in Java vol4.
The author uses his own packages to reduce typing. However the author did not use Eclipse and this is where the problem commes in..
This is an example of the code in the book.
import java.util.*;
import static net.mindview.util.print.*;
public class HelloWorld {
public static void main(String[] args) {
System.out.println("hello world");
print("this does not work");
}
this is the contents of print.Java
//: net/mindview/util/Print.java
// Print methods that can be used without
// qualifiers, using Java SE5 static imports:
package net.mindview.util;
import java.io.*;
public class Print {
// Print with a newline:
public static void print(Object obj) {
System.out.println(obj);
}
// Print a newline by itself:
public static void print() {
System.out.println();
}
// Print with no line break:
public static void printnb(Object obj) {
System.out.print(obj);
}
// The new Java SE5 printf() (from C):
public static PrintStream
printf(String format, Object... args) {
return System.out.printf(format, args);
}
} ///:~
The error I get the most is in the statement.
Import static net.mindview.util.print.*;
On this staement the Eclipse IDE says it cannot resolve net
also on the
print("this does not work");
The Eclipse IDE says that the class print() does not exist for the class HelloWorld.
I have been trying to get these to work, but with only limited success, The autor uses another 32 of these packages through the rest of the book.
I have tried to add the directory to the classpath, but that seems to only work if you are using the JDK compiler. I have tried to add them as libraries and i have tried importing them into a package in a source file in the project. I have tried a few other things but cant remember them all now.
I have been able to make one of the files work, the print.java file I gave the listing for in this message. I did that by creating a new source folder then making a new package in that foldeer then importing the print.java file into the package.
But the next time I try the same thing it does not work for me.
What I need is a way to have eclipse load all these .java files at start up so when I need them for the exercises in the book they will be there and work for me, or just an easy way to make them work everytime.
I know I am not the only one that has had this problem I have seen other questions about it on google searches and they were also asking about the Thinking In Java book.
I have searched this site and others and am just not having any luck.
Any help with this or sugestions are welcome and very appreciated.
thank you
Ok I have tried to get this working as you said, I have started a new project and I removed the static from the import statement, I then created a new source folder, then I created a new package in the source folder. Then I imported the file system and selected the the net.mindview.util folder.
Now the immport statement no longer gives me an error. But the the print statement does, the only way to make the print statement work is to use its fully qualified name. Here is the code.
import net.mindview.util.*;
public class Hello2 {
public static void main(String[] args) {
Hello2 test = new Hello2();
System.out.println();
print("this dooes not work");
net.mindview.util.Print.print("this stinks");
}
}
The Error on the print statement is:
The method print(String) is undefined for the type Hello2
and if I try to run it the error I get is:
Exception in thread "main" java.lang.Error: Unresolved compilation problem:
The method print(String) is undefined for the type Hello2
at Hello2.main(Hello2.java:6)
The Statement::::: net.mindview.util.Print.print("this stinks") is the fully qualified print statement and it does not throw an error but it does totally defeat the purpose of the print.java file..
If you have any questions please ask Ill get back to you as soon as I can.
I've had similar issues. I solved it by following the steps below:
Click File->New->Java Project. Fill in UtilBuild for the ProjectName. Chose the option "Use project folder as root and click 'Finish'.
Right-click on UtilBuild in the PackageExplorer window and click New->package. For the Package Name, fill in net.mindview.util
Navigate within the unzipped Thinking In Java (TIJ) folder to TIJ->net\mindview\util. Here you will find all the source code (.java) files for util.
Select all the files in the net\mindview\util folder and drag them to the net.mindview.util package under UtilBuild in Eclipse. Chose the 'Copy Files' option and hit 'OK'.
You will probably already have the 'Build Automatically' option checked. If not, go to Project and click 'Build Automatically'. This will create the .class files from the .java source files.
In Eclipse, right-click on the project you were working on (the one where you couldn't get that blasted print() method to work!) Click Properties and Java Build Path->Libraries. Click 'Add Class Folder...' check the box for UtilBuild (the default location for the .class files).
I think the confusion here arises due to CLASSPATH. If you use Eclipse to build and run your code then Eclipse manages your CLASSPATH. (You don't have to manually edit CLASSPATH in the 'Environment Variables' part of your computer properties, and doing so changes nothing as far as Eclipse Build and Run are concerned.)
In order to call code that exists outside your current project (I will name this 'outside code' for convenience) you need to satisfy three things:
A. You need to have the .class files for that code (as .class files or inside a JAR)
B. You need to indicate in your source code where to look for the 'outside code'
C. You need to indicate where to start looking for the 'outside code'
In order to satisfy these requirements, in this example we:
A. Build the project UtilBuild which creates the .class files we need.
B. Add the statement import static net.mindview.util.Print.*; in our code
C. Add the Class Folder library in Eclipse (Java Build Path->Libraries).
You can investigate the effect of Step C by examining the .classpath file that lives directly in your project folder. If you open it in notepad you will see a line similar to the following:
<classpathentry kind="lib" path="/UtilBuild>
You should combine this with your import statement to understand where the compiler will look for the .class file. Combining path="/UtilBuild" and import static net.mindview.util.Print.*; tells us that the compiler will look for the class file in:
UtilBuild/net/mindview/util
and that it will take every class that we built from the Print.java file (Print.*).
NOTE:
There is no problem with the keyword static in the statement
import static net.mindview.util.Print.*;
static here just means that you don't have to give specify the class name from Print.java, just the methods that you want to call. If we omit the keyword static from the import statement, then we would need to qualify that print() method with the class it belongs to:
import net.mindview.util.Print.*;
//...
Print.print("Hello");
which is slightly more verbose than what is achieved with the static import.
OPINION:
I think most people new to Java will use Eclipse at least initially. The Thinking in Java book seems to assume you will do things via command line (hence it's guidance to edit environment variables in order to update CLASSPATH). This combined with using the util folder code from very early in the book I think is a source of confusion to new learners of the language. I would love to see all the source code organised into an Eclipse project and available for download. Short of that, it would be a nice touch to include the .class files in just the 'net/mindview/util' folder so that things would be a little easier.
U should import package static net.mindview.util not static net.mindview.util.Print
and you should extend the class Print to use its method.......
You should remove the static keyword from your import decleration, this: import static net.mindview.util.print.*; becomes this: import net.mindview.util.print.*;
If that also does not work, I am assuming you did the following:
Create your own project;
Start copying code directly from the book.
The problem seems to be that this: package net.mindview.util; must match your folder structure in your src folder. So, if your src folder you create a new package and name it net.mindview.util and in it you place your Print class, you should be able to get it working.
For future reference, you should always make sure that your package decleration, which is at the top of your Java class, matches the package in which it resides.
EDIT:
I have seen your edit, and the problem seems to have a simple solution. You declare a static method named print(). In java, static methods are accessed through the use of ClassName.methodName(). This: print("this dooes not work"); will not work because you do not have a method named print which takes a string argument in your Hello2 class. In java, when you write something of the sort methodName(arg1...), the JVM will look for methods with that signature (method name + parameters) in the class in which you are making the call and any other classes that your calling class might extend.
However, as you correctly noted, this will work net.mindview.util.Print.print("this stinks");. This is because you are accessing the static method in the proper way, meaning ClassName.methodName();.
So in short, to solve your problem, you need to either:
Create a method named print which takes a string argument in your Hello2 class;
Call your print method like so: Print.print("this stinks");
Either of these two solutions should work for you.
In my case I've dowloaded and decompressed the file TIJ4Example-master.zip. in eclipse workspace folder. The three packages : net.mindview.atunit, net.mindview.simple and net.mindview.util are in this point of the project :
and java programs runs with no problems (on the right an example of /TIJ4Example/src/exercises/E07_CoinFlipping.java)