I have a doubt about -cp and when should I use it. This is my scenario:
I have two .java, the first one:
package autos.tests.paquete;
public class MainAutos {
public static void main(String args[]) {
int x = Integer.parseInt(args[0]);
Respotar objeto1 = new Respotar (x);
int mostrar = objeto1.repostar();
System.out.println(mostrar);
}
}
And the second one:
package autos.tests.paquete;
public class Respotar {
int gasolina;
public Respotar (int gasolina) {
this.gasolina=gasolina;
}
public int repostar (){
int gasolina = this.gasolina +20;
return gasolina;
}
}
Well, I am at root directory, and there, I have that directory: autos/tests/paquete
with both .java.
So I compile:
javac autos/tests/paquete/*.java
And execute from root directory:
java autos.tests.paquete.MainAutos 10
And it works, now here go my doubts:
1) I execute with java -cp . autos.tests.paquete.Main autos 10 and the behaviour is the same.
2) I move the Respotar.class from auto/tests/paquete to another directory, I compile with
java autos.tests.paquete.MainAutos 10 and it works.
3) I move the MainAutos.class from auto/tests/paquete to another directory, I compile with
java autos.tests.paquete.MainAutos 10 and it says: Error: Could not find or load main class autos.tests.paquete.MainAutos
4) I compile with java -cp . autos.tests.paquete.MainAutos (I have the .class on the current directory I am compiling, so I think I have to use -cp .) and it says the same:
Error: Could not find or load main class autos.tests.paquete.MainAutos
Thank you in advance, I hope someone can enlighten me, regards
java -cp is used to set any libraries such as jar file to your current classpath.
For the examples you have shown, you can do it in the below simple way
From you root directory compile your java files using the below command
javac -d . *.java
This would created those appropriate packages and place the class files under them
Then run you code using the command like this from the same root location
java autos.tests.paquete.MainAutos
Related
I'm a relative Java newbie so apologies if the question appears somewhat basic. I've googled high and low for an answer here and I'm not finding anything that's helping.
Problem:
Whilst I'm able to integrate external packages into my Java programs from an IDE environment, I am trying to do run a very basic program from the command line that calls on a separate, basic package file that I have written - and am simply doing all this as I want to have a bottom-up understanding of how package files are related to a main program by Java.
I have a program that sits on my desktop named MyProgram.java:
import org.somepackage;
public class MyProgram {
public static void main(String arguments[]) {
System.out.println("Programme up and running...");
Human myHuman = new Human();
myHuman.scream();
}
Still on the Desktop, I then have another folder which I've named src, inside of which I have created the necessary subfolders corresponding to the package name, i.e. ./src/org/somepackage - and in this location, I have the Human.java file which defines the Human class with the following contents:
package org.somepackage;
public class Human {
public void scream() {
System.out.println("I exist!!");
}
}
I then created a classes folder, again on the Desktop, and ran the following compile command on the command line:
javac -d ./classes/ ./src/org/packagename/Human.java
This ran fine and created - as expected - the Human.class file within the ./classes/org/packagename/ location.
However, where I fall down is when I then try to compile MyProgram.java on the command line, i.e.
javac -cp ".:./classes/" MyProgram.java
As you'll see, my class path contains a reference to the current location (".") for the MyProgram.java file, and it contains a reference to the classes folder ("./classes/") which is the base location for the org.somepackage package inside whose subfolders (./classes/org/somepackage/) on can find the Human.class file.
At this stage, I was simply expecting the java engine to compile MyProgram.java into the program MyProgram.class - but, instead, I get an error:
MyProgram.java:1: error: package org does not exist
I've been following the instructions listed here:
https://www3.ntu.edu.sg/home/ehchua/programming/java/J9c_PackageClasspath.html
and I don't appear to be deviating from the instructions - yet I'm unable to locate an explanation on Stackoverflow or anywhere else as to a possible reason for this compile failure. If anyone has an idea, your help would be very much appreciated.
Thanks,
Your mistake is here
import org.somepackage; <--
public class MyProgram {
public static void main(String arguments[]) {
System.out.println("Programme up and running...");
Human myHuman = new Human();
myHuman.scream();
}
you forgot to import class actually, you need to write this name
import org.somepackage.Human; import all package content import org.somepackage.*; or write full qualified name of class in your code
org.somepackage.Human myHuman = new org.somepackage.Human();
myHuman.scream();
correct mistake:
import org.somepackage.Human;
public class MyProgram {
public static void main(String arguments[]) {
System.out.println("Programme up and running...");
Human myHuman = new Human();
myHuman.scream();
}
after that compile your Human.java by this command:
javac -d classes Human.java
and MyProgram.java
javac -d classes -cp "classes" MyProgram.java
and run MyProgram by
java -cp "classes" MyProgram
I am trying to create a package of library functions for a main Java program but I am have some issues.
I don't know much about about Java packages and I am going through some documentary online.
I have created my directory as such
./Program/Program.java
./Program/TestFunc.java
./Program/classes/library/
The contents of TestFunc.java are
package library;
public class TestFunc {
public void message01() {
System.out.println("called message01");
}
public void message02() {
System.out.println("called message02");
}
}
I compiled it as I read in the documentation
javac -d ./Program/classes TestFunc.java
Which gives me
./Program/classes/library/TestFunc.class
Then I try to call it in Program.java
import library.*;
public class Program {
public static void main(String[] args) {
System.out.println("Starting Script");
}
}
When I try to compile using
javac -d ./Program/classes Program.java
I get the error
package library does not exist
Why is this?
You've used -d which says where to put the output, but you haven't told it that the same directory should also be used for input on the classpath. Use the -cp option for that:
javac -d classes -cp classes Program.java
(It's not clear whether you're trying to do this from inside the Program directory, or above it - your source filename appears to be inside the Program directory, but you're specifying the output directory as if you were in the directory above...)
I am using enthuware to practice mock questions for classpath and packages. Here is the question.
//in file ./Foo.java
public class Foo {
public static void main(String[] args) {
System.out.println("In Foo");
}
}
//in file ./com/Foo.java
package com;
public class Foo {
public static void main(String[] args) {
System.out.println("In com.Foo");
}
}
Which of the given statements are correct assuming that the command lines are executed from the current directory with default classpath set to ./classes?
The options given are
Executing java -classpath .:./com Foo will print "In com.Foo"
Executing java -classpath ./com:. Foo will print "In com.Foo"
Executing java Foo will print "In com.Foo"
java -classpath . com.Foo will not execute.
Executing java -classpath ./com:. com.Foo will print "In com.Foo"
Correct option given is option-5. The strange problem is when i try to execute option-5 from my command line it gives me the following error.
Can someone tell me what is wrong? I am not able to figure out the reason. Plus what does this
./com classpath mean?
Command Change
I noticed one strange thing, if i change the classpath and run the command as
java -classpath . com.Foo
It states in com.Foo. As soon as i change the command to add this path ./com. It gives the above mentioned error.
Thanks.
The "./com" is a relative path to the current path.
If you have your class in [current_path]/com/Foo.class
You can run your class with the following command:
java -classpath ./com com.Foo
If you are in the com folder [current_path: com]/Foo.class you can execute the following command:
java -classpath . com.Foo
Is it possible to first compile a set of Java source code files into bytecode, and later run that bytecode-- not directly, but by adding commands to another java program-- such that this new java program (in its various classes/functions) runs the previously compiled java bytecode?
If this is possible, then what is/are the required command(s) to do this?
Absolutely - and that's what libraries are all about! They're typically distributed as jar files, but you don't have to use jar files in order to reuse the code.
All you need to do is make sure that it's on the classpath at both compile-time and execution time.
For example, create the following files:
lib\p1\Hello.java:
package p1;
public class Hello {
public static void sayHi(String name) {
System.out.println("Hi " + name + "!");
}
}
app\p2\Greeter.java:
package p2;
import p1.Hello;
public class Greeter {
public static void main(String[] args) {
Hello.sayHi(args[0]);
}
}
Now let's compile our "library":
$ cd lib
$ javac -d . p1/Hello.java
$ cd ..
And now, by adding that to the classpath, we can use it in our "app":
$ javac -d . -cp ../lib p2/Greeter.java
$ java -cp .:../lib p2.Greeter Jon
Hi Jon!
(This all works on Windows with the one change of using ";" instead of ":" in the joint classpath on the last line.)
Im using Java SDK 1.7 on Windows 7 via cmd.exe . Up until a few hours ago everything was working correctly when suddenly I was unable to run my compiled class files, consistently presented with the error in the title.
I seem to be able to compile my My.java file however I am unable to run the resulting class file (My.class). I am constantly given the error "Error: Could not find or load main class My.class". I have tried this with multiple other class files all resulting in the same problem.
My 'Path' environment variable is set to "C:\Program Files (x86)\Java\jdk1.7.0_05\bin" if you were wondering
I have tried reinstalling, creating and setting a classpath variable (no luck), and even directly using the
java -cp . My.class
command.
I have tried these posts all to no avail, hence why I'm posting:
Error: Could not find or load main class
Error: Could not find or load main class- Novice
Could not find or load main class
Java 1.7.0_03 Error: Could not find or load main class
If it makes any difference my code is :
import javax.swing.JOptionPane;
class My {
public static void main(String[] args) {
final double x = 3.2;
int i = (int)x;
double m = 0;
if (x < 4) {
String saySomething = JOptionPane.showInputDialog(i);
System.out.println(saySomething);
}
else {
String saySomething = JOptionPane.showInputDialog(i);
System.out.println("Hello World");
}
while (m < 10) {
System.out.print(" While Loop ");
m++;
};
for (i=1; i < 10; i++) {
System.out.println("For Loop");
};
}
}
You should specify the classname instead of the file of the class to load. The difference is a simple matter of removing the .class extension.
I would use an IDE and you shouldn't get these issues. Compile and run is just a click of the mouse.
BTW to run your program from the command line
java -cp . My
You don't add .class
Position yourself in a directory of your project (you need to have src and bin directories there, assuming you keep sources in src and binaries in bin)
java -cp bin My
I myself was facing the same problem. It was happening because I was being remiss in typing the name of the class properly. In my instance, I was typing
java doubler
instead of
java Doubler