How do I call a simple "Hello World" Java program in MATLAB?

I have a simple Java program, and I would like to know how to call it in MATLAB.

Answer 1

[MathWorks Support Team]

Here is an example. This assumes you already have knowledge of Java.

The following is a Java program:

public class HelloWorld {
  public static void main( String args[] )
  {
    System.out.println( "Hello World!" );
  }
}

To call this program in MATLAB:

0. Make sure you have the version of JRE/JDK on your system that is supported with your version of MATLAB. You can get this version by executing this on the MATLAB command prompt:

>> version -java

1. Outside of MATLAB: use Java to compile this class, so you have file HelloWorld.class

2. Locate the classpath.txt file for the MATLAB installation. The location of this file can be found by typing the following command in MATLAB command window:

which classpath.txt

3. Open the 'classpath.txt' with a text editor as Administrator. Add the full path for the directory with the HelloWorld.class to the end of the 'classpath.txt' file as a single line and save the file.

4. Restart MATLAB.

5. In MATLAB: to create an object of class HelloWorld, type:

o = HelloWorld

6. In MATLAB: to execute main() of object o, type:

javaMethod('main', o, '')

Alternately one may also add the directory in which the class files are to the dynamic path. Use the JAVAADDPATH command to add the directory (which contains the HelloWorld.class file) to JAVA's dynamic classpath. This also obviates the need to restart MATLAB. Once this is done the code can be invoked as follows:

o = HelloWorld;
javaMethod('main', o);

For more information on calling Java class from MATLAB, see chapter 3 of the "External Interfaces" documentation. This documentation can be accessed from the following URL:

https://www.mathworks.com/help/pdf_doc/matlab/matlab_external.pdf

[Eric Barnhill]

I would advise that at this time many users will have Java 1.7 on their machine by default, and this won't work (certainly doesn't on Ubuntu 14.04).

To make it work you will have to compile with backwards compatibility and this (at least on Ubuntu 14.04) requires using -source, -target, and -bootclasspath options. The -bootclasspath will require the rt.jar file from the 1.6 JDK. So an example might be:

javac -source 1.6 -target 1.6 -bootclasspath '/usr/lib/jvm/java-6-openjdk-amd64/jre/lib/rt.jar' HelloWorld.java

YMMV...

[Sepehr Farhand]

Fyi, it works without -bootclasspath option for me. Just use:

javac -source 1.6 -target 1.6 HelloWorld.java

[Ruixin Zhou]

Alternative to step 2 and 3:

If you don't have Admin rights, create a javaclasspath.txt in the preferences folder (located at prefdir) then add those desired paths in it.

Check out the link below:

https://www.mathworks.com/help/matlab/matlab_external/static-path-of-java-class-path.html

Answer 2

[sai m]

step 5 is notworking ??? Undefined function or variable 'Helloworld'. please help me

[Titus Edelhofer]

Did you type Helloworld or HelloWorld? Note the capital "W" ...

Titus

[Sunil Nayak]

It's not working....step 5....

[vishwa joshi]

step 5 is not working..Undefined function or variable 'HelloWorld'.

[Elad]

I know it's basic, but I always fall for it - maybe you forgot to addpath to the Java class? (you were suppose to take care of the path on parts 2-3 class-wise, but sometimes just adding the path as a "full MATLAB" path would be easier and cost little)

[Rutuja Shirali]

Please make sure that you are adding the full path for the directory with the HelloWorld.class to the end of the 'classpath.txt' file as a single line and save the file. Also if you are using the JAVAADDPATH command, make sure that you are adding the directory (do not include HelloWorld.class in the path in this case) to JAVA's dynamic classpath.

[Mohammad Reza]

I had the same issue "Undefined function or variable 'HelloWorld'" until I figured out the version of java in matlab and the one used to compile the HelloWorld.java program must be identical.

I have the latest java installed on my machine but Matlab seems to have its own java installation. But it is possible to "tell" matlab to use my version of java by setting an environment variable. So I start matlab as follows:

c:> set MATLAB_JAVA=C:\Program Files\Java\jre1.8.0_45 c:> "C:\Program Files\MATLAB\R2014a\bin\matlab.exe"

jre1.8.0_45 is the one used to compile the HellowWorld.java

And now HelloWorld is reconised as an Object and you can run the following to get the Hello World message

>> o.main('')

Answer 3

[Akshay Jain]

I am trying to call the HelloWorld code listed above from Matlab 2015b, but am unable to do this. I am using eclipse to write and compile the java code on Windows 10. After compiling the code, I have the HelloWorld.class file in /my_package/bin/my_package/ folder. I have tried the following:

- Changed the MATLAB_JAVA environment variable to the java version used in eclipse as described at http://www.mathworks.com/matlabcentral/answers/130359-how-do-i-change-the-java-virtual-machine-jvm-that-matlab-is-using-on-windows. Verified the version update by using java -version command. Tried adding the .class folder location both by static and dynamic method. However, when I run [M,X,J] = inmem command, my java class does not show in the J variable. As a result, when I run o = HelloWorld command, matlab says: Undefined function or variable 'HelloWorld'.

- Also tried compiling the Java code with javaSE-1.7 (the default java version in matlab is Java 1.7.0_60-b19) and removing MATLAB_JAVA environment variable. Still the same problem.

I eventually want to call an opensource library written in Java. But for now, I would be happy if I can at least call the HelloWorld class from Matlab.

Thanks in advance, Akshay

[Akshay Jain]

It was a very simple fix. I was writing the java class as part of the package, but trying to access it as if it wasn't. Earlier, I was using:

javaaddclasspath('/my_package/bin/my_package')

and I changed it to:

javaaddclasspath('/my_package/bin')

and then called the class using o = my_package.my_class

and it worked perfectly.

Akshay

Answer 4

[Supriyo Srimani]

This program just prints Hello world...if i have some logic when user give a input in java could it be executed in matlab??? Please give a example like adding two numbers...

[Prannay Jain]

'javaMethod' function provided by MATLAB also takes arguments as user input. For example, if you have a class 'Addnumber' which contains function 'addtwonum' and 'obj' be its object as follows:

obj = Addnumber;
javaMethod('addtwonum', obj, 1, 2);

This will add 1 and 2.

Refer to this documentation link of 'javaMethod' for more details:

https://www.mathworks.com/help/matlab/ref/javamethod.html

[Mehmet Mahmudoglu]

Addnumber Class must be defined with package otherwise it is not working

>> obj=numberpack.Addnumber

package numberpack;
public class Addnumber {
    public double addtwonum( double a1,double a2)
    {
        return a1+a2;
    }
}

Answer 5

[Vanessa King]

Are we still meant to add the directory to classpath.txt in R2018a? Classpath.txt explicitly says to not modify it as it is autogenerated. Is it adequate to just use the javaaddpath method to add it to the dynamic Java class path?

Answer 6

[Amhar Amhar]

sir i want to design fir filter using java in matlab.please help me inthis