how to pass java output as a matlab variable? - java

I have written a Java program which is invoked using system() function, thus it runs on the command window of Matlab. Now I want to know if there's another way to run a Java program other than running it on command window? Can it be run on any user made GUI in Matlab? Another problem is, I want to know if my program has some string value as output, which is generally displayed on command window, how can i store it in variable in Matlab?
Hope to hear from you very soon.

The Hello World solution by The MathWorks provides some insights on how to run a simple 'Hello World' java application inside MATLAB. You may change the Java code a bit, in order to have a method that returns a String.
public class HelloWorld
{
public String hello()
{
String helloWorld = "Hello World!";
return helloWorld;
}
}
Once this simple class is compiled and on the MATLAB JVM classpath create an instance and invoke the method with the following two commands.
o = HelloWorld
output = o.hello;
The String returned by the HelloWorld instance is assigned to the MATLAB variable output.
There is no need for a system command with Java code in MATLAB. You have direct access to the JVM from inside MATLAB. For an application with a complex GUI, break out to Java.
Undocumented Java is a valuable source on MATLAB, Java and GUIs.

Yes the classpath set is correct.
I modified the code, using it without main..
class HelloWorld
{
public String Hello()
{
String helloWorld="Hello World!";
return helloWorld;
}
}
Now, as per guided i try to create instance obj in Matlab, with following command:
o = HelloWorld;
Here I get following err:
??? No constructor 'HelloWorld' with
matching signature found.
The next command indicated it this:
output = o.hello;
which wouldnt work unless instance is created.

Related

Read c++ console output from method call using Java

Currently, I am trying to read the console output of my program when there is a method call. How it works is, my Java program calls the JNI wrapper to invoke the C++ method call. My C++ is using std::cout. The payload.invoke will invoke my c++ library API. Inside the c++ API there are a few cout statements. That is the statement I want to read it as a variable in Java.
The current PrintStream only supports out, print, println and printf.Each time when there is a method call, there are bunch of logs being printed out in command prompt. Now, I would like my java program to read those output which my C++ prints out.
This is my current Java code:
public void execute() {
try {
Matcher m = Pattern.compile("([^()]*)[(]([^()]*)[)]").matcher(getPayloadString());
m.find();
boolean passed;
if (m.group(2).isEmpty()) {
// this is where it invoke the method
passed = (boolean) payload.invoke(testcase);
System.out.println("Boolean is: " + passed + "\n");
}else {
passed = (boolean) payload.invoke(testcase, ObjectCreator.convertParameters(m.group(2)));
System.out.println("Boolean2 is: " + passed + "\n");
}
String output = passed ? "PASS" : "FAIL";
// I want to use the console output string variable here -> xxxx
TestCase.printResult(payload.getName(), output, m.group(2), xxxx, "");
} catch (Exception ex) {
ex.printStackTrace();
}
}
Are there any ways to do it? Previously I tried using one of the tutorial from here. but that seems not working. Can someone help me out with this please. Thanks in advance.
If you have full control of the .cpp codebase it is possible for you to change the std::cout to become a ostream& instead.
By referencing a base class you can easily switch the implementation.
Do a JNI-Stream whichh extends the ostream& and just overwrite the operators you need to send JNI wrapped callbacks to java.
Then depending on if you run native or Java style use your JNI-Stream instead of the std::cout.
I would do something as seen in the link below to package and send the data to Java:
JNI - How to callback from C++ or C to Java?
that is my 2 cents.
Read carefully the JNI tutorial.
https://www.baeldung.com/jni
Follow the tutorial.
Basically you need to create a java class with a static block to load the C++ library:
static {
System.loadLibrary("native");
}
Then you can create your own static java method and invoke a method on the library.
If what you need is read the output from a system call, it's a different matter.
This can help you:
Java Runtime.getRuntime(): getting output from executing a command line program

How to run java class with custom variables from terminal

My scenario:
I have a Main.java file that simply does System.out.println("Hello").
I run it by first, compiling with javac Main.java and then excecuting the command java Main.
Now what I want is that instead of printing "Hello", it will print whatever the user wants, but I don't want to change the source code whenever I want a different output. So I replaced the System.out.println("Hello") with System.out.println(${MESSAGE}). But this gives error "Cannot resolve symbol MESSAGE".
Ultimately, I want a Main.class file and run with something like java Main -env MESSAGE=whateverIPutHere and it should print out whateverIPutHere.
Is it possible?
You can use system properties
public final class Test {
public static void main(String[] args) {
System.out.println(System.getProperty("port") + " port");
}
}
And then compile and run
javac Test.java
java -Dport=8080 Test
Output is : 8080 port
Now what I want is that instead of printing "Hello", it will print whatever the user wants, but I don't want to change the source code.
Simply not possible without changing code.
System.out.println("Hello")
Prints that string. End of story. And:
System.out.println(${MESSAGE})
is simply not valid Java. If you want to read an environment variable, see here how to do that.
But then: that is really a detour here. You can simply pass arguments on the command line:
java Main "some string" "and another one"
and then retrieve those two strings via the String args[] parameter that your main method receives!
The real answer here: you learn a new language by researching how that language works. You don't assume how syntax might look like, based on experiences from other languages. Meaning: $ENV_VAR is a "shell language" concept. Your idea: "maybe Java has the same" is a very inefficient strategy to go about this.
You can use the input arguments:
public static void main(String[] args) {
System.out.println(args[0]);
}
And then call it like this: java Main whateverIPutHere
Simple as that! args is an array containing all the arguments that you pass in the command line.
In this example, we are printing all the arguments passed from the command-line. For this purpose, we have traversed the array using for loop. The arguments passed in command line is captured by args argument.
class test{
public static void main(String args[]){
for(int i=0;i<args.length;i++)
System.out.println(args[i]);
}
}
compile by > javac test.java
run by > java test sonoo jaiswal 1 3 abc
Output:
sonoo
jaiswal
1
3
abc
You can either read it from args as mentioned above, or, if you know how to add a library to your project, try args4j. You'll get way cleaner code as you can use it to separate commandline argument processing to a dedicated class.

Calling jython script from java

I am currently working on a simple hello world program using jython and java.
The program is designed in way that a jython method accepts a name parameter and returns welcome message.
My problem is whenever I am accessing the jython method from java, it shows nullponter exception
My jython Script (JythonHello.py):
class JythonHello:
def __init__(self, name):
self.name = name
def sayHello(self):
return "Hello "+ self.name
and my java code:
public static void main(String[] args) {
PythonInterpreter interpreter = new PythonInterpreter();
interpreter.execfile("src/jython/JythonHello.py");
PyObject callFunction = interpreter.get("sayHello");
PyObject result = callFunction.__call__(new PyString("Boban"));
String msg = (String) result.__tojava__(String.class);
System.out.println("output: " + msg);
}
Any suggestions?
Looking at your code; your python code defines a class and a member method:
class JythonHello:
def __init__(self, name): ...
def sayHello(self): ...
And it seems that you intend to call that method:
PyObject callFunction = interpreter.get("sayHello");
PyObject result = callFunction.__call__(new PyString("Boban"));
But please note: sayHello() doesn't take any arguments. That self parameter is an indication that you have to call it on an object; but without any other parameters!
So, in pure python you would say:
helloVar = JythonHello("Boban")
helloVar.sayHello()
But your java code tries to call it like
sayHello("Boban")
So, the real answer is: step back; and re-think what you really intend to do; and then write code that works that way.
I would start by not adding the "class" part on the python side; instead try to simply invoke a function that takes a string argument for example!
And finally: could it be that you are on the wrong path altogether? The main point of jython is to write simply python code to do "debug" work within a running JVM. You are writing complicated Java code to use a bit of python code on the other hand ...

Using Java class in Matlab

I've been struggling with this problem for two days now and no resource I've found have been able to solve it.
I am trying to call a java class (added the link at the bottom) from Matlab (version 7.13.0.564 (R2011b)). I've compiled the java class using java 1.6 into a .class file and also added the path to the folder where the file is situated using javaaddpath (I've of course checked that the path is correct in the list of dynamic paths). However, when I try to call the class from Matlab using javaMethod('main','PerlinNoiseGenerator','') I get the error:
"No class PerlinNoiseGenerator can be located on Java class path"
I would be extremely grateful if someone with experience in calling java from Matlab could put together a short tut on how to do this. I am probably going to distribute my code so I kinda need to set the java path dynamically and from what I've read it really should be possible although I've seen post that indicate that it could be the cause of the problem.
http://svn.j3d.org/code/tags/Xj3D-M10/src/java/org/j3d/texture/procedural/PerlinNoiseGenerator.java
Usually I create jar files that contain java classes. I also had problems loading individual java classes before. In your case I did the following on xubuntu 13.04 x64 and Matlab 2013a x64 to load your particular class:
Compile it using java 6 (not the default 7) with option -d . to create a set of package folders, as your class defines a package org/j3d/texture/proecedural/ etc:
/usr/lib/jvm/java-6-openjdk-amd64/bin/javac -d . PerlinNoiseGenerator.java
This will compile the class and make in the current director (thus .) the set of package folders.
Make jar file containing your class again using jar from java 6. I named it javaNoise.jar:
/usr/lib/jvm/java-6-openjdk-amd64/bin/jar cf javaNoise.jar ./org/j3d/texture/procedural/PerlinNoiseGenerator.class
In Matlab, in the directory where javaNoise.jar is:
javaaddpath('./javaNoise.jar');
Create object of your java class:
png=org.j3d.texture.procedural.PerlinNoiseGenerator()
% results in: png = org.j3d.texture.procedural.PerlinNoiseGenerator#3982a033
To test it, I just generated some 1D noise:
png.noise1(1.2)
ans = -0.0960
Hope this helps.
P.S.
javaMethod('main','PerlinNoiseGenerator','') wont work because this class has no main method:-).
Your notation to the compiler of the constructor is a polymorphic class meaning "use appropriate constructor that is called at runtime".
public PerlinNoiseGenerator()
public PerlinNoiseGenerator(int seed)
The first form with no argument can be called but is irrelevent because the line with this(DEFAULT_SEED) attempts to call itself but only one constructor is allowed used
Second constructor has int for an argument but requires being loaded by an already loaded class.
Use the first version and change the case sensitive name of the one with the argument and remove this(DEFAULT_SEED) from it replace with the method name(the one you changed from a constructor that has the argument).
e.g. public perlinNoiseGenerator(int seed)
note: by convention java code method names start with a lower-case letter.
A final note, java arguments from the command line come in as "String" data type through the "main" method, a starter method for applications (gui or command prompt).
The first argument on the main method argument is the first commandline argument.
public static void main(String[] Args){
new PerlinNoiseGenerator(Args); // recursive class call
}//end main method
int[] args; // global
public PerlinNoiseGenerator(String[] Args){
int arglength=Args.length();
args = new int[arglength];
for(int cnt=0;cnt<arglength;cnt++){
Args[cnt].trim();
args[cnt]=new Integer(Args[cnt]).intValue();
}//enfor
perlinNoiseGenerator(args[0]); // call method
}//end constructor

How to call a user defined Matlab from Java using matlabcontrol.jar

I am trying to call a user defined Matlab Function(M file) which takes 3 arguments(Java Strings) from my Java application which is developed in Eclipse. At the moment I am able to call proxy.eval and proxy.feval methods with the functions/commands like disp or sqr. But when i try to invoke a user-defined function it says on the matlab console that there is no such function defined like that and on the Java console MatlabInvocationException occurs.
Then I tried with a simple user-defined function which takes no arguments and just has single line disp('Hello') but still the result is same. So I think rather than a type conversion problem there is something wrong with how user-defined functions are getting invoked.
Please can anyone help me soon? I am meeting the deadline very soon for this project. I would be so thankful if someone can come up with a solution. (Mr Joshuwa Kaplan, is there any guide on solving an issue like this in your posts? I tried but found nothing)
Thanks in advance
You must have any user-defined m-files on the MATLAB search path, just as if you were working normally inside MATLAB.
I tested with the following example:
C:\some\path\myfunc.m
function myfunc()
disp('hello from MYFUNC')
end
HelloWorld.java
import matlabcontrol.*;
public class HelloWorld
{
public static void main(String[] args)
throws MatlabConnectionException, MatlabInvocationException
{
// create proxy
MatlabProxyFactoryOptions options =
new MatlabProxyFactoryOptions.Builder()
.setUsePreviouslyControlledSession(true)
.build();
MatlabProxyFactory factory = new MatlabProxyFactory(options);
MatlabProxy proxy = factory.getProxy();
// call builtin function
proxy.eval("disp('hello world')");
// call user-defined function (must be on the path)
proxy.eval("addpath('C:\\some\\path')");
proxy.feval("myfunc");
proxy.eval("rmpath('C:\\some\\path')");
// close connection
proxy.disconnect();
}
}
We compile and run the Java program:
javac -cp matlabcontrol-4.0.0.jar HelloWorld.java
java -cp ".;matlabcontrol-4.0.0.jar" HelloWorld
a MATLAB session will open up, and display the output:
hello world
hello from MYFUNC
You could also add your folder to the path once, then persist it using SAVEPATH. That way you won't have to do it each time.

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