Call a method from one class from within another class

Question

I am trying to call the method from one class using a method within a second class (which should return a list), however, the second class method is not being recognised. I don't receive any visible errors or warnings.

My first class:

import java.util.*;

public class testClass {

        public static List<Object> makeStuff() {

            int a = 2;
            double b = 3.1;             
            return Arrays.asList(a, b);         
        }
    }

And the second

import java.util.*;

public class otherClass {

    public List<Object> outputStuff() {

        // create some other stuff here which will be appended to id
        List<Object> id = testClass.makeStuff();
        return id ;
    }

    public void main(String[] args) {

    }
}

From How to access a method from a class from another class? I thought this would work as the first method is static. Where am I making the mistake please?

Extra info if required: I am actually interfacing this code with R using rJava, but receive an error indicating that the java is wrong.

This returns the values as expected for the first class/method

library(rJava)
.jinit()
.jaddClassPath("C:\\Users\\david\\eclipse-workspace\\SOtest\\bin")    
myJavaClass <- .jnew("testClass")
x <- J(myJavaClass, "makeStuff")
x
# [1] "Java-Object{[2, 3.1]}"

but not for the second

.jinit()
.jaddClassPath("C:\\Users\\david\\eclipse-workspace\\SOtest\\bin")    
myJavaClass <- .jnew("otherClass")
x <- J(myJavaClass, "outputStuff")

throws the error

Error in .jcall("RJavaTools", "Ljava/lang/Object;", "invokeMethod", cl, : java.lang.NoSuchMethodException: otherClass.outputStuff()

Thank you for your comment @KlitosKyriacou. So I should update to `public static List — user2957945, Jan 10 '19 at 17:40
You only need `public static void main` if you want to execute your application from Java. You don't need to define main at all if you want to call a Java class from R. — Klitos Kyriacou, Jan 10 '19 at 18:14

ygbgames · Answer 1 · 2019-01-10T17:48:46.143

1

Nothing wrong with the code. Main method should be

public static void main(String a[]){ new otherClass().otherstuff();}

That should be enough. We can call static methods from non-static methods by Class name.

So to call otherstuff from main either make otherstuff as static or create new instance of otherClass and call directly.

edited Jan 10 '19 at 17:48

answered Jan 10 '19 at 17:41

ygbgames

191
9

Thank you for your answer. Is there any benefit in applying `static` to both the `outputStuff()` method and the `main` call or is it okay just on the `main`? Re your edit; the code seemed to run (no errors in java or R) without the additional `new otherClass().otherstuff();` – user2957945 Jan 10 '19 at 17:50
2

_Nothing wrong with the code._ **but** change all these things. How does this explain the error message they get from rjava? – Sotirios Delimanolis Jan 10 '19 at 18:01

score 0 · Answer 2 · answered Jan 10 '19 at 17:46

0

You can make your outputStuff static and it should work, as that one is the method you are trying to invoque

answered Jan 10 '19 at 17:46

DSantiagoBC

464
2
11

score -1 · Answer 3 · answered Jan 10 '19 at 18:22

-1

Main method should be public static void main(String[] args). Calling static method from non static method is perfectly fine.

answered Jan 10 '19 at 18:22

Sonia D'Souza

21
3

Call a method from one class from within another class

3 Answers3