How to call only the return value to Java from a C file function (Java Native Interface)

Question

I have a Java file that declares a native method average() which is defined in my C file. The method takes the average of two numbers, prints "In C, the numbers are n1, n2" (n1 and n2 being the input numbers), and returns the average as a double.

The Java file then calls the average() method, prints "In Java, the average is (n1 + n2)/2" and then prints the average x 2.

Here is an example with the numbers 3 and 2 (running my Java file):

In C, the numbers are 3 and 2

In Java, the average is 2.5

In C, the numbers are 3 and 2

5.0

I would like to multiply only the return value from the C file (the average value) by 2. I don't want the string "In C, the numbers are 3 and 2" to print again. How can I print only the return value (average) from the C file and not have the string "In C, the numbers are 3 and 2" print again? I know I can just create another method but that would be very repetitive.

My Java file follows:

public class TestJNIPrimitive {
    
    static {
        System.loadLibrary("myjni"); //myjni.dll (Windows)
    }

    private native double average(int n1, int n2);
    public static void main(String args[]) {
        
        System.out.println("In Java, the average is " + new TestJNIPrimitive().average(3,2));
        double result = new TestJNIPrimitive().average(3,2);
        System.out.println(result*2);
    }
}

My C file follows:

JNIEXPORT jdouble JNICALL Java_TestJNIPrimitive_average(JNIEnv *env, jobject thisObj, jint n1, jint n2) {
    
    printf("In C, the numbers are %d and %d\n", n1, n2);
    jdouble result;
    result = ((jdouble)n1 + n2) / 2.0;
    // jint is mapped to int, jdouble is mapped to double
    return result;
}

Thanks for the help!

How about adding a 3rd parameter? (which allows to suppress this line from being printed) — maraca
– maraca, Commented Jul 22, 2015 at 23:22
new TestJNIPrimitive().average(3,2); calls your function. You call it twice, it gets called twice. — Stack Exchange Broke The Law
– Stack Exchange Broke The Law, Commented Jul 22, 2015 at 23:23

Jashaszun · Accepted Answer · 2015-07-22 23:21:24Z

5

Unless I'm missing something, you can simply not call your C average function twice, and just store the result after you call it once:

public static void main(String args[]) {
    double result = new TestJNIPrimitive().average(3,2);
    System.out.println("In Java, the average is " + result);
    System.out.println(result*2);
}

answered Jul 22, 2015 at 23:21

Jashaszun

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