justik justik - 1 year ago 126
C++ Question

C++ functors to Java, overloading operator ()()

Suppose a simple example of functors in C++:

class Test2 {
double a;

Test2 (double a_) : a(a_){}
double operator () () {return 10*a;}

template <typename Function>
double test ( Function function ) {return function();}

int main(int argc, char* argv[]) {
double a = test( Test2(5) );
return 0;

Is there any way to implement this construction in Java (for example using the interface Functor)? Could you give me a short example? Thanks for your help.

Answer Source

In Java 8, you can use the DoubleSupplier interface to get a double value from an object:

public class Test implements DoubleSupplier {
    private double a;
    public Test(double a) { this.a = a; }
    public double getAsDouble() { return 10 * a; }

    public static double test(DoubleSupplier ds) { 
        return ds.getAsDouble();
    public static void main(String[] args) {
        double a = test(new Test(5));

If you aren't using Java 8, then you could just make your own interface to implement from:

public interface MyDoubleSupplier {
    double getAsDouble();
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