How to pass a function as a parameter in java

Question

I have a matrix class and I want to create a function called map in this class, that looks something like this

  public void map(T fn) {
      //do something
   }

the idea behind the function is that as a parameter fn, which could be any method and apply this method to each number in the matrix.

the Idea behind it was for the coding train videos on a neural network but I tried to manipulate it in Java and it didn't work.I search lambda expressions but when I use it it's only let me pass method like Math.cos(arg) with an argument inside. but I want it to pass without any arguments the current function looks like this

interface lambda<T> {
double foo(T fn);
}

public void map(T fn) {
    for(int i = 0 ; i< this.getRow() ; i++) {
        for(int j = 0 ; j< this.getCol(); j++) {
            int a = i;
            int b = j;
            lambda mycode = (T) -> setData(matrix[a][b], fn);
            matrix[i][j] = mycode.foo(fn);
        }
    }
}
private double setData(double data, T fn) {
    data = (double) fn;
    return data;
}

when I send a call from my main class I can only call with arguments like

matrix.map(Math.cos(matrix.getData(0,0))

but it apply on each value in the matrix the cos of data at 0,0

I wish it will look more like this

matrix.map(Math.cos())

or even

Matrix.map(function())

is there a way in java to pass methods as parameters without entering arguments in them?

I tried to switch math.cos it with math::cos but I get an error "The target type of this expression must be a functional interface" — eyal mazuz, Feb 03 '18 at 06:13

Shmulik Klein · Accepted Answer · 2018-02-03T06:40:39.320

Java 8 was shipped with enormous number of functional interfaces. You can use one of those to describe the expected function to be passed as parameter in your method.

In your case, for passing a function which excepts a single double parameter and applys a logic, use java.util.function.DoubleUnaryOperator:

static double foo(DoubleUnaryOperator fn){
    double aNumber = 2.0;  
    return fn.applyAsDouble(aNumber);
}

And then create a lambda (or an object which implements DoubleUnaryOperator) and pass it to the foo method:

foo(d -> d * 3);

or

public class Fn implements DoubleUnaryOperator {
    @Override
    public double applyAsDouble(double d) {
        return d * 3;
    }
}

Ravi · Answer 2 · 2018-02-03T06:29:46.823

1

Use :: operator, to pass method reference, such as

Math::cos

Also, you need to make sure, method where you are passing method reference, should accept functional interface

That said,

void map(Consumer<T> fn)

edited Feb 03 '18 at 06:29

answered Feb 03 '18 at 06:23

Ravi

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score 0 · Answer 3 · answered Feb 03 '18 at 06:35

Functional programming in Java is backed by functional interfaces (interfaces with only one abstract method) and lamdas. There are several built-in functional interfaces provided by java. You can use Function / DoubleFunction those interfaces has one abstract method Function#apply. You can also create your own functional interfaces but it is better to use the existing solution provided by API if it fulfills your requirements. Please refer Below is the code sample you may get clarity.

public void map(DoubleFunction<Double> fn) {
  //do something
  setData(10.0, Math::cos);
 //do something
}
private double setData(double data,  DoubleFunction<Double> fn) {
    data = fn.apply(data);
    return data;
}

How to pass a function as a parameter in java

3 Answers3

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