What is the best way to reuse a for-loop?

Question

I am currently working on a Matrix class that (will) allow you to perform operations on it. However, because I am using a 2D array to represent the matrix inside the class, I've often found myself using the following loop (to do something with all elements):

    for (int y = 0; y < this.matrix.length; y++) {
        for (int x = 0; x < this.matrix[0].length; x++) {
            // do something
        }
    }

What is a good way to reuse this loop, yet have it perform a different piece of code inside the loop?

I've thought about the functional Interfaces, but haven't quite figured those out.

Answer 1

If you have this in a method

void iterateMatrix(int[][] matrix) {
    for (int y = 0; y < matrix.length; y++) {
        for (int x = 0; x < matrix[0].length; x++) {
            // do something
        }
    }
}

you can define a functional interface for the "do something"

interface MatrixVisitor {
    void accept(int x, int y, int value);
}

It's a "functional interface" because it only contains one method; you can mark it with @FunctionInterface , but that's only a marker and not necessary.

You can use this via

void iterateMatrix(int[][] matrix, MatrixVisitor consumer) {
    for (int y = 0; y < matrix.length; y++) {
        for (int x = 0; x < matrix[0].length; x++) {
            consumer.accept(x, y, matrix[x][y]);
        }
    }
}

The effect of it being a functional interface is that you can "implement" it using a lambda, like

iterateMatrix(this.matrix, (x, y, value) -> System.out.printf("value at %d/%d: %d%n", x, y, value));

The question remains whether this is the "best way", it's a rather trivial loop so I don't know if reusing it is worth it.

Answer 2

You say you are not familiar with functional interfaces, so here's an alternative approach using normal classes, which allows you to use Java's for-each loop.

I assumed your matrix values were doubles, but if they aren't then switch out the doubles for whatever type you're using.

static class MatrixElement {
    private final int x;
    private final int y;
    private final double value;

    // ctor, getters
}

static Iterable<MatrixElement> matrixIterable(double[][] matrix) {
    final List<MatrixElement> elements = new ArrayList<>();
    for (int y = 0; y < matrix.length; y++) {
        for (int x = 0; x < matrix[y].length; x++) {              
            elements.add(new MatrixElement(x, y, matrix[x][y]));
        }
    }
    return elements;
}

Sample usage:

double[][] matrix = {{1.1, 2.2}, {3.3, 4.4}};
for (MatrixElement element : matrixIterable(matrix)) {
    System.out.println("(" + element.getX() + ", " + element.getY() + ") " + element.getValue());
}

Prints

(0, 0) 1.1
(1, 0) 3.3
(0, 1) 2.2
(1, 1) 4.4