derivation COS

public static class Cos implements CustomInterface {

        public String getName() {
            return "cos";
        }

        public double getValue(double[] args) {
            return Math.cos(args[0]);
        }

        public Function derivate(Function[] args) {
            Function defFunction = new AddAndMultiply(new Function[][]{{
                            new CustomFunction(new Sin(), new Function[]{args[0]})}},
                    new boolean[][]{{true}});

            if (args[0] instanceof VariableX) {
                return defFunction;
            }
            return new AddAndMultiply(new Function[][]{{
                            args[0].derivate(),
                            defFunction
                        }});
        }
    }