如何在java中通过递归连接字符串来生成等式？

public class EquationGenerator {

    public static void main(String[] args) {
        generateEquations("1", "2", "+");
    }

    public static void generateEquations(String num1, String num2, String operator) {
        if (num1.length() == 0 && num2.length() == 0) {
            System.out.println(operator);
            return;
        }

        if (num1.length() > 0) {
            generateEquations(num1.substring(1), num2, operator + num1.charAt(0));
        }

        if (num2.length() > 0) {
            generateEquations(num1, num2.substring(1), operator + num2.charAt(0));
        }
    }
}