Calc.java

import java.util.Scanner;
import java.util.Stack;
import java.util.Random;

public class Calc
{
    public static void main(String[] args)
    {
    // whether each element in rpn string is a number or operator
    // five possibilities
    final boolean[][] isOp = {{false,false,true,false,true,false,true},
                  {false,false,true,false,false,true,true},
                  {false,false,false,false,true,true,true},
                  {false,false,false,true,false,true,true},
                  {false,false,false,true,true,false,true}};
    // possible ops and nums
    final char[] ops = {'+', '-', '*', '/'};
    char[] nums = {'1', '5', '6', '7'};

    Random rand = new Random();
    Stack<Double> stk = new Stack<Double>();
    char[] str = new char[8];
    double result = 0;
    while (result < 20.9 || 21.1 < result) { // allow floating point errors

        // fisher-yates shuffle on nums
        for (int i = 3; i > 0; i--) {
        int j = rand.nextInt(i + 1);
        char tmp = nums[j];
        nums[j] = nums[i];
        nums[i] = tmp;
        }

        int ord = rand.nextInt(5); // select order of ops and nums
        for (int i = 0, nnum = 0; i < 7; i++)
        // can select any operator
        // nums can only be used once (hence shuffling)
        str[i] = isOp[ord][i] ? ops[rand.nextInt(4)] : nums[nnum++];

        // run rpn through stack calculator
        for (int i = 0; i < 7; i++) {
        switch (str[i]) {
        case '+':
            stk.push(stk.pop() + stk.pop());
            break;
        case '-':
            stk.push(- stk.pop() + stk.pop());
            break;
        case '*':
            stk.push(stk.pop() * stk.pop());
            break;
        case '/':
            stk.push(1 / stk.pop() * stk.pop());
            break;
        default:
            stk.push((double)(str[i] - 0x30));
        }
        }
        result = stk.pop();
    }
    System.out.println(result);
    System.out.println(str);
    }
}