// represent an operato key on a calculator
abstract class Operator {
  abstract int evaluate(int n1, int n2);
}

class Plus extends Operator{
  Plus(){}
  int evaluate(int n1, int n2) {
    return n1 + n2;
  }
}

class Subtract extends Operator{
  Subtract(){}	
  int evaluate(int n1, int n2) {
    return n1 - n2;
  }
}

class Multiply extends Operator{
  Multiply(){	}
  int evaluate(int n1, int n2) {
    return n1 * n2;
  }
}

class Divide extends Operator{
  Divide(){}
  int evaluate(int n1, int n2) {
    return n1 / n2;
  }
}

class Clear extends Operator{
  Clear(){}
  int evaluate(int n1, int n2) {
    return 0;
  }
}

// to repesent a simple calculator
class Calculator {
  int display;   //current display on calculator
  int num1;      //current number that's been hit
  Operator op;   //operator
  int num2;      //next number that's been hit	 
			 
  // Create a Calculator instance
  // Initial values are (0, 0, new Plus(), 0)
  Calculator(int display, int num1, Operator op, int num2) {
    this.display = display;
    this.num1 = num1;
    this.op = op;
    this.num2 = num2;
  }

  // consumes an int and produces a new Calculator
  Calculator digit(int n){
    return new Calculator((this.num2 * 10) + n, 
                           this.num1,
			   this.op,
		           (this.num2 * 10) + n);
  }

  // consumes an operator and produces a new Calculator
  Calculator operation(Operator op){
    return new Calculator(this.op.evaluate(this.num1, this.num2),
			  this.op.evaluate(this.num1, this.num2),
			  op,
			  0);
  } 
}




class Client {
  Client(){}

  Operator p = new Plus();
  Operator s = new Subtract();
  Operator m = new Multiply();
  Operator d = new Divide();

  Operator clear = new Clear();

  boolean testPlus     = this.p.evaluate(3, 5) == 8;
  boolean testSubtract = this.s.evaluate(5, 3) == 2;
  boolean testMultiply = this.m.evaluate(3, 5) == 15;
  boolean testDivide   = this.d.evaluate(30, 5) == 6;

  boolean testClear     = this.clear.evaluate(3, 5) == 0;

  boolean testOperators(){
    return this.testPlus
        && this.testSubtract
        && this.testMultiply
        && this.testDivide

        && this.testClear;
  }

  Calculator c = new Calculator(0, 0, this.p, 0);
  boolean testCalc1 = this.c.digit(8).display == 8;
  boolean testCalc2 = this.c.digit(8).num2 == 8;

  Calculator c2 = this.c.digit(8);
  Calculator c3 = this.c2.digit(2);
  boolean testCalc3 = (this.c3.display == 82) &&
                      (this.c3.num1 == 0) &&
                      (this.c3.op.equals(this.p)) &&
                      (this.c3.num2 == 82);

  Calculator c4 = this.c2.operation(this.m);
  boolean testCalc4 = (this.c4.display == 8) &&
                      (this.c4.num1 == 8) &&
                      (this.c4.op.equals(this.m)) &&
                      (this.c4.num2 == 0);

  Calculator c5 = this.c2.operation(this.p).digit(3).operation(this.p);
  boolean testCalc5 = (this.c5.display == 11) &&
                      (this.c5.num1 == 11) &&
                      (this.c5.op.equals(this.p)) &&
                      (this.c5.num2 == 0);

  Calculator c6 = this.c2.operation(this.m).digit(3).digit(4).operation(this.p);
  boolean testCalc6 = (this.c6.display == 272) &&
                      (this.c6.num1 == 272) &&
                      (this.c6.op.equals(this.p)) &&
                      (this.c6.num2 == 0);

  boolean calcTestAll = this.testCalc1 &&
                        this.testCalc2 &&
                        this.testCalc3 &&
                        this.testCalc4 &&
                        this.testCalc5 &&
                        this.testCalc6;

  /* Run the tests in the Interactions:
  > Client c = new Client
  > c.testOperators()
  > c.calcTestAll
*/

}