// expr-eval-tree.java: java code for abstract parse
//      tree with evaluation
// Modern Compiler Implementation in Java, p. 92

public abstract class Exp
{
   public abstract int eval();
}

public class PlusExp extends Exp
{
   private Exp e1, e2;
   public PlusExp(Exp a1, Exp a2) { e1 = a1; e2 = a2; }
   public int eval()
   {
      return e1.eval() + e2.eval();
   }
}

public class MinusExp extends Exp
{
   private Exp e1, e2;
   public MinusExp(Exp a1, Exp a2)
   {
      e1 = a1; e2 = a2;
   }
   public int eval()
   {
      return e1.eval() - e2.eval();
   }
}

public class TimesExp extends Exp
{
   private Exp e1, e2;
   public TimesExp(Exp a1, Exp a2)
   {
      e1 = a1; e2 = a2;
   }
   public int eval()
   {
      return e1.eval() * e2.eval();
   }
}

public class DivideExp extends Exp
{
   private Exp e1, e2;
   public DivideExp(Exp a1, Exp a2)
   {
      e1 = a1; e2 = a2;
   }
   public int eval()
   {
      return e1.eval() / e2.eval();
   }
}

public class Identifier extends Exp
{
   private String name;
   public Identifier(String n) { name = n; }
   public int eval()
   {
      return lookup(name);
   }
}

public class IntegerLiteral extends Exp
{
   private String val;
   public IntegerLiteral(String v) { val = v; }
   public int eval()
   {
      return Integer.parseInt(val);
   }
}