/* Abstract syntax trees for online calculator 

   For the online calculator program, writing a tree structure may be
   redundant, but it illustrates the general technique
*/

interface Exp   // interface for all expressions
{
    public double eval();
}

class dlit implements Exp // class for simple double expressions
{
    double value;
    public dlit(double x) {value=x;}
    public double eval() { return value; }
}


class opexp   // superclass for all binary operator expressions
{
    Exp left, right; // left and right subtrees
    public opexp(Exp l, Exp r) {left=l;  right=r;}
}

class plusexp extends opexp implements Exp
{ 
    public plusexp(Exp l, Exp r) { super(l,r); }
    public double eval() { return left.eval() + right.eval(); }
}

class minusexp extends opexp implements Exp
{ 
    public minusexp(Exp l, Exp r) { super(l,r); }
    public double eval() { return left.eval() - right.eval(); }
}

class multexp extends opexp implements Exp
{ 
    public multexp(Exp l, Exp r) { super(l,r); }
    public double eval() { return left.eval() * right.eval(); }
}

class divexp extends opexp implements Exp
{ 
    public divexp(Exp l, Exp r) { super(l,r); }
    public double eval() { return left.eval() / right.eval(); }
}

class negexp implements Exp  // negative values
{ 
    Exp sub;  // subexpression
    public negexp(Exp s) { sub=s; }
    public double eval() { return -1 * sub.eval(); }
}