package composite.three;

public class ExprFactory2 {
  private ExprFactory2() {}
  static public Expr newConst(int v) {
    return new Const(v);
  }
  static public Expr newPlus(Expr l, Expr r) {
    return new PlusOp(l, r);
  }
//  static public Expr newMinus(Expr l, Expr r) {
//    return new BinOp(l, new OpSub(), r);
//  }
//  static public Expr newMult(Expr l, Expr r) {
//    return new BinOp(l, new OpMul(), r);
//  }
//  static public Expr newQuot(Expr l, Expr r) {
//    return new BinOp(l, new OpDiv(), r);
//  }

  private static final class Const implements Expr {
    private final int v;
    public Const(int v) {
      this.v = v;
    }
    public int eval() {
      return v;
    }
    public String toString() {
      return Integer.toString(v);
    }
  }

  private static abstract class BinOp implements Expr {
    private final Expr l;
    private final Expr r;
    private final String opString;
    public BinOp(Expr l, String opString, Expr r) {
      if ((l == null) || (r == null)) {
        throw new IllegalArgumentException();
      }
      this.l = l;
      this.r = r;
      this.opString = opString;
    }
    protected abstract int run (int x, int y);
    // to be template method, there must a template method
    // template method = method of abstract class, that calls an abstract method
    public int eval() {
      return this.run(l.eval(), r.eval());
    }
    public String toString() {
      return l.toString() + " " + r.toString() + " " + opString;
    }
  }
  private static final class PlusOp extends BinOp {
    public PlusOp(Expr l, Expr r) { super (l, "+", r); }
    public int run(int x, int y) { return x+y; }
  }
}