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file:enumeration/Expr.java [source] [doc-public] [doc-private]
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package enumeration;

public interface Expr {
  void printPostorder();
  int evaluate();
}

class Const implements Expr {
  private final int v;
  public Const(int v) {
    this.v = v;
  }

  public int evaluate() {
    return v;
  }
  public void printPostorder() {
    System.out.print(v + " ");
  }
}

class BinOp implements Expr {
  private final Expr l;
  private final Expr r;
  private final Op op;

  public BinOp(Expr l, Op op, Expr r) {
    if ((l == null) || (op == null) || (r == null)) {
      throw new IllegalArgumentException();
    }
    this.op = op;
    this.l = l;
    this.r = r;
  }

  public int evaluate() {
    return op.eval(l.evaluate(), r.evaluate());
  }
  public void printPostorder() {
    l.printPostorder();
    r.printPostorder();
    System.out.print(op + " ");
  }
}

file:enumeration/Op.java [source] [doc-public] [doc-private]
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package enumeration;

public abstract class Op {
  private final String name;
  Op(String name) { this.name = name; }
  public String toString() { return name; }

  public abstract int eval(int x, int y);

  public static final Op ADD = new OpAdd();
  public static final Op SUB = new OpSub();
  public static final Op MUL = new OpMul();
  public static final Op DIV = new OpDiv();
}

final class OpAdd extends Op {
  OpAdd() {  super("+"); }
  public int eval(int x, int y) { return x+y; }
}
final class OpSub extends Op {
  OpSub() {  super("-"); }
  public int eval(int x, int y) { return x-y; }
}
final class OpMul extends Op {
  OpMul() {  super("*"); }
  public int eval(int x, int y) { return x*y; }
}
final class OpDiv extends Op {
  OpDiv() {  super("/"); }
  public int eval(int x, int y) { return x/y; }
}

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