package Symbol; class Binder { Object value; Symbol prevtop; Binder tail; Binder(Object v, Symbol p, Binder t) { value=v; prevtop=p; tail=t; } } /** * The Table class is similar to java.util.Dictionary, except that * each key must be a Symbol and there is a scope mechanism. */ public class Table { private java.util.Dictionary dict = new java.util.Hashtable(); private Symbol top; private Binder marks; public Table(){} /** * Gets the object associated with the specified symbol in the Table. */ public Object get(Symbol key) { Binder e = (Binder)dict.get(key); if (e==null) return null; else return e.value; } /** * Puts the specified value into the Table, bound to the specified Symbol. */ public void put(Symbol key, Object value) { dict.put(key, new Binder(value, top, (Binder)dict.get(key))); top = key; } /** * Remembers the current state of the Table. */ public void beginScope() {marks = new Binder(null,top,marks); top=null;} /** * Restores the table to what it was at the most recent beginScope * that has not already been ended. */ public void endScope() { while (top!=null) { Binder e = (Binder)dict.get(top); if (e.tail!=null) dict.put(top,e.tail); else dict.remove(top); top = e.prevtop; } top=marks.prevtop; marks=marks.tail; } /** * Returns an enumeration of the Table's symbols. */ public java.util.Enumeration keys() {return dict.keys();} }
