X-Git-Url: http://git.megacz.com/?p=sbp.git;a=blobdiff_plain;f=src%2Fedu%2Fberkeley%2Fsbp%2FUnion.java;h=77a158181a309578ca9e6354ca54db520370d0d6;hp=334874285e7f83896b9db2c4338d32f399676e89;hb=75d0fa39d405292f4b831a6d1743f2aeea01ebd4;hpb=0e17670bcfa7b0fe8eb3a2cac81f4b080a09fc98

diff --git a/src/edu/berkeley/sbp/Union.java b/src/edu/berkeley/sbp/Union.java
index 3348742..77a1581 100644
--- a/src/edu/berkeley/sbp/Union.java
+++ b/src/edu/berkeley/sbp/Union.java
@@ -7,11 +7,19 @@ import java.util.*;
 import java.lang.reflect.*;
 import java.lang.ref.*;
 
-/** <font color=green>an element which can produce one of several alternatives</font> */
+/**
+ *  <font color=green>an element which can produce one of several alternatives</font>.
+ *  <p>
+ *
+ *  Unlike the other Elements, Union is not immutable once
+ *  constructed.  To simulate this desirable feature, it is immutable
+ *  <i>once examined</i> by taking its iterator or calling contains().
+ */
 public class Union extends Element implements Iterable<Sequence> {
 
     private final String name;
     private final boolean synthetic;
+    private boolean viewed = false;
 
     private final List<Sequence> alternatives = new ArrayList<Sequence>();
 
@@ -31,11 +39,21 @@ public class Union extends Element implements Iterable<Sequence> {
         this.synthetic = synthetic;
     }
 
-    public boolean contains(Sequence s) { return alternatives.contains(s); }
-    public Iterator<Sequence> iterator() { return alternatives.iterator(); }
+    public boolean contains(Sequence s) {
+        viewed = true;
+        return alternatives.contains(s);
+    }
+
+    /** iterator over this Union's Sequences */
+    public Iterator<Sequence> iterator() {
+        viewed = true;
+        return alternatives.iterator();
+    }
 
     /** adds an alternative */
     public void add(Sequence s) {
+        if (viewed)
+            throw new RuntimeException("attempt to add a Sequence to a Union that has already been examined");
         if (alternatives.contains(s)) return;
         alternatives.add(s);
     }
@@ -64,6 +82,7 @@ public class Union extends Element implements Iterable<Sequence> {
         return "(anon_union)";
     }
     public String toString() {
+        viewed = true;
         if (name != null) return name;
         StringBuffer sb = new StringBuffer();
         sb.append("(");
@@ -71,7 +90,10 @@ public class Union extends Element implements Iterable<Sequence> {
         sb.append(")");
         return sb.toString();
     }
+
+    /** display this union in long/expanded form */
     public StringBuffer toString(StringBuffer sb) {
+        viewed = true;
         if (synthetic) return sb;
         boolean first = true;
         String before = StringUtil.pad(15, getName()) + " = ";
@@ -87,6 +109,7 @@ public class Union extends Element implements Iterable<Sequence> {
     }
     
     private void bodyToString(StringBuffer sb, String before, String between) {
+        viewed = true;
         boolean first = true;
         for(Sequence s : this) {
             // FIXME: what to do here about printing out negated sequences?