fix build breakage due to missing isEmpty() methods

classpath/java/util/Collections.java

@@ -45,6 +45,10 @@ public class Collections {
       synchronized (lock) { return collection.size(); }
     }
 
+    public boolean isEmpty() {
+      return size() == 0;
+    }
+
     public synchronized boolean contains(T e) {
       synchronized (lock) { return collection.contains(e); }
     }
@@ -77,7 +81,6 @@ public class Collections {
     public void addAll(Collection<T> c) {
       synchronized (lock) { ((Set<T>)collection).addAll(c); }
     }
-
   }
 
   static class SynchronizedIterator<T> implements Iterator<T> {

classpath/java/util/HashMap.java

@@ -293,6 +293,10 @@ public class HashMap<K, V> implements Map<K, V> {
       return HashMap.this.size();
     }
 
+    public boolean isEmpty() {
+      return HashMap.this.isEmpty();
+    }
+
     public void addAll(Collection<Entry<K, V>> c) {
       throw new UnsupportedOperationException();
     }
@@ -323,6 +327,10 @@ public class HashMap<K, V> implements Map<K, V> {
       return HashMap.this.size();
     }
 
+    public boolean isEmpty() {
+      return HashMap.this.isEmpty();
+    }
+
     public boolean contains(K key) {
       return containsKey(key);
     }
@@ -354,6 +362,10 @@ public class HashMap<K, V> implements Map<K, V> {
       return HashMap.this.size();
     }
 
+    public boolean isEmpty() {
+      return HashMap.this.isEmpty();
+    }
+
     public boolean contains(V value) {
       return containsValue(value);
     }

classpath/java/util/HashSet.java

@@ -22,6 +22,10 @@ public class HashSet<T> implements Set<T> {
     return map.size();
   }
 
+  public boolean isEmpty() {
+    return map.isEmpty();
+  }
+
   public boolean contains(T element) {
     return map.containsKey(element);
   }

classpath/java/util/TreeSet.java

@@ -63,6 +63,10 @@ public class TreeSet<T> implements Collection<T> {
     return size;
   }
 
+  public boolean isEmpty() {
+    return size == 0;
+  }
+
   public boolean contains(T value) {
     return !set.find(new Cell(value, null)).fresh();
   }