drop confusing first(cond) methods; introduce reduce(f); wrap iterator

platform/platform-tests/testSrc/com/intellij/util/containers/TreeTraverserTest.java

@@ -377,9 +377,10 @@ public class TreeTraverserTest extends TestCase {
   }
 
   public void testJoin() {
+    assertNull(JBIterable.<String>of().join(", ").reduce((a, b) -> a + b));
     assertEquals("", JBIterable.of().join(", ").reduce("", (a, b) -> a + b));
-    assertEquals("a", JBIterable.of("a").join(", ").reduce("", (a, b) -> a + b));
-    assertEquals("a, b, c", JBIterable.of("a", "b", "c").join(", ").reduce("", (a, b) -> a + b));
+    assertEquals("a", JBIterable.of("a").join(", ").reduce((a, b) -> a + b));
+    assertEquals("a, b, c", JBIterable.of("a", "b", "c").join(", ").reduce((a, b) -> a + b));
   }
 
   public void testSplits1() {

platform/util/src/com/intellij/util/containers/JBIterable.java

@@ -100,7 +100,7 @@ public abstract class JBIterable<E> implements Iterable<E> {
     Multi(Iterable<? extends E> iterable) { super(iterable);}
 
     public Iterator<E> iterator() {
-      return ((Iterable<E>)content).iterator();
+      return JBIterator.from(((Iterable<E>)content).iterator());
     }
   }
 
@@ -571,24 +571,6 @@ public abstract class JBIterable<E> implements Iterable<E> {
     return iterator.hasNext() ? iterator.next() : null;
   }
 
-  /**
-   * Returns the first element if it is an instance of the specified class, otherwise null.
-   */
-  @Nullable
-  public final <T> T first(@NotNull Class<T> type) {
-    E first = first();
-    return first != null && type.isInstance(first) ? (T)first : null;
-  }
-
-  /**
-   * Returns the first element if it satisfies the condition, otherwise null.
-   */
-  @Nullable
-  public final E first(@NotNull Condition<? super E> condition) {
-    E first = first();
-    return first != null && condition.value(first) ? first : null;
-  }
-
   /**
    * Returns the first element if it is the only one, otherwise null.
    */
@@ -639,7 +621,20 @@ public abstract class JBIterable<E> implements Iterable<E> {
   }
 
   /**
-   * Returns the index of the first matching element.
+   * Perform calculation over this iterable.
+   */
+  public final E reduce(@NotNull PairFunction<E, ? super E, E> function) {
+    boolean first = true;
+    E cur = null;
+    for (E e : this) {
+      if (first) { cur = e; first = false; }
+      else cur = function.fun(cur, e);
+    }
+    return cur;
+  }
+
+  /**
+   * Returns the the first matching element.
    */
   public final E find(@NotNull Condition<? super E> condition) {
     return filter(condition).first();