Observable.concat: refactor varargs to overloads

ReactiveX#359 Varargs cause compiler warnings

As part of this I also changed this:

```java
public static <T> Observable<T> concat(Observable<Observable<? extends T>> observables)
```

to

```java
public static <T> Observable<T> concat(Observable<Observable<T>> observables)
```

I documented the reasoning of this at ReactiveX#360 (comment)

Author: benjchristensen

rxjava-core/src/main/java/rx/Observable.java

@@ -979,14 +979,232 @@ public static <T> Observable<T> merge(Observable<? extends T>... source) {
      * <p>
      * <img width="640" src="https://raw.github.com/wiki/Netflix/RxJava/images/rx-operators/concat.png">
      * 
-     * @param source
-     *            a series of Observables
+     * @param observables
+     *            an Observable of Observables
+     * @return an Observable that emits items that are the result of combining the items emitted by
+     *         the {@code source} Observables, one after the other
+     * @see <a href="http://msdn.microsoft.com/en-us/library/system.reactive.linq.observable.concat(v=vs.103).aspx">MSDN: Observable.Concat Method</a>
+     */
+    public static <T> Observable<T> concat(Observable<Observable<T>> observables) {
+        return create(OperationConcat.concat(observables));
+    }
+    
+    /**
+     * Returns an Observable that emits the items emitted by two or more Observables, one after the
+     * other.
+     * <p>
+     * <img width="640" src="https://raw.github.com/wiki/Netflix/RxJava/images/rx-operators/concat.png">
+     * 
+     * @param t1
+     *            an Observable to be concatenated
+     * @param t2
+     *            an Observable to be concatenated
+     *            an Observable to be concatenated
+     * @return an Observable that emits items that are the result of combining the items emitted by
+     *         the {@code source} Observables, one after the other
+     * @see <a href="http://msdn.microsoft.com/en-us/library/system.reactive.linq.observable.concat(v=vs.103).aspx">MSDN: Observable.Concat Method</a>
+     */
+    @SuppressWarnings("unchecked")
+    // suppress because the types are checked by the method signature before using a vararg
+    public static <T> Observable<T> concat(Observable<? extends T> t1, Observable<? extends T> t2) {
+        return create(OperationConcat.concat(t1, t2));
+    }
+
+    /**
+     * Returns an Observable that emits the items emitted by two or more Observables, one after the
+     * other.
+     * <p>
+     * <img width="640" src="https://raw.github.com/wiki/Netflix/RxJava/images/rx-operators/concat.png">
+     * 
+     * @param t1
+     *            an Observable to be concatenated
+     * @param t2
+     *            an Observable to be concatenated
+     * @param t3
+     *            an Observable to be concatenated
+     *            an Observable to be concatenated
+     * @return an Observable that emits items that are the result of combining the items emitted by
+     *         the {@code source} Observables, one after the other
+     * @see <a href="http://msdn.microsoft.com/en-us/library/system.reactive.linq.observable.concat(v=vs.103).aspx">MSDN: Observable.Concat Method</a>
+     */
+    @SuppressWarnings("unchecked")
+    // suppress because the types are checked by the method signature before using a vararg
+    public static <T> Observable<T> concat(Observable<? extends T> t1, Observable<? extends T> t2, Observable<? extends T> t3) {
+        return create(OperationConcat.concat(t1, t2, t3));
+    }
+
+    /**
+     * Returns an Observable that emits the items emitted by two or more Observables, one after the
+     * other.
+     * <p>
+     * <img width="640" src="https://raw.github.com/wiki/Netflix/RxJava/images/rx-operators/concat.png">
+     * 
+     * @param t1
+     *            an Observable to be concatenated
+     * @param t2
+     *            an Observable to be concatenated
+     * @param t3
+     *            an Observable to be concatenated
+     * @param t4
+     *            an Observable to be concatenated
+     * @return an Observable that emits items that are the result of combining the items emitted by
+     *         the {@code source} Observables, one after the other
+     * @see <a href="http://msdn.microsoft.com/en-us/library/system.reactive.linq.observable.concat(v=vs.103).aspx">MSDN: Observable.Concat Method</a>
+     */
+    @SuppressWarnings("unchecked")
+    // suppress because the types are checked by the method signature before using a vararg
+    public static <T> Observable<T> concat(Observable<? extends T> t1, Observable<? extends T> t2, Observable<? extends T> t3, Observable<? extends T> t4) {
+        return create(OperationConcat.concat(t1, t2, t3, t4));
+    }
+
+    /**
+     * Returns an Observable that emits the items emitted by two or more Observables, one after the
+     * other.
+     * <p>
+     * <img width="640" src="https://raw.github.com/wiki/Netflix/RxJava/images/rx-operators/concat.png">
+     * 
+     * @param t1
+     *            an Observable to be concatenated
+     * @param t2
+     *            an Observable to be concatenated
+     * @param t3
+     *            an Observable to be concatenated
+     * @param t4
+     *            an Observable to be concatenated
+     * @param t5
+     *            an Observable to be concatenated
+     * @return an Observable that emits items that are the result of combining the items emitted by
+     *         the {@code source} Observables, one after the other
+     * @see <a href="http://msdn.microsoft.com/en-us/library/system.reactive.linq.observable.concat(v=vs.103).aspx">MSDN: Observable.Concat Method</a>
+     */
+    @SuppressWarnings("unchecked")
+    // suppress because the types are checked by the method signature before using a vararg
+    public static <T> Observable<T> concat(Observable<? extends T> t1, Observable<? extends T> t2, Observable<? extends T> t3, Observable<? extends T> t4, Observable<? extends T> t5) {
+        return create(OperationConcat.concat(t1, t2, t3, t4, t5));
+    }
+
+    /**
+     * Returns an Observable that emits the items emitted by two or more Observables, one after the
+     * other.
+     * <p>
+     * <img width="640" src="https://raw.github.com/wiki/Netflix/RxJava/images/rx-operators/concat.png">
+     * 
+     * @param t1
+     *            an Observable to be concatenated
+     * @param t2
+     *            an Observable to be concatenated
+     * @param t3
+     *            an Observable to be concatenated
+     * @param t4
+     *            an Observable to be concatenated
+     * @param t5
+     *            an Observable to be concatenated
+     * @param t6
+     *            an Observable to be concatenated
+     * @return an Observable that emits items that are the result of combining the items emitted by
+     *         the {@code source} Observables, one after the other
+     * @see <a href="http://msdn.microsoft.com/en-us/library/system.reactive.linq.observable.concat(v=vs.103).aspx">MSDN: Observable.Concat Method</a>
+     */
+    @SuppressWarnings("unchecked")
+    // suppress because the types are checked by the method signature before using a vararg
+    public static <T> Observable<T> concat(Observable<? extends T> t1, Observable<? extends T> t2, Observable<? extends T> t3, Observable<? extends T> t4, Observable<? extends T> t5, Observable<? extends T> t6) {
+        return create(OperationConcat.concat(t1, t2, t3, t4, t5, t6));
+    }
+
+    /**
+     * Returns an Observable that emits the items emitted by two or more Observables, one after the
+     * other.
+     * <p>
+     * <img width="640" src="https://raw.github.com/wiki/Netflix/RxJava/images/rx-operators/concat.png">
+     * 
+     * @param t1
+     *            an Observable to be concatenated
+     * @param t2
+     *            an Observable to be concatenated
+     * @param t3
+     *            an Observable to be concatenated
+     * @param t4
+     *            an Observable to be concatenated
+     * @param t5
+     *            an Observable to be concatenated
+     * @param t6
+     *            an Observable to be concatenated
+     * @param t7
+     *            an Observable to be concatenated
      * @return an Observable that emits items that are the result of combining the items emitted by
      *         the {@code source} Observables, one after the other
      * @see <a href="http://msdn.microsoft.com/en-us/library/system.reactive.linq.observable.concat(v=vs.103).aspx">MSDN: Observable.Concat Method</a>
      */
-    public static <T> Observable<T> concat(Observable<? extends T>... source) {
-        return create(OperationConcat.concat(source));
+    @SuppressWarnings("unchecked")
+    // suppress because the types are checked by the method signature before using a vararg
+    public static <T> Observable<T> concat(Observable<? extends T> t1, Observable<? extends T> t2, Observable<? extends T> t3, Observable<? extends T> t4, Observable<? extends T> t5, Observable<? extends T> t6, Observable<? extends T> t7) {
+        return create(OperationConcat.concat(t1, t2, t3, t4, t5, t6, t7));
+    }
+
+    /**
+     * Returns an Observable that emits the items emitted by two or more Observables, one after the
+     * other.
+     * <p>
+     * <img width="640" src="https://raw.github.com/wiki/Netflix/RxJava/images/rx-operators/concat.png">
+     * 
+     * @param t1
+     *            an Observable to be concatenated
+     * @param t2
+     *            an Observable to be concatenated
+     * @param t3
+     *            an Observable to be concatenated
+     * @param t4
+     *            an Observable to be concatenated
+     * @param t5
+     *            an Observable to be concatenated
+     * @param t6
+     *            an Observable to be concatenated
+     * @param t7
+     *            an Observable to be concatenated
+     * @param t8
+     *            an Observable to be concatenated
+     * @return an Observable that emits items that are the result of combining the items emitted by
+     *         the {@code source} Observables, one after the other
+     * @see <a href="http://msdn.microsoft.com/en-us/library/system.reactive.linq.observable.concat(v=vs.103).aspx">MSDN: Observable.Concat Method</a>
+     */
+    @SuppressWarnings("unchecked")
+    // suppress because the types are checked by the method signature before using a vararg
+    public static <T> Observable<T> concat(Observable<? extends T> t1, Observable<? extends T> t2, Observable<? extends T> t3, Observable<? extends T> t4, Observable<? extends T> t5, Observable<? extends T> t6, Observable<? extends T> t7, Observable<? extends T> t8) {
+        return create(OperationConcat.concat(t1, t2, t3, t4, t5, t6, t7, t8));
+    }
+
+    /**
+     * Returns an Observable that emits the items emitted by two or more Observables, one after the
+     * other.
+     * <p>
+     * <img width="640" src="https://raw.github.com/wiki/Netflix/RxJava/images/rx-operators/concat.png">
+     * 
+     * @param t1
+     *            an Observable to be concatenated
+     * @param t2
+     *            an Observable to be concatenated
+     * @param t3
+     *            an Observable to be concatenated
+     * @param t4
+     *            an Observable to be concatenated
+     * @param t5
+     *            an Observable to be concatenated
+     * @param t6
+     *            an Observable to be concatenated
+     * @param t7
+     *            an Observable to be concatenated
+     * @param t8
+     *            an Observable to be concatenated
+     * @param t9
+     *            an Observable to be concatenated
+     * @return an Observable that emits items that are the result of combining the items emitted by
+     *         the {@code source} Observables, one after the other
+     * @see <a href="http://msdn.microsoft.com/en-us/library/system.reactive.linq.observable.concat(v=vs.103).aspx">MSDN: Observable.Concat Method</a>
+     */
+    @SuppressWarnings("unchecked")
+    // suppress because the types are checked by the method signature before using a vararg
+    public static <T> Observable<T> concat(Observable<? extends T> t1, Observable<? extends T> t2, Observable<? extends T> t3, Observable<? extends T> t4, Observable<? extends T> t5, Observable<? extends T> t6, Observable<? extends T> t7, Observable<? extends T> t8, Observable<? extends T> t9) {
+        return create(OperationConcat.concat(t1, t2, t3, t4, t5, t6, t7, t8, t9));
     }
 
     /**

rxjava-core/src/main/java/rx/operators/OperationConcat.java

@@ -60,7 +60,7 @@ public static <T> OnSubscribeFunc<T> concat(final Observable<? extends T>... seq
         return concat(Observable.from(sequences));
     }
 
-    public static <T> OnSubscribeFunc<T> concat(final List<? extends Observable<? extends T>> sequences) {
+    public static <T> OnSubscribeFunc<T> concat(final Iterable<? extends Observable<? extends T>> sequences) {
         return concat(Observable.from(sequences));
     }
 

rxjava-core/src/test/java/rx/ConcatTests.java

@@ -0,0 +1,57 @@
+package rx;
+
+import static org.junit.Assert.*;
+
+import java.util.Arrays;
+import java.util.List;
+
+import org.junit.Test;
+
+public class ConcatTests {
+
+    @Test
+    public void testConcatSimple() {
+        Observable<String> o1 = Observable.from("one", "two");
+        Observable<String> o2 = Observable.from("three", "four");
+
+        List<String> values = Observable.concat(o1, o2).toList().toBlockingObservable().single();
+
+        assertEquals("one", values.get(0));
+        assertEquals("two", values.get(1));
+        assertEquals("three", values.get(2));
+        assertEquals("four", values.get(3));
+    }
+
+    @Test
+    public void testConcatWithObservableOfObservable() {
+        Observable<String> o1 = Observable.from("one", "two");
+        Observable<String> o2 = Observable.from("three", "four");
+        Observable<String> o3 = Observable.from("five", "six");
+
+        Observable<Observable<String>> os = Observable.from(o1, o2, o3);
+
+        List<String> values = Observable.concat(os).toList().toBlockingObservable().single();
+
+        assertEquals("one", values.get(0));
+        assertEquals("two", values.get(1));
+        assertEquals("three", values.get(2));
+        assertEquals("four", values.get(3));
+    }
+
+    @Test
+    public void testConcatWithIterableOfObservable() {
+        Observable<String> o1 = Observable.from("one", "two");
+        Observable<String> o2 = Observable.from("three", "four");
+        Observable<String> o3 = Observable.from("five", "six");
+
+        @SuppressWarnings("unchecked")
+        Iterable<Observable<String>> is = Arrays.asList(o1, o2, o3);
+
+        List<String> values = Observable.concat(Observable.from(is)).toList().toBlockingObservable().single();
+
+        assertEquals("one", values.get(0));
+        assertEquals("two", values.get(1));
+        assertEquals("three", values.get(2));
+        assertEquals("four", values.get(3));
+    }
+}