2.x: Enhance NPE messages

src/main/java/io/reactivex/internal/observers/ToNotificationObserver.java

@@ -40,7 +40,7 @@ public void onSubscribe(Disposable s) {
     public void onNext(T t) {
         if (t == null) {
             s.dispose();
-            onError(new NullPointerException());
+            onError(new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources."));
         } else {
             try {
                 consumer.accept(Notification.<Object>createOnNext(t));
@@ -71,4 +71,4 @@ public void onComplete() {
             RxJavaPlugins.onError(ex);
         }
     }
-}
+}

src/main/java/io/reactivex/internal/operators/flowable/FlowableError.java

@@ -36,7 +36,7 @@ public void subscribeActual(Subscriber<? super T> s) {
             error = t;
         }
         if (error == null) {
-            error = new NullPointerException();
+            error = new NullPointerException("Callable returned null throwable. Null values are generally not allowed in 2.x operators and sources.");
         }
         EmptySubscription.error(error, s);
     }

src/main/java/io/reactivex/internal/operators/flowable/FlowableGenerate.java

@@ -174,7 +174,7 @@ public void cancel() {
         @Override
         public void onNext(T t) {
             if (t == null) {
-                onError(new NullPointerException());
+                onError(new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources."));
                 return;
             }
             actual.onNext(t);
@@ -183,7 +183,7 @@ public void onNext(T t) {
         @Override
         public void onError(Throwable t) {
             if (t == null) {
-                t = new NullPointerException();
+                t = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
             }
             terminate = true;
             actual.onError(t);

src/main/java/io/reactivex/internal/operators/flowable/FlowableGroupBy.java

@@ -462,7 +462,7 @@ public void subscribe(Subscriber<? super T> s) {
 
         public void onNext(T t) {
             if (t == null) {
-                error = new NullPointerException();
+                error = new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources.");
                 done = true;
             } else {
                 queue.offer(t);
@@ -659,4 +659,4 @@ public void clear() {
             queue.clear();
         }
     }
-}
+}

src/main/java/io/reactivex/internal/operators/maybe/MaybeErrorCallable.java

@@ -45,7 +45,7 @@ protected void subscribeActual(MaybeObserver<? super T> observer) {
         }
 
         if (ex == null) {
-            ex = new NullPointerException();
+            ex = new NullPointerException("Callable returned null throwable. Null values are generally not allowed in 2.x operators and sources.");
         }
 
         observer.onError(ex);

src/main/java/io/reactivex/internal/operators/observable/ObservableBuffer.java

@@ -205,7 +205,7 @@ public void onNext(T t) {
                 if (b == null) {
                     buffers.clear();
                     s.dispose();
-                    actual.onError(new NullPointerException());
+                    actual.onError(new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources."));
                     return;
                 }
 

src/main/java/io/reactivex/internal/operators/observable/ObservableError.java

@@ -34,7 +34,7 @@ public void subscribeActual(Observer<? super T> s) {
             error = t;
         }
         if (error == null) {
-            error = new NullPointerException();
+            error = new NullPointerException("Callable returned null throwable. Null values are generally not allowed in 2.x operators and sources.");
         }
         EmptyDisposable.error(error, s);
     }

src/main/java/io/reactivex/internal/operators/observable/ObservableGenerate.java

@@ -134,7 +134,7 @@ public boolean isDisposed() {
         @Override
         public void onNext(T t) {
             if (t == null) {
-                onError(new NullPointerException());
+                onError(new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources."));
                 return;
             }
             actual.onNext(t);
@@ -143,7 +143,7 @@ public void onNext(T t) {
         @Override
         public void onError(Throwable t) {
             if (t == null) {
-                t = new NullPointerException();
+                t = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
             }
             terminate = true;
             actual.onError(t);

src/main/java/io/reactivex/internal/operators/observable/ObservableGroupBy.java

@@ -260,7 +260,7 @@ public void subscribe(Observer<? super T> s) {
 
         public void onNext(T t) {
             if (t == null) {
-                error = new NullPointerException();
+                error = new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources.");
                 done = true;
             } else {
                 queue.offer(t);

src/main/java/io/reactivex/internal/operators/observable/ObservableZip.java

@@ -268,7 +268,7 @@ public void onSubscribe(Disposable s) {
         public void onNext(T t) {
             if (t == null) {
                 s.get().dispose();
-                onError(new NullPointerException());
+                onError(new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources."));
                 return;
             }
             if (!queue.offer(t)) {

src/main/java/io/reactivex/internal/operators/single/SingleError.java

@@ -39,7 +39,7 @@ protected void subscribeActual(SingleObserver<? super T> s) {
         }
 
         if (error == null) {
-            error = new NullPointerException();
+            error = new NullPointerException("Callable returned null throwable. Null values are generally not allowed in 2.x operators and sources.");
         }
 
         EmptyDisposable.error(error, s);

src/main/java/io/reactivex/observers/SafeObserver.java

@@ -84,7 +84,7 @@ public void onNext(T t) {
         }
 
         if (t == null) {
-            Throwable ex = new NullPointerException();
+            Throwable ex = new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources.");
             try {
                 s.dispose();
             } catch (Throwable e1) {
@@ -167,7 +167,7 @@ public void onError(Throwable t) {
         }
 
         if (t == null) {
-            t = new NullPointerException();
+            t = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
         }
 
         try {

src/main/java/io/reactivex/observers/SerializedObserver.java

@@ -89,7 +89,7 @@ public void onNext(T t) {
         }
         if (t == null) {
             s.dispose();
-            onError(new NullPointerException());
+            onError(new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources."));
             return;
         }
         synchronized (this) {

src/main/java/io/reactivex/plugins/RxJavaPlugins.java

@@ -259,14 +259,14 @@ public static void onError(Throwable error) {
             } catch (Throwable e) {
                 // Exceptions.throwIfFatal(e); TODO decide
                 if (error == null) {
-                    error = new NullPointerException();
+                    error = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
                 }
                 e.printStackTrace(); // NOPMD
                 uncaught(e);
             }
         } else {
             if (error == null) {
-                error = new NullPointerException();
+                error = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
             }
         }
         error.printStackTrace(); // NOPMD

src/main/java/io/reactivex/processors/AsyncProcessor.java

@@ -87,7 +87,7 @@ public void onNext(T t) {
     @SuppressWarnings("unchecked")
     void nullOnNext() {
         value = null;
-        Throwable ex = new NullPointerException();
+        Throwable ex = new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources.");
         error = ex;
         for (AsyncSubscription<T> as : subscribers.getAndSet(TERMINATED)) {
             as.onError(ex);
@@ -98,7 +98,7 @@ void nullOnNext() {
     @Override
     public void onError(Throwable t) {
         if (t == null) {
-            t = new NullPointerException();
+            t = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
         }
         if (subscribers.get() == TERMINATED) {
             RxJavaPlugins.onError(t);

src/main/java/io/reactivex/processors/BehaviorProcessor.java

@@ -175,7 +175,7 @@ public void onSubscribe(Subscription s) {
     @Override
     public void onNext(T t) {
         if (t == null) {
-            onError(new NullPointerException());
+            onError(new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources."));
             return;
         }
         if (done) {
@@ -191,7 +191,7 @@ public void onNext(T t) {
     @Override
     public void onError(Throwable t) {
         if (t == null) {
-            t = new NullPointerException();
+            t = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
         }
         if (done) {
             RxJavaPlugins.onError(t);

src/main/java/io/reactivex/processors/PublishProcessor.java

@@ -189,7 +189,7 @@ public void onNext(T t) {
             return;
         }
         if (t == null) {
-            onError(new NullPointerException());
+            onError(new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources."));
             return;
         }
         for (PublishSubscription<T> s : subscribers.get()) {
@@ -205,7 +205,7 @@ public void onError(Throwable t) {
             return;
         }
         if (t == null) {
-            t = new NullPointerException();
+            t = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
         }
         error = t;
 

src/main/java/io/reactivex/processors/ReplayProcessor.java

@@ -265,7 +265,7 @@ public void onSubscribe(Subscription s) {
     @Override
     public void onNext(T t) {
         if (t == null) {
-            onError(new NullPointerException());
+            onError(new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources."));
             return;
         }
         if (done) {
@@ -283,7 +283,7 @@ public void onNext(T t) {
     @Override
     public void onError(Throwable t) {
         if (t == null) {
-            t = new NullPointerException();
+            t = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
         }
         if (done) {
             RxJavaPlugins.onError(t);
@@ -1230,4 +1230,4 @@ public int size() {
             return s;
         }
     }
-}
+}

src/main/java/io/reactivex/subjects/AsyncSubject.java

@@ -86,7 +86,7 @@ public void onNext(T t) {
     @SuppressWarnings("unchecked")
     void nullOnNext() {
         value = null;
-        Throwable ex = new NullPointerException();
+        Throwable ex = new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources.");
         error = ex;
         for (AsyncDisposable<T> as : subscribers.getAndSet(TERMINATED)) {
             as.onError(ex);
@@ -97,7 +97,7 @@ void nullOnNext() {
     @Override
     public void onError(Throwable t) {
         if (t == null) {
-            t = new NullPointerException();
+            t = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
         }
         if (subscribers.get() == TERMINATED) {
             RxJavaPlugins.onError(t);

src/main/java/io/reactivex/subjects/BehaviorSubject.java

@@ -172,7 +172,7 @@ public void onSubscribe(Disposable s) {
     @Override
     public void onNext(T t) {
         if (t == null) {
-            onError(new NullPointerException());
+            onError(new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources."));
             return;
         }
         if (done) {
@@ -188,7 +188,7 @@ public void onNext(T t) {
     @Override
     public void onError(Throwable t) {
         if (t == null) {
-            t = new NullPointerException();
+            t = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
         }
         if (done) {
             RxJavaPlugins.onError(t);

src/main/java/io/reactivex/subjects/PublishSubject.java

@@ -175,7 +175,7 @@ public void onNext(T t) {
             return;
         }
         if (t == null) {
-            onError(new NullPointerException("Subject got a null value. Null values are generally not allowed in 2.x operators and sources."));
+            onError(new NullPointerException("onNext got a null value. Null values are generally not allowed in 2.x operators and sources."));
             return;
         }
         for (PublishDisposable<T> s : subscribers.get()) {
@@ -191,7 +191,7 @@ public void onError(Throwable t) {
             return;
         }
         if (t == null) {
-            t = new NullPointerException();
+            t = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
         }
         error = t;
 

src/main/java/io/reactivex/subjects/ReplaySubject.java

@@ -248,7 +248,7 @@ public void onSubscribe(Disposable s) {
     @Override
     public void onNext(T t) {
         if (t == null) {
-            onError(new NullPointerException());
+            onError(new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources."));
             return;
         }
         if (done) {
@@ -266,7 +266,7 @@ public void onNext(T t) {
     @Override
     public void onError(Throwable t) {
         if (t == null) {
-            t = new NullPointerException();
+            t = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
         }
         if (done) {
             RxJavaPlugins.onError(t);

src/main/java/io/reactivex/subjects/UnicastSubject.java

@@ -184,7 +184,7 @@ public void onNext(T t) {
             return;
         }
         if (t == null) {
-            onError(new NullPointerException());
+            onError(new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources."));
             return;
         }
         queue.offer(t);
@@ -198,7 +198,7 @@ public void onError(Throwable t) {
             return;
         }
         if (t == null) {
-            t = new NullPointerException();
+            t = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
         }
         error = t;
         done = true;

src/main/java/io/reactivex/subscribers/SafeSubscriber.java

@@ -73,7 +73,7 @@ public void onNext(T t) {
         }
 
         if (t == null) {
-            Throwable ex = new NullPointerException();
+            Throwable ex = new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources.");
             try {
                 s.cancel();
             } catch (Throwable e1) {
@@ -155,7 +155,7 @@ public void onError(Throwable t) {
         }
 
         if (t == null) {
-            t = new NullPointerException();
+            t = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
         }
 
         try {

src/main/java/io/reactivex/subscribers/SerializedSubscriber.java

@@ -76,7 +76,7 @@ public void onNext(T t) {
         }
         if (t == null) {
             subscription.cancel();
-            onError(new NullPointerException());
+            onError(new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources."));
             return;
         }
         synchronized (this) {