As per the recent spec amendments per https://bugs.openjdk.java.net/browse/JDK-8286675
"NullPointerException if the end of the last contained stream has not been reached and b is null."

Expectation :
So this clarifies that When last contained stream has been reached and the b is null shall not throw NPE instead it should return -1.

Actual Behavior :
When last contained stream has been reached and the b is null throws NPE, To result in expected behavior there is a need for one more invocation of read() before this.

For complete context, refer the attached sample with has comments.