GPLv2 has patent grant, read the license. GPLv3 has more broad patent grant, but that is irrelevant in case of OpenJDK because Oracle is one who distribute it. If Oracle move OpenJDK to GPLv3, it would make no difference. They can't sue you now, and they can't if it is GPLv3. They can only sue non-GPL implementation like Dalvik.
What you meant probably is the case of Mono. But that is different issue. Mono is not distributed by patent holder (Microsoft). It is by Novell. So Microsoft is third party and they can troll under GPLv2, but couldn't under GPLv3. But that has nothing to do with Java. Java is distributed by patent holder and it is irrelevant which version of GPL it uses.
There is no reason to believe that Oracle is lying about this. Everybody with half of clue can see that Oracle can't contain Java and they can't have it all for themselves. That ship have sailed. Java is free now and there is no going back. Oracle can play nice of others, or they can completely lose control of Java. There is no other choice. Google made this to themselves by not using GPL'd Java, but they can still try to invalidate those patents and use OpenJDK in future.