IIRC, in the early days of Stack Overflow, this used to be the other way around. Is there a way to see historical trends in questions with a specific tag?
For C# and Java, the plot shows that Java overtook C#, question-count wise, in April 2014:
A few upvoters asked about comparing Java and Android. Here is that plot:
And one comparing Java and Swing:
Sure - just write yourself a little query: http://data.stackexchange.com/stackoverflow/query/201292#graph
That's just questions per month, not the cumulative totals - calculating when the number of Java questions first surpassed C# questions is left as an exercise for the reader.