Ternary Operator and unexpected NullPointerException

问题

I am getting NullPointerException from the below line sometimes.

System.out.println("Date::"+ row != null ? row.getLegMaturityDate() : "null");

After adding brackets, it is fine.

System.out.println("Date::"+ (row != null ? row.getLegMaturityDate() : "null"));

Please clarify me the behavior. Thanks in advance.

回答1:

"Date::" + row is never null, although row sometimes is.

That is, "Date::"+ row != null is equivalent to ("Date::"+ row) != null which is always true.

回答2:

It's a matter of operator precedence. Christoffer Hammarström has the executive summary. See this page http://bmanolov.free.fr/javaoperators.php for more detail.

来源：https://stackoverflow.com/questions/8168130/ternary-operator-and-unexpected-nullpointerexception