ternary operation behaves weirdly [duplicate]

问题

This question already has answers here:

Java ternary operator precedence ? Different outputs given (3 answers)

Strange Null pointer exception case: ternary conditional operator not working with string concatenation (6 answers)

Closed 4 months ago.

I have a difficulty in understanding how the ternary operation works in the below code.

public static void main(String[] args) {
        try{
            throw new ArithmeticException("Exception Testing...");
        }catch(Exception e){
            msg = "First Statement : " + e.getCause() != null ? e.getMessage() : null;  //Here e.getCause() is null
            System.out.println(msg);  //  prints "Exception Testing..."
         }
    }

In the first Statement block(Line 4), e.getcause() is null so it should print First Statement: null instead it prints only Exception Testing....

My question is,

1)Why TRUE block got executed in the ternary operation instead of returning null and also,

2)Why First Statement: is not printed with the msg Exception Testing...?

Thanks in advance.

回答1:

Because of operator precedence, + is applied before ?:, so you are checking whether:

"First Statement : " + e.getCause()

is null - it's not.

Add parentheses:

 msg = "First Statement : " + (e.getCause() != null ? e.getMessage() : null);

来源：https://stackoverflow.com/questions/57672066/ternary-operation-behaves-weirdly