Conditional operator in concatenated string

Question

I\'d like know why the following program throws a NPE

public static void main(String[] args) {
    Integer testInteger = null;
    String test = \"test\" + t         


        

Answer 1

I believe you need to add the parenthesis. Here is a working example which produces "http://localhost:8080/catalog/rest"

public static String getServiceBaseURL(String protocol, int port, String hostUrl, String baseUrl) {
    return protocol + "://" + hostUrl + ((port == 80 || port == 443) ? "" : ":" + port) + baseUrl;
}

Answer 2

This is an example of the importance of understanding operator precedence.

You need the parentheses otherwise it is interpreted as follows:

String test = ("test" + testInteger) == null ? "(null)" : testInteger.toString();

See here for a list of operators and their precedence. Also note the warning at the top of that page:

Note: Use explicit parentheses when there is even the possibility of confusion.

Answer 3

Without the brackets it's doing this effectively: String test = ("test" + testInteger) == null ? "(null)" : testInteger.toString(); Which results in an NPE.

Answer 4

Because it's evaluated as "test" + testInteger (which is "testnull", and therefore NOT null), meaning your testInteger == null test will never return true.