Question

I’m using the properties -D method to give arguments to a jar file on the command line. Here’s the code:

        String prop1 = "one";  

        String val1 = System.getProperty(prop1);  

        System.out.println(val1);

This works, but when I try to implement this in a larger program, strange things happen. Without arguments, this will print null, which is fine. But if I then do something like boolean test = val1.equals(null); , I get problems that don’t seem to relate to the code I’ve provided. Any thoughts?

Answer 1

You are encoutering a NullPointerException because you try to evaluate a method on a null reference. If val1 is null, you cannot invoke a method on it. If you want to test for null, you should test with ==.

if (val1==null)
   // doSomething

or

if(val1!=null && val.equals("aValue")) {
    // Do something
}