空指针异常

Question

There is a possiblity that this may be a dupicate question. I initialize a String variable to null.I may or may not update it with a value.Now I want to check whether this variable is not equal to null and whatever I try I get a null pointer exception.I can't afford to throw nullpointer exception as it is costly.Is there any workaround that is efficient.TIA

Answer 1

If you use

if (x == null)

you will not get a NullPointerException .

I suspect you're doing:

if (x.y == null)

which is throwing because x is null, not because xy is null.

If that doesn't explain it, please post the code you're using to test for nullity.

Answer 2

I guess you are doing something like this,

  String s = null;

  if (s.equals(null))

You either check for null like this

  if (s == null)

A better approach is to ignore the null and just check for the expected value like this,

  if ("Expected value".equals(s))

In this case, the result is always false when s is null.

Answer 3

String is immutable

@Test(expected = NullPointerException.class)
public void testStringEqualsNull() {
    String s = null;
    s.equals(null);
}

@Test
public void testStringEqualsNull2() {
    String s = null;
    TestCase.assertTrue(s == null);
}

Answer 4

I am comparing s==null only

can you show the code snippet that you have written s==null will never throw a NPE

Answer 5

if you are checking whether "s" is null, then do not apply a dot(.) after "s". Doing that would throw NullPOinterException, as applying dot(.) means that you are trying to access on a pointer location which is basically null at the moment !

Also try to use library functions that check whether a string is null or empty. you may use StringUtils.isEmpty(s) from apache library which checked both