You cannot use the dereference (dot, '.') operator to access instance variables or call methods on an instance if that instance is null. Doing so will yield a NullPointerException.

It is common practice to use something you know to be non-null for string comparison. For example, "something".equals(stringThatMayBeNull).

Copyif (myString != null && !myString.isEmpty()) {
  // doSomething
}

As further comment, you should be aware of this term in the equals contract:

From Object.equals(Object):

For any non-null reference value x, x.equals(null) should return false.

The way to compare with null is to use x == null and x != null.

Moreover, x.field and x.method() throws NullPointerException if x == null.

if(dates[i] != null);
                    ^

the extra ; causes the following block to always execute (regardless of the evaluation of the if statement), since it ends the if statement. Remove it.

1st one is better (and the only option), because 2nd one will throw NPE, when your value is actually null. As simple as that.

Try this out:

String str = null;
str.equals(null);  // will throw `NPE`.

So basically, the test which you wanted to perform itself triggers a NullPointerException in the 2nd case. So, it is no choice.

string == null compares if the object is null. string.equals("foo") compares the value inside of that object. string == "foo" doesn't always work, because you're trying to see if the objects are the same, not the values they represent.

Longer answer:

If you try this, it won't work, as you've found:

CopyString foo = null;
if (foo.equals(null)) {
    // That fails every time. 
}

The reason is that foo is null, so it doesn't know what .equals is; there's no object there for .equals to be called from.

What you probably wanted was:

CopyString foo = null;
if (foo == null) {
    // That will work.
}

The typical way to guard yourself against a null when dealing with Strings is:

CopyString foo = null;
String bar = "Some string";
...
if (foo != null && foo.equals(bar)) {
    // Do something here.
}

That way, if foo was null, it doesn't evaluate the second half of the conditional, and things are all right.

The easy way, if you're using a String literal (instead of a variable), is:

CopyString foo = null;
...
if ("some String".equals(foo)) {
    // Do something here.
}

If you want to work around that, Apache Commons has a class - StringUtils - that provides null-safe String operations.

Copyif (StringUtils.equals(foo, bar)) {
    // Do something here.
}

Another response was joking, and said you should do this:

Copyboolean isNull = false;
try {
    stringname.equalsIgnoreCase(null);
} catch (NullPointerException npe) {
    isNull = true;
}

Please don't do that. You should only throw exceptions for errors that are exceptional; if you're expecting a null, you should check for it ahead of time, and not let it throw the exception.

In my head, there are two reasons for this. First, exceptions are slow; checking against null is fast, but when the JVM throws an exception, it takes a lot of time. Second, the code is much easier to read and maintain if you just check for the null pointer ahead of time.

If Java 7+, use Objects.equals(); its documentation explicitly specifies that:

[...] if both arguments are null, true is returned and if exactly one argument is null, false is returned. Otherwise, equality is determined by using the equals method of the first argument.

which is what you want.

If you don't, your method can be rewritten to:

Copyreturn s1 == null ? s2 == null : s1.equals(s2);

This works because the .equals() contract guarantees that for any object o, o.equals(null) is always false.

There's a key difference between a null array and an empty array. This is a test for null.

int arr[] = null;
if (arr == null) {
  System.out.println("array is null");
}

"Empty" here has no official meaning. I'm choosing to define empty as having 0 elements:

arr = new int[0];
if (arr.length == 0) {
  System.out.println("array is empty");
}

An alternative definition of "empty" is if all the elements are null:

Object arr[] = new Object[10];
boolean empty = true;
for (int i=0; i<arr.length; i++) {
  if (arr[i] != null) {
    empty = false;
    break;
  }
}

or

Object arr[] = new Object[10];
boolean empty = true;
for (Object ob : arr) {
  if (ob != null) {
    empty = false;
    break;
  }
}

Since Java 7 you can use the static method java.util.Objects.equals(Object, Object) to perform equals checks on two objects without caring about them being null.

If both objects are null, it will return true, if one is null and the other isn't, it will return false. Otherwise, it will return the result of calling equals on the first object with the second as argument.

You should use equals() rather than compareTo(). compareTo() raises NullPointerException if passed null.

Java doc of Comparable

Note that null is not an instance of any class, and e.compareTo(null) should throw a NullPointerException even though e.equals(null) returns false.

          if (stringArray[i] == null) {
                continue;
            } else if (stringArray[i].equals(stringArray[j]) && i != j) {
                // duplicate
                duplicates++;
            }

So complete source code should look like. I have not changed any logic inside your for loop.

    String line = "DO UNTO OTHERS AS YOU WOULD HAVE THEM DO UNTO YOU";
    String[] stringArray = line.split("\\s");//Use split StringTokenizer use is discouraged in new code
    int duplicates = 0;
    int wordCount = stringArray.length;
    for (int j = 0; j < stringArray.length; j++) {
        for (int i = j + 1; i < stringArray.length; i++) {
            if (stringArray[i] == null) {
                stringArray[i] = "null";
            } else if (stringArray[i].equals(stringArray[j]) && i != j) {
                // duplicate
                System.out.print("\n" + stringArray[j]);
                duplicates++;
            }
        }
    }
    wordCount -= duplicates;
    System.out.print("\nNumber of words, not including duplicates: "
            + wordCount);

Using "abc".equals(...) is a reasonable precaution against a condition you didn't anticipate, but doesn't really solve any problems. Maybe this particular method doesn't care that myString is null (the string comparison returns false, but that's it), but what about elsewhere? Now there's this null value working its way through your logic that's not supposed to be there.

In this particular case, it might not matter, but in another, letting this invalid value pass through part of the logic could lead to an incorrect state later on (e.g. an address with the city missing). Fail early!

If myString should not be null, then write the code so that it can't be. Add contracts to your methods, or at least some in-method argument validation to ensure invalid values are dealt with before the method's logic ever executes. Then be sure your tests cover these cases.

If you add a string to null, the null is converted to the string "null". For instance, null + " hi there" gives "null hi there".

So if wordInd[length] is null, and you execute

wordInd[length] += " " + word + " ";

Then you are concatenating null to a string, giving you a string starting with "null ".

Try checking for null:

if (wordInd[length]==null) {
    wordInd[length] = word;
} else {
    wordInd[length] += " "+word;
}

No, because they are not the same. "" is very different from null.

Now if you want this behavior write a custom method:

static boolean checkIfEmptyOrNull(String s1, String s2) {
     //check for empty and null
     return myBooleanValue;
}

Mark this as static because this should be a helper (and should probably go in a helper class as well)

You may also understand the difference between null and an empty string this way:

Original image by R. Sato (@raysato)