Java NullPointerException in if statement

Question

I currently get the NullPointerException in my following code.

else if (isEmpty(href))
    System.err.println("This is an empty link: " + href);

Where the isEmpty function is the following.

public boolean isEmpty(String HREF){
    if (HREF.equals("") || HREF.equals("null") || HREF == null || HREF.isEmpty())  
        return true;
    else 
        return false;
}

Is it because I can not compare a string to null? What can I do to make this function work?

Answer 1

Your isEmpty test is just doing things in the wrong order. Change this:

if (HREF.equals("") || HREF.equals("null") || HREF == null || HREF.isEmpty())  
    return true;

to:

if (HREF == null || HREF.equals("") || HREF.equals("null") || HREF.isEmpty())  
    return true;

If HREF is actually null, then the first test will short-circuit the rest of the if and you won't get a NPE.

(By the way, explicitly testing HREF.equals("") and also calling HREF.isEmpty() is redundant. You only need to do one or the other.)

By the by-the-way, I would recommend, as a matter of style simplifying your method to a single return statement:

public boolean isEmpty(String HREF){
    return HREF == null || HREF.isEmpty()) || HREF.equals("null");
}

(Also, if testing against the string "null" was an attempt to check against a null value, you can drop that condition as well. In fact, you could then use something like Apache Commons' StringUtils.isEmpty(CharSequence) method to do what you want.)

Answer 2

changing the order in your if will solve it.

public boolean isEmpty(String HREF) {
    return HREF == null || HREF.equals("") || HREF.equals("null") ||  HREF.isEmpty();
}

this way, when HREF == null, none of the other tests will be evaluated.

Answer 3

When HREF is null , referencing it with dot operator will throw a null pointer exception .

Different ways to solve this is as follows

  public boolean isEmpty(String HREF){
      if (HREF == null ||HREF.equals("") || HREF.equals("null") ||    HREF.isEmpty())  
          return true;
      else 
          return false;
 }

this is by just re arranging your conditions, by short cut of boolean operations, this will avoid referencing a null pointer in subsequent checks.

OR

  public boolean isEmpty(String HREF){
      //handling null case first of all
      if(HREF == null)   return true;

      if(HREF.equals("") || HREF.equals("null") ||    HREF.isEmpty())  
          return true;
      else 
          return false;
 }

Also avoid redundant use of checks like HREF.equals("") and HREF.isEmpty() are similar.

Another best practice to check equals with a string object is to use the constant in the beginning as in "".equals(HREF) and "null".equals(HREF), here we are pretty sure that "null" is a valid string and which is not null

Answer 4

You cannot call methods of the null here HREF.equals("") Common practice is to change the order "".equals(HREF) . This is null safe. And you will not need HREF.isEmpty() part this way. So the code will become

public boolean isEmpty(String HREF){
     return HREF == null || "".equals(HREF) || "null".equals(HREF);
}

I have removed the unnecessary if here as we are returning the result of boolean expression.

Answer 5

If HREF is null, then doing this:

HREF.equals("")

will throw a NPE.

Do the null check first:

HREF != null && /* rest of conditions */