Those sneaky NullPointerExceptions!

There is a form of driving called “defensive driving”. Defensive drivers anticipate accidents and bad drivers. This lowers the chances of getting into accidents. It has saved me from several bad accidents.

Similarly, we can avoid the scary NullPointerException in Java if we know how to anticipate it.

For example, what is wrong with this if check?

if (aString.equals("yes")) { /* do stuff */ }

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It’s assuming that aString has been initialized. If a string is not initialized, it will be null! Calling .equals() on a null object is not going to end well. This happens more often than you might expect. It is better to write this:

if ("yes".equals(aString)) { /* do stuff */ } 

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I edited a POJO recently that was meant to represent a deserialized JSON object. The business requirements said I needed to remove any leading or trailing whitespace. Thankfully, the String.trim() method can do this for me in a single line! I placed it in the setter like this:

public class Person {
  private String firstName;

  public String getFirstName() { return this.firstName; }

  public void setFirstName(String firstName) { 
    this.firstName = firstName.trim(); 
  }
}

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Seems innocent enough.

Imagine my frustration when converting a JSON to this object was throwing a com.fasterxml.jackson.databind.JsonMappingException with a not-so-helpful explanation of “N/A”. What was happening???

Apparently, if firstName is null in JSON, setFirstName(firstName) was calling trim() on a null object!

What I need to do is check to make sure that the String parameter in setFirstName(firstName) is not null. There’s more than one way to check, but I chose a ternary operator:

public class Person {
  private String firstName;

  public String getFirstName() { return this.firstName; }

  public void setFirstName(String firstName) { 
    this.firstName = (firstName == null) ? null : firstName.trim(); 
  }
}

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How about you? Have you run into the dreaded NullPointerException recently? How did you fix it?

原文链接：Those sneaky NullPointerExceptions!