if statement checks for null but still throws a NullPointerException

Question

In this code   public class Test        public static void testFun String str             if  str    null   str length      0                 System out println  String is empty               else                 System out println  String is not empty                           public static void main String    args             testFun null             We pass a null value to the function testFun  Compiles fine  but gives a NullPointerException in runtime  which I did not expect  Why is it throwing an exception  rather than evaluating the if condition to true and printing  String is empty      Suppose the value of the actual argument being passed to testFun is generated from some process  Assume that mistakenly a null value is returned by that process and is fed to testFun  If such is the case  how does one validate that the value passed to the function is null or not   One  weird  solution may be by assigning the formal parameter to some variable inside the function and then testing it  But if there are many variables passed to the function  that might become tedious and unfeasible  So  how does one check for null values in such a scenario

User · Answer

The problem is that you are using the bitwise or operator: |. If you use the logical or operator, ||, your code will work fine.

See also:
http://en.wikipedia.org/wiki/Short-circuit_evaluation
Difference between & and && in Java?

User · Answer

The   and  amp  check both the sides everytime      if  str    null   str length      0    here we have high possibility to get NullPointerException   Logical    and  amp  amp  check the right hand side only if necessary   but with logical operator     no chance to get NPE because it will not check RHS

User · Answer

Change Below line   if  str    null   str length      0      into   if  str    null    str isEmpty        now your code will run corectlly  Make sure str isEmpty   comes after str    null because calling isEmpty   on null will cause NullPointerException  Because of Java uses Short-circuit evaluation when str    null is true it will not evaluate str isEmpty

User · Answer

You can use StringUtils   import org apache commons lang3 StringUtils   if  StringUtils isBlank str      System out println  String is empty       else     System out println  String is not empty         Have a look here also  StringUtils isBlank   vs String isEmpty    isBlank examples   StringUtils isBlank null         true StringUtils isBlank              true   StringUtils isBlank              true   StringUtils isBlank  bob         false   StringUtils isBlank    bob       false

User · Answer

The edit shows exactly the difference between code that works and code that doesn t   This check always evaluates both of the conditions  throwing an exception if str is null    if  str    null   str length      0      Whereas this  using    instead of    is short-circuiting - if the first condition evaluates to true  the second is not evaluated   See section 15 24 of the JLS for a description of     and section 15 22 2 for binary    The intro to section 15 24 is the important bit though      The conditional-or operator    operator is like      15 22 2   but evaluates its right-hand operand only if the value of its left-hand operand is false

User · Answer

The problem here is that in your code the program is calling  null length    which is not defined if the argument passed to the function is null  That s why the exception is thrown

[java] if statement checks for null but still throws a NullPointerException

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