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            Programs must not catch {{java.lang.NullPointerException}}. A {{NullPointerException}} exception thrown at runtime indicates the existence of an underlying {{null}} pointer dereference that must be fixed in the application code (see rule [EXP11-J. Never dereference null pointers].) Handling the underlying null pointer dereference by catching the {{NullPointerException}} rather than fixing the underlying problem is inappropriate for several reasons. First, _avoiding_ the exception by catching {{NullPointerException}} executesadds significantlysignificant moreperformance slowlyoverhead than simply adding the necessary checks \[[Bloch 2008|AA. Bibliography#Bloch 08]\]. Second, when there are multiple expressions in a {{try}} block that are capable of throwing a {{NullPointerException}}, it is difficult or impossible to determine which expression is responsible for the exception because the {{NullPointerException}} {{catch}} block handles any {{NullPointerException}} thrown from any location in the {{try}} block. Third, programs rarely remain in an expected and usable state after a {{NullPointerException}} has been thrown. Attempts to continue execution after first catching and logging (or worse, suppressing) the exception rarely succeed.

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Omitting the null check means that the program fails more quickly than if the program had returned false and let lets an invoking method discover the null value. A function that throws a NullPointerException without a null check must provide a precondition that the argument being passed to it is not null.

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Code Block

bgColor	#ccccff

import java.io.IOException;

public class DivideException {
  public static void main(String[] args) {
    try {
      division(200, 5);
      division(200, 0); // Divide by zero        
    } catch (ArithmeticException ae) { 
      throw new DivideByZeroException();  // DivideByZeroException extends Exception so is checked 
    } catch (IOException ieex) { 
      ExceptionReporter.report(ex);
    }	    
  }

  public static void division(int totalSum, int totalNumber) throws ArithmeticException, IOException  {  
    int average  = totalSum / totalNumber; 
    // Additional operations that may throw IOException...
    System.out.println("Average: "+ average);   	
  }
}

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Code Block

bgColor	#ccccff

class DoSomethingException extends Exception {
  public DoSomethingException(Throwable cause) {
    super( cause);
  }
    
  // other methods
};

private void doSomething() throws DoSomethingException {
  try {
    // code that might throw an Exception
  } catch (Throwable t) {
    throw new DoSomethingException(t);
  }
}

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<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="d892c0202692932e-548223df-4b954a48-a840a3df-5e9855fff2b384eead2437fe"><ac:plain-text-body><![CDATA[	[[J2SE 2011	AA. Bibliography#J2SE 11]]	Catching Multiple Exception Types and Rethrowing Exceptions with Improved Type Checking	]]></ac:plain-text-body></ac:structured-macro>
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