In this topic:
Add an assertion to fix the error Create a .jkb file and describe the method as the check Bind the Check record to the checker Test the knowledge base

Java tuning tutorial 3 - Tuning NPE.RET to reduce false positives

Let's say we have a snippet:

package com.klocwork.jdefects.checkers.dfa.binding_walkthrough;


public class CheckSample1 {
   public String toString() {   
      final Object o = get();   
      return o.toString();  
   }
 
   private Object get() {   
      if (hashCode() > 0 ) {      
         return new Object();  
      }    
      return null;  
   }
}

An analysis with kwcheck detects the NPE.RET issue, where null comes from the get()and is dereferenced at o.toString().

Add an assertion to fix the error

To fix the NPE.RET issue, add assertNotNull(o); to the code:

package com.klocwork.jdefects.checkers.dfa.binding_walkthrough;


public class CheckSample1 {  
   public String toString() {   
      final Object o = get();
      assertNotNull(o);
      return o.toString(); 
   }
 
   private Object get() {    
      if (hashCode() > 0 ) {    
         return new Object();    
      }  
      return null;  
   }
}

Create a .jkb file and describe the method as the check

After we add the assertion and re-run the analysis, NPE.RET is still reported because the call to assertNotNull is the call to the library method, about which Klocwork does not have any specific information.

Create a text file with the .jkb file extension, for example, check.jkb.
In the file, describe the method as the check:

package junit.framework;


class Assert { 
   public static void assertNotNull(@Check Object object);
}

Note: This step is also captured for Eclipse and IntelliJ IDEA users.

Bind the Check record to the checker

After you add @Check, bind the data to the issue, using @Bind("NPE.RET"):

package junit.framework;


@Bind("NPE.RET")
class Assert { 
   public static void assertNotNull(@Check Object object);

}

Test the knowledge base

To test your knowledge base:

Import the knowledge base into your project using kwcheck:
```
kwcheck import check.jkb
```
Run the analysis:
```
kwcheck run
```

When the code is analyzed using this knowledge base, Klocwork now knows assertNotNull is the check; consequently, NPE.RET is not reported.