How to avoid NPE while Auto-Unboxing

Auto-Boxing/Unboxing is introduced in Java 5 version. It supports automatic conversion of primitive type (int, boolean, float, double) to their equivalent Objects ( Integer, Boolean, Float, Double) in assignment , method and constructor invocation and other way also. Here is the examples

Auto-Boxing

Integer i = 10; // Auto-boxing converts primitive int value to Integer Object

Auto-Unboxing

int i = 0;

i= new Integre(10); // Auto-Unboxing converts Integer Object to primitive int

There is serious problem with using Auto-bocing/Unboxing, it may result null pointer exception. Take an example of following program

package testproject;

public class TestAutoUnboxing {

private static TestAutoUnboxing test= new TestAutoUnboxing(); //cause Null Pointer Exception
public static final Boolean YES = true; // Autoboxing for
private boolean amIHappy = YES; // Instance Variable

public static void main(String[] args){
System.out.println("Inside Main");
}
}

This example will cause null pointer exception. Here test is an Object of same class TestAutoUnboxing and it is private and static, it is like Singleton Object. As execution of class is sequential, test object will get instantiated and for that all instance variable has to instatiate. So amIHappy set to value of YES, but YES is not yet initialized to value true. So YES is hold neither true nor false. So non initialized Boolean object is assigned to primitive variable amIHappy auto-unboxing will cause null pointer exception. Here is the exception trace

java.lang.ExceptionInInitializerError
Caused by: java.lang.NullPointerException
at testproject.TestAutoUnboxing.<init>(TestAutoUnboxing.java:7)
at testproject.TestAutoUnboxing.<clinit>(TestAutoUnboxing.java:5)
Could not find the main class: testproject.TestAutoUnboxing. Program will exit.
Exception in thread "main" Java Result: 1

Exception stack trace shows that exception trace starts at line 5 and ends at 7. Line no 5 is test object initialization and line 7 is amIHappy instance variable initialization . To avoid this NPE sequence of statements need to be changed as below.

package testproject;

public class TestAutoUnboxing {

//private static TestAutoUnboxing test= new TestAutoUnboxing(); //cause Null Pointer Exception
public static final Boolean YES = true; // Autoboxing for
private boolean amIHappy = YES; // Instance Variable
private static TestAutoUnboxing test= new TestAutoUnboxing();
public static void main(String[] args){
System.out.println("Inside Main");
}
}

Now this program will run successfully without any error. Here By the time test get initialized static variable YES would have been initialized and auto-unboxing while assigning value to amIHappy instance variable goes fine without any error.

Best practice of avoiding such problems, make use of boolean primitive type instead of mix of object and primitive variable.