NPE while AutoBoxing

Auto-boxing and Unboxing is a feature of Java which allows us to assign primitives to wrappers and vice versa. But, at times while boxing there could be a chance of NPE if null checks are not done.

Auto-boxing:

  Double price = 10.5; // primitve -> wrapper

Unboxing:

  Double price = new Double(10.5); 
  double price1 = price; // wrapper -> primitive

Direct assignment of null to primitives is not allowed in Java
- ```
  double price = null; // Doesn't compile
```

However, a null value of wrapper type can be assigned to a primitive type, which would cause NPE.

  Double price = null; // Assume this value is passed in from caller
  double localPrice = price; // compiles but results in NPE
  /*- Error: 
  Exception in thread "main" 
  java.lang.NullPointerException: Cannot invoke "java.lang.Double.doubleValue()" 
  because "price" is null 
  */

Conisder the following snippet

private static void computePrice(double price) {
  // operation involving price
}
// caller
Double d = null; // assume we got null by calling another service or function
computePrice(d); // compiles, but throws NPE

In sum, it’s always good to do null checks in the code, and have a dedicated unit test case when_null_value_is_passed_in_to_<method-name>

References