Unknown NullPointerException in Java
java
Solution
This statement:
return demo==null?1:demo.getValue();
is actually being converted into something like this:
int tmp = demo == null ? 1 : demo.getValue().intValue();
return (Integer) tmp;
The type of the conditional expression is determined to be `int` (not `Integer`) based on the rules laid out in the JLS, section 15.25, using binary numeric promotion (5.6.2). The conversion from the null reference to `int` then triggers the `NullPointerException`.
Problem
The following snippet uses simple Java code. ``` package pkg; final public class Main { final private class Demo { private Integer value = null; public Integer getValue() { return value; } } private Integer operations() { Demo demo = new Demo(); return demo==null?new Integer(1):demo.getValue(); } public static void main(String[] args) { Main main=new Main(); System.out.println("Value = " + String.valueOf(main.operations())); } } ``` The above code works as expected with no problem and displays `Value = null` on the console. In the following `return` statement, ``` return demo==null?new Integer(1):demo.getValue(); ``` since the object `demo` of type `Demo` is not `null`, the expression after `:` which is `demo.getValue()` is executed which invokes `getValue()` within the inner `Demo` class which returns `null` and finally, it is converted to String and displayed on the console. But when I modify the `operations()` method something like the one shown below, ``` private Integer operations() { Demo demo = new Demo(); return demo==null?1:demo.getValue(); } ``` it throws `NullPointerException`. How? I mean when I use this `return` statement ``` return demo==null?new Integer(1):demo.getValue(); ``` it works (doesn't throw `NullPointerException`) and when I use the following something similar `return` statement ``` return demo==null?1:demo.getValue(); ``` it causes `NullPointerException`. Why?