When programming java.lang.NullPointerException occurs, what causes this error? how to solve?

 When we programming with java, we can see 'java.lang.NullPointerException' error often. NullPointerException is kinds of RuntimeException.

 This error occurs as a Java program attempts to use a null when an object is required. Let's see what this means.

        Integer invalid_intObject = null;

        System.out.println(invalid_intObject.intValue());

 This codes makes NullPointerException because invalid_intObject is object but it's null. Below is valid code.  

        Integer invalid_intObject = 0;

        System.out.println(invalid_intObject.intValue());

 But in real programming world, this is not that simple to debug. So, what's in real programming world? we always call functions that we did not make.

        Integer invalid_intObject = null;

        // .. some code here

        // .. my logic here

        usefulFunction(invalid_intObject);

 This is real world. UserfulFuncion could be standard library or opensource library. and Exception could be like below:

    Exception in thread "main" java.lang.NullPointerException

        at app.App.usefulFunction(App.java:22)

        at app.App.main(App.java:11)

 Actually in real world, the Exception line could be more than 10 lines. then we should track the Exception lists till my code appears.

 we should see App.java file line 22. that's 

    public static void usefulFunction(Integer intObject){

        System.out.println(intObject.intValue());           // line 22

    }

in my app.App.main(App.java) file line 11 is

        usefulFunction(invalid_intObject);          // line 11

now, we can solve the problem by assigning just before the error. ex) 'invalid_intObject = 0;'