Null pointer exception

Precondition for null pointer exception : “ Null reference ” Access instance 【 object 】 Relevant data , Null pointer exceptions will occur !

Let's first look at the following example code :

public class NullPointerTest {
    
    public static void main(String[] args) {
    
        //  Create customer object 
        Customer o = new Customer();
        //  Visit this customer's id
        System.out.println(" Customer's id yes  "+o.id);
        //  Back to the Id assignment 
        o.id = 9521; //  Lifetime code 
        System.out.println(" Customer's id yes  "+o.id);
        o = null;  //  representative o Do not save memory address 
        System.out.println(o.id);
    }
}
class Customer{
    
    //  Customer id
    int id; //  Access instance variables in member variables , You should create the object first , adopt " quote ." Mode of access 1
}

Running results :

 Customer's id yes  0
 Customer's id yes  9521
Exception in thread "main" java.lang.NullPointerException
	at com.lxz.study01.NullPointerTest.main(NullPointerTest.java:19)

A null pointer exception is reported in the last line of the running result , This is because of the code :

o = null;  //  representative o Do not save memory address 

So that the local variable does not save the instance object in the stack area Customer The address of , Thus, it cannot find the error that the corresponding object in the heap area reports null pointer exception ！
The corresponding memory graph is :

Last , Mention it java Of Garbage collector GC:
stay java In language , Garbage collector mainly aims at heap memory , When one java When an object does not have any references to the object ,GC Will consider recycling the garbage data , The recycled data will not come back .