CODEWITHSUNDEEP
java
홍윤표
홍윤표

Reputation: 31

Null & Pointer in Java

package ex;

class Item{
    String text = "hello";
}

class A {
    Item item;

    private A() {}

    private static class LazyHolder {
        public static final A INSTANCE = new A();
    }

    public static A getInstance() {
        return LazyHolder.INSTANCE;
    }
}



public class Main {
    public static void main(String[] args) {
        A a = A.getInstance();

        Item n0 = a.item;

        a.item = new Item();

        Item n1 = a.item;

        a.item.text = "world";

        Item n2 = a.item;

        if(n0 != null)
        {System.out.println(n0.text);}
        else{System.out.println("null");};
        // This print "null"

        System.out.println(n1.text);
        // This print "world" 

        System.out.println(n2.text);
        // This print "world"
    }
}

Hello I'm a student studying java alone. And i have a question.

As you see, n1 & n2 are not null, n0 is null. And n1.text and n2.text both has "world".

So when I saw this result, i got a conclusion, but i don't know what i think is true.

This is my question: If some field have null, Does it mean that the filed has no pointer?

re-question:

Can i understand that n0 "has" pointer to null, n1 and n2 has pointer to Item type Instance?

Upvotes: 0

Views: 308

Answers (2)

EJoshuaS - Stand with Ukraine
EJoshuaS - Stand with Ukraine

Reputation: 12181

If some field have null, Does it mean that the filed has no pointer?

No it doesn't because a null pointer is still a pointer - it's just a pointer that's assigned null.

Keep in mind that being assigned a null pointer is quite different than being completely uninitialized. For example, the following won't even compile:

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

The following will compile, and it'll literally print null:

public static void main(String []args){
     String s = null;
     System.out.println(s);
 }

Some similar examples of trying to use something assigned null may throw a NullPointerException, which is actually one of the most discussed exceptions on Stack Overflow.

Upvotes: 2

divyang4481
divyang4481

Reputation: 1783

there is not pointer feature in JAVA, so if any object is not initialized with any value then reference to NULL

Upvotes: 0

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