Assign null to null in Java

Question

I got this piece of code from a commercial product. I believe that it will always throw the java.lang.NullPointerException exception when this.result is null. Am I right to say that?

private void closeCurrentResult() throws SQLException {
    this.lastUpdateCount = -1;
    if(this.result != null) {
        this.result.close();
    }

    this.result = null;
}

if this.result is null, then

this.result = null

is equivalent to

null = null;

while causes the NPE.

After read through some of the replies, I think the thing becomes clear. I also did a test as the code shown below:

public class NullTest {
    public static void main(String[] args) {
        ABC a = new ABC(null);
        a.test();
    }

    static class ABC {
        Object result;

        ABC(Object result) {
            this.result = result;
        }

        void test() {
            if (this.result != null) {
                System.out.println("do something");
            }

            this.result = null;
        }
    }
}

And it works just fine.

Since result is not being dereferenced (you're not calling a method on result), no NPE will be thorwn. Also, assigning it a new value (whatever it is) will not cause the NPE even if the original reference was null. — kuporific
– kuporific, Commented May 3, 2015 at 14:18
I really recommend you to read an introductory book on Java :) — uraimo
– uraimo, Commented May 3, 2015 at 14:26
@kuporific Can references really be null? I assume that the values held by a reference can be null but not a reference itself. — Chetan Kinger
– Chetan Kinger, Commented May 3, 2015 at 14:28

Thomas Junk · Accepted Answer · 2015-05-03 14:32:30Z

2

Let's go through it step by step:

first this.lastUpdateCount = -1; does nothing but setting a global member variable to -1.

if(this.result != null)

This one (guard clause) tests, whether result is null if not so this.result.close(); the close() method on result is called. The guard says, it's safe to do so. After that this.result = null; sets result to null anyways.

After this codeblock, two things are assured:

1) this.result is null

2) And if this.result had something open it is now close

if this.result is null, then this.result = null is equivalent to null = null;

this.result = is an assignment statement: You are assigning something to this.result, whose value may be null. This operation is NPE-safe.

edited May 3, 2015 at 14:32

answered May 3, 2015 at 14:18

Thomas Junk

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Comments

wings · Accepted Answer · 2015-05-03 15:18:32Z

2

No.. this.result = null simply assigns null reference to this.result.

It will throw NPE only when this == null, which is...never.

NPE means you invoked a method or referred a property on a null reference. That is to say, when your code is like this:

obj.someProperty or obj.someMethod() when obj itself is null.

BTW, null reference is a special reference (instead of a special value or some compile-time mechanism).

All the null reference is identical (same reference).
```
String a = null;
Person p = null;
assert a == b;
```

null reference is the "default value" for reference-type variables.

class Person{
    String name;  // `name` is null until assigned otherwise.
}

NPE will be thrown when unboxing a null reference variable.
```
Integer a = null;
int b = a;  // throws NPE.
```

Many JDK methods threat null reference specially.(Which is good and bad.)

String a  = null;
System.out.println(a);  
// will NOT throw NPE because:
// PrintStream.java:
public void println(String var1) {
    synchronized(this) {
        this.print(var1);
        this.newLine();
    }
}

Another example:

    String b = null;
    String a = String.valueOf(b);
    // a is String "null" instead of null reference. Because:
    // String.java:
    public static String valueOf(Object var0) {
        return var0 == null?"null":var0.toString();
    }

edited May 3, 2015 at 15:18

answered May 3, 2015 at 14:28

wings

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1 Comment

David Over a year ago

Yes, understand better now. Thanks for your explanation.

uraimo · Accepted Answer · 2015-05-03 14:16:57Z

1

No, you are not.

This:

if(this.result != null) {
    this.result.close();
}

Call close() only if this.result is not null, correct. This would have thrown a NullPointerException if the check would have been this.result != null.

The last line:

this.result = null;

Simply set this.result as null, nothing wrong here either.

answered May 3, 2015 at 14:16

uraimo

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3 Comments

David Over a year ago

if this.result is null, it is like to assign null to null, this does't work right?

Chetan Kinger Over a year ago

result is a reference to a null. The reference itself is not null.

uraimo Over a year ago

No, that DOES work, when you assign null to a variable you are just changing its value, as @ChetanKinger points out.

kuporific · Accepted Answer · 2015-05-03 14:25:33Z

0

The result is like a label to a memory address. When result is not null, it's pointing to some memory address where there is an object.

When you use the assignment operator =, as in result = null, you're saying, "make result point to null and I don't care what it was originally pointing at". So whatever result is currently pointing to does not matter, even if it's null, reassigning result to null will not throw a NPE.

answered May 3, 2015 at 14:25

kuporific

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Comments

techPackets · Accepted Answer · 2015-05-03 14:38:02Z

0

The code you posted won't throw a null pointer exception. But after setting the result as null you try to access any method it will throw a null pointer exception.

private void closeCurrentResult() throws SQLException {
    this.lastUpdateCount = -1;
    if(this.result != null) {
        this.result.close();
    }
    this.result = null;
    this.result.someMethod(); //Will throw a Null Pointer Exception
}

answered May 3, 2015 at 14:38

techPackets

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