What Is Null in Java

What is null in Java?

Is null an instance of anything?

No, there is no type which null is an instanceof.

15.20.2 Type Comparison Operator `instanceof`

RelationalExpression:
    RelationalExpression instanceof ReferenceType
At run time, the result of the instanceof operator is true if the value of the RelationalExpression is not null and the reference could be cast to the ReferenceType without raising a ClassCastException. Otherwise the result is false.

This means that for any type E and R, for any E o, where o == null, o instanceof R is always false.

What set does 'null' belong to?

JLS 4.1 The Kinds of Types and Values

There is also a special null type, the type of the expression null, which has no name. Because the null type has no name, it is impossible to declare a variable of the null type or to cast to the null type. The null reference is the only possible value of an expression of null type. The null reference can always be cast to any reference type. In practice, the programmer can ignore the null type and just pretend that null is merely a special literal that can be of any reference type.

What is null?

As the JLS quote above says, in practice you can simply pretend that it's "merely a special literal that can be of any reference type".

In Java, null == null (this isn't always the case in other languages). Note also that by contract, it also has this special property (from java.lang.Object):

public boolean equals(Object obj)

For any non-null reference value x, x.equals(null) should return false.

It is also the default value (for variables that have them) for all reference types:

JLS 4.12.5 Initial Values of Variables

Each class variable, instance variable, or array component is initialized with a default value when it is created:

For all reference types, the default value is null.

How this is used varies. You can use it to enable what is called lazy initialization of fields, where a field would have its initial value of null until it's actually used, where it's replaced by the "real" value (which may be expensive to compute).

There are also other uses. Let's take a real example from java.lang.System:

public static Console console()

Returns: The system console, if any, otherwise null.

This is a very common use pattern: null is used to denote non-existence of an object.

Here's another usage example, this time from java.io.BufferedReader:

public String readLine() throws IOException

Returns: A String containing the contents of the line, not including any line-termination characters, or null if the end of the stream has been reached.

So here, readLine() would return instanceof String for each line, until it finally returns a null to signify the end. This allows you to process each line as follows:

String line;
while ((line = reader.readLine()) != null) {
   process(line);
}

One can design the API so that the termination condition doesn't depend on readLine() returning null, but one can see that this design has the benefit of making things concise. Note that there is no problem with empty lines, because an empty line "" != null.

Let's take another example, this time from java.util.Map<K,V>:

V get(Object key)

Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

If this map permits null values, then a return value of null does not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key to null. The containsKey operation may be used to distinguish these two cases.

Here we start to see how using null can complicate things. The first statement says that if the key isn't mapped, null is returned. The second statement says that even if the key is mapped, null can also be returned.

In contrast, java.util.Hashtable keeps things simpler by not permitting null keys and values; its V get(Object key), if returns null, unambiguously means that the key isn't mapped.

You can read through the rest of the APIs and find where and how null is used. Do keep in mind that they aren't always the best practice examples.

Generally speaking, null are used as a special value to signify:

Uninitialized state
Termination condition
Non-existing object
An unknown value

How is it represented in the memory?

In Java? None of your concern. And it's best kept that way.

Is `null` a good thing?

This is now borderline subjective. Some people say that null causes many programmer errors that could've been avoided. Some say that in a language that catches NullPointerException like Java, it's good to use it because you will fail-fast on programmer errors. Some people avoid null by using Null object pattern, etc.

This is a huge topic on its own, so it's best discussed as answer to another question.

I will end this with a quote from the inventor of null himself, C.A.R Hoare (of quicksort fame):

I call it my billion-dollar mistake. It was the invention of the null reference in 1965. At that time, I was designing the first comprehensive type system for references in an object oriented language (ALGOL W). My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler. But I couldn't resist the temptation to put in a null reference, simply because it was so easy to implement. This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years.

The video of this presentation goes deeper; it's a recommended watch.

What does null mean?

Formally, null is a singleton member of the null type, which is defined to be the subtype of every other Java type.

null is a reference type and its value is the only reference value which doesn't refer to any object. Therefore there is no representation of null in memory. The binary value of a reference-typed variable whose value is null is simply zero (all zero bits). Even though this is not explicitly specified, it follows from the general initialization semantics of objects and any other value would cause major problems to an implementation.

Best way to check for null values in Java?

Method 4 is best.

if(foo != null && foo.bar()) {
   someStuff();
}

will use short-circuit evaluation, meaning it ends if the first condition of a logical AND is false.

Difference between null and empty () Java String

"" is an actual string, albeit an empty one.

null, however, means that the String variable points to nothing.

a==b returns false because "" and null do not occupy the same space in memory--in other words, their variables don't point to the same objects.

a.equals(b) returns false because "" does not equal null, obviously.

The difference is though that since "" is an actual string, you can still invoke methods or functions on it like

a.length()

a.substring(0, 1)

and so on.

If the String equals null, like b, Java would throw a NullPointerException if you tried invoking, say:

b.length()

If the difference you are wondering about is == versus equals, it's this:

== compares references, like if I went

String a = new String("");
String b = new String("");
System.out.println(a==b);

That would output false because I allocated two different objects, and a and b point to different objects.

However, a.equals(b) in this case would return true, because equals for Strings will return true if and only if the argument String is not null and represents the same sequence of characters.

Be warned, though, that Java does have a special case for Strings.

String a = "abc";
String b = "abc";
System.out.println(a==b);

You would think that the output would be false, since it should allocate two different Strings. Actually, Java will intern literal Strings (ones that are initialized like a and b in our example). So be careful, because that can give some false positives on how == works.

What is the difference between null, 0 and nothing?

null means that a variable contains a reference to a space in memory that does not contain an object.

0 is a numeric data type with a value of 0.

Nothing doesn't really exist, however I think you may be viewing this as an empty String "" which is simply a String data type that does not contain a value.

If your looking at this from a Javascript paradigm it may be confusing. In Java variables are not truthy and falsey therefore the type is considered in comparisons.

Is null a Java keyword?

No.It is not a keyword.

Difference between null and empty string

String s1 = ""; means that the empty String is assigned to s1.
In this case, s1.length() is the same as "".length(), which will yield 0 as expected.

String s2 = null; means that (null) or "no value at all" is assigned to s2. So this one, s2.length() is the same as null.length(), which will yield a NullPointerException as you can't call methods on null variables (pointers, sort of) in Java.

Also, a point, the statement

String s1;

Actually has the same effect as:

String s1 = null;

Whereas

String s1 = "";

Is, as said, a different thing.

What Is Null in Java