Why is a empty variable declaration of type String "null" and not "" (blank or empty)

Question

Declaring a variable as such:

private String concatenation;

would have System.out.println(concatenation) show this:

null

Which would be a string of length 4, and not a null(of boolean type as in null or !null) value. This would make sense if I delcared it … = "null";, but I didn't.

but if it is declared as:

private String concatenation = "";

then, System.out.println(concatenation) would show this:

` ` // No clue how to get a space to show up in SO so I am surrounding it with ticks

Why does Java not just create it as an Object with an empty string, why do I have to define it as being empty.

Oliver Charlesworth · Accepted Answer · 2013-06-09 20:30:49Z

16

It's not "null", it's null. But println displays it like that because it's defined to do so (see the Javadocs).

As for why it's not initialized to an empty string, that's because String is a class type, and the default value for references to class instances is always null.

There is a big difference between no object at all (i.e. null) and an object in an "empty" state.

edited Jun 9, 2013 at 20:30

answered Jun 9, 2013 at 20:24

Oliver Charlesworth

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2 Comments

Steve Kuo Over a year ago

println is a method of PrintStream. String.valueOf(Object) (your JavaDoc link) is not called as part of println

Oliver Charlesworth Over a year ago

@SteveKuo: If you follow the links in the documentation for PrintStream.println, that is where you'll end up...

tgkprog · Accepted Answer · 2013-06-09 20:30:45Z

1

This is because that is the way system out println works -> it prints null when the object send to it is null (or its a reference with no current reference)

You could over write it :

String concatenation = null;
System.out.println(concatenation ==null ? "" : concatenation )

This is sending an empty string to the println function instead of null. FYI : The function actually calls toString on most object to get what should be displayed, unless the object it self is a string .

More theory, try:

System.out.println(concatenation == null);

and see what happens. –( jlordo)

answered Jun 9, 2013 at 20:30

tgkprog

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Comments

Sello · Accepted Answer · 2013-06-09 21:50:18Z

0

As previously said String is a class and variable concatenation is a reference variable and reference variables have default value of null, seing this output tells you that whatever reference variable that was, its not initialised, points to no object, so an empty string != null, these are two different values that mean different things. I think if it where to automatically create an empty string,it'l be creating unnecessary objects..

answered Jun 9, 2013 at 21:50

Sello

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user2277872 · Accepted Answer · 2013-06-09 20:43:07Z

The String is not an object, it is a class. It is needed just like any other class if you're wanting to create an object off it. An example, if you did:

int i;

you would eventually have to initialize and set a value for i. Just like with String. You cannot have an object holding NULL (no specific spot allocated in memory), that will tell the program there is no such memory inside that object. That is why you get null being printed out. If you do "", it is blank, BUT, it is also initialized because the object has a spot in the memory system for the "" to be placed, and eventually searched and accessed later. I have had a bit of struggle with this throughout my Java programming, but I eventually learned that if I have my Strings, int(s), and etc initialized (set a value to the object for memory reference later) then nothing would cause a problem

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