You may also understand the difference between null and an empty string this way:
Original image by R. Sato (@raysato)
Answer from mikiqex on Stack OverflowYou may also understand the difference between null and an empty string this way:
Original image by R. Sato (@raysato)
"" 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.
Videos
First let's clarify something: You mention that after assigning null to the variable you could forget to initialize it, but by assigning null to it you are in effect initializing it.
public static void main (String args[]){
String s;
System.out.println(s); // compiler error variable may not be initialized
}
vs
public static void main (String args[]){
String s=null;
System.out.println(s); // no compiler error
System.out.println(s.equals("helo")); // but this will generate an exception
}
So after you do String s=null; there's is no way that you could forget to initialize because you did initialize it.
That being clear, I would recommend you to use a "smart default". In your case perhaps the empty string "" would be a good default value if you want to avoid NullPointerException. In the other hand, sometimes it is desirable that the program produce an exception because it indicates something wrong happened under the hood that should not have happened.
In general you want to keep declaration and initialisation as close as possible to minimise exactly the type of problem you're talking about.
There is also the issue of redundant initialisation where the value null you're assigning is never used which is extra code that harms readability even if the redundant assignment is optimised away by the compiler.
Sometimes assigning some sort of default value is unavoidable, for example if you declare before a try catch, initialise inside and use it afterwards. For other types you can often find a more natural default value such as an empty list.
I was using IntelliJ and it said that a string needed to be initialized in order for the program to work. It suggests I initialize it this way
String example = null;
instead of
String example = "";
Do you agree with this? I've found conflicting information as to whether or not this is a good idea.
You can use Objects.toString() (standard in Java 7):
Objects.toString(gearBox, "")
Objects.toString(id, "")
From the linked documentation:
public static String toString(Object o, String nullDefault)Returns the result of calling
toStringon the first argument if the first argument is not null and returns the second argument otherwise.Parameters:
o- an object
nullDefault- string to return if the first argument isnullReturns:
the result of callingtoStringon the first argument if it is notnulland the second argument otherwise.See Also:
toString(Object)
For java 8 you can use Optional approach:
Optional.ofNullable(gearBox).orElse("");
Optional.ofNullable(id).orElse("");