Difference between null and empty Java String

Question

What is the difference between null and the     empty string    I have written some simple code   String a       String b   null   System out println a    b      false System out println a equals b       false   Both statements return false  It seems  I am not able to find what is the actual difference between them

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as a curiosity     String s1   null      String s2    quot hello quot         s1   s1   s2           System out println  s      nullhello

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Difference between null  amp  empty string  For example  you have a variable named x  If you write in JS   var x        this means that you have assigned a value which is empty string  length is 0   Actually this is like something but which is feel of nothing    On the other hand    var y   null   this means you ve not assigned a value to y that clearly said by writing null to y at the time of declaration  If you write y length  it will throw an error which indicates that no value assigned to y and as a result can t read length of y

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Amazing answers  but I d like to give from a different perspective  String a    quot StackOverflow quot   String a1    quot StackOverflow quot     quot  quot   String a2    quot StackOverflow quot    null   System out println a    a1      true System out println a    a2      false   So this can tell us  quot  quot  and null point to the different object references

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When you write  String a        It means there is a variable  a  of type string which points to a object reference in string pool which has a value     As variable a is holding a valid string object reference  all the methods of string can be applied here   Whereas when you write  String b   null   It means that there is a variable b of type string which points to an unknown reference  And any operation on unknown reference will result in an NullPointerException   Now  let us evaluate the below expressions   System out println a    b      false  because a and b both points to different object reference  System out println a equals b       false  because the values at object reference pointed by a and b do not match   System out println b equals a       NullPointerException  because b is pointing to unknown reference and no operation is allowed

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String is an Object and can be null  null means that the String Object was not instantiated      is an actual value of the instantiated Object String like  aaa    Here is a link that might clarify that point http   download oracle com javase tutorial java concepts object html

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What your statements are telling you is just that    isn t the same as null - which is true      is an empty string  null means that no value has been assigned   It might be more enlightening to try   System out println a length        0 System out println b length        error  b is not an object      is still a string  meaning you can call its methods and get meaningful information   null is an empty variable - there s literally nothing there

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In Java a reference type assigned null has no value at all   A string assigned    has a value  an empty string  which is to say a string with no characters in it   When a variable is assigned null it means there is no underlying object of any kind  string or otherwise

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null means nothing  it means you have never set a value for your variable but empty means you have set   quot  quot  value to your String for instance see the following example  String str1  String str2    quot  quot    Here str1 is null meaning that you have defined it but not set any value for it yet  but you have defined str2 and set empty value for it so it has a value even that value is  quot  quot   but

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This image might help you to understand the differences    The image was collected from ProgrammerHumor

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here a is an Object but b null  is not an Object it is a null reference  System out println a instanceof Object      true  System out println b instanceof Object      false   here is my similar answer

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null means the name isn t referencing any instantiated object      means an empty string   Here a is referencing some object which happens to be an empty string  b isn t referencing any object as it s null

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A string can be empty or have a null value  If a string is null  it isn t referring to anything in memory  Try s length   gt 0  This is because if a string is empty  it still returns a length of 0  So if you enter nothing for the same  then it will still continue looping since it doesn t register the string as null  Whereas if you check for length  then it will exit out of it s loop

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There is a pretty significant difference between the two   The empty string    is  the string that has no characters in it    It s an actual string that has a well-defined length   All of the standard string operations are well-defined on the empty string - you can convert it to lower case  look up the index of some character in it  etc   The null string null is  no string at all    It doesn t have a length because it s not a string at all   Trying to apply any standard string operation to the null string will cause a NullPointerException at runtime

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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

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String s null   String is not initialized for null  if any string operation tried it can throw null pointer exception   String t  null    It is a string literal with value string  null  same like t    xyz   It will not throw null pointer   String u      It is as empty string   It will not throw null pointer

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I call it my billion-dollar mistake  It was the invention of the null reference in 1965  - https   en wikipedia org wiki Tony Hoare  With respect to real world both can be assumed same  Its just a syntax of a programming language that creates a difference between two as explained by others here  This simply creates overhead like when checking comparing whether string variable has something  you have to first check if its not null and then actual string comparing ie two comparisons  This is a waste of processing power for every string comparisons   Objects equals   checks for null before calling  equals

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The empty string is distinct from a   null reference in that in an   object-oriented programming language a   null reference to a string type   doesn t point to a string object and   will cause an error were one to try to   perform any operation on it  The empty   string is still a string upon which   string operations may be attempted    From the wikipedia article on empty string

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String s       s length     String s   null  s length      A reference to an empty string    points to an object in the heap - so you can call methods on it    But a reference pointing to null has no object to point in the heap and thus you ll get a NullPointerException

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This concept can be better understood from mathematics  Have you ever tried dividing a number  not zero  by 0 using a calculator e g 7 0  You will get a result that looks like something this  undefined  not a number  null etc  This means that the operation is impossible  for some reasons  let s leave those reasons to be discussed another day    Now  perform this  0 7  You will get the output  0  This means that the operation is possible and can be executed  but you the answer is just 0 because nothing is left after the division  There is a valid output and that output is zero   In the first example  not only was the output invalid  the operation was not possible to execute  This is akin to null string in java  The second example is akin to empty string

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and null both are different   the first one means as part of string variable declaration the string constant has been created in the string pool and some memory has been assigned for the same   But when we are declaring it with null then it has just been instantiated jvm   but no memory has been allocated for it  therefore if you are trying to access this object by checking it with    - blank variable    it can t prevent nullpointerexception   Please find below one use-case   public class StringCheck    public static void main String   args           TODO Auto-generated method stub      String s1    siddhartha       String s2           String s3   null       System out println  length s1    s1 length         System out println  length s2    s2 length            this piece of code will still throw nullpointerexception        if s3                 System out println  length s3    s3 length

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In simple term       is an empty String null is an empty String Variable

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You may also understand the difference between null and an empty string this way     Original image by R  Sato   raysato

[java] Difference between null and empty ("") Java String

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