Java null check why use instead of equals

Question

In Java I am told that when doing a null check one should use    instead of  equals    What are the reasons for this

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If an Object variable is null  one cannot call an equals   method upon it  thus an object reference check of null is proper

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You could always do  if  str    null    str equals null     This will first check the object reference and then check the object itself providing the reference isnt null

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foo equals null    What happens if foo is null     You get a NullPointerException

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In addition to the accepted answer  https   stackoverflow com a 4501084 6276704    Since Java 1 7  if you want to compare two Objects which might be null  I recommend this function   Objects equals onePossibleNull  twoPossibleNull       java util Objects      This class consists of static utility methods for operating on   objects  These utilities include null-safe or null-tolerant methods   for computing the hash code of an object  returning a string for an   object  and comparing two objects       Since    1 7

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If we use    equals method  if obj equals null         Which mean null equals null  when obj will be null    When your obj will be null it will throw Null Point Exception   so we should use     if obj    null    it will compare the references

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I have encountered this case last night  I determine that simply that       Don t exist equals   method for null    So  you can not invoke an inexistent method if you don t have    --    That is reason for why we use    to check null

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So I never get confused and avoid problems with this solution   if str trim   length    lt  0           is null

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If you try calling equals on a null object reference  then you ll get a null pointer exception thrown

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According to sources it doesn t matter what to use for default method implementation   public boolean equals Object object        return this    object      But you can t be sure about equals in custom class

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here is an example where str    null but str equals null  when using org json   JSONObject jsonObj   new JSONObject   field  null      Object field   jsonObj get  field     System out println field    null               gt  true  System out println  field equals null        gt  true  System out println  field getClass           gt  org json JSONObject Null      EDIT  here is the org json JSONObject Null class          JSONObject NULL is equivalent to the value that JavaScript calls null     whilst Java s null is equivalent to the value that JavaScript calls    undefined      private static final class Null                   A Null object is equal to the null value and to itself                 param object                   An object to test for nullness          return true if the object parameter is the JSONObject NULL object or                null               Override     public boolean equals Object object            return object    null    object    this

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You code breaks Demeter s law  That s why it s better to refactor the design itself  As a workaround  you can use Optional    obj   Optional ofNullable object1       map o - gt  o getIdObject11         map o - gt  o getIdObject111         map o - gt  o getDescription         orElse  quot  quot    above is to check to hierarchy of a object so simply use Optional ofNullable object1    if you have only one object to check Hope this helps

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Because equal is a function derived from Object class  this function compares items of the class  if you use it with null it will return false cause cause class content is not null  In addition    compares reference to an object

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Object equals is null safe  however be aware that if two objects are null  object equals will return true so be sure to check that the objects you are comparing aren t null  or hold null values  before using object equals for comparison    String firstname   null  String lastname   null   if Objects equals firstname  lastname        System out println  equal       else       System out println  not equal         Example snippet above will return equal

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In Java 0 or null are simple types and not objects    The method equals   is not built for simple types  Simple types can be matched with

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They re two completely different things     compares the object reference  if any  contained by a variable   equals   checks to see if two objects are equal according to their contract for what equality means  It s entirely possible for two distinct object instances to be  equal  according to their contract  And then there s the minor detail that since equals is a method  if you try to invoke it on a null reference  you ll get a NullPointerException   For instance   class Foo       private int data       Foo int d            this data   d              Override     public boolean equals Object other            if  other    null    other getClass      this getClass                 return false                    return   Foo other  data    this data                In a real class  you d override  hashCode  here as well       Foo f1   new Foo 5   Foo f2   new Foo 5   System out println f1    f2      outputs false  they re distinct object instances  System out println f1 equals f2       outputs true  they re  equal  according to their definition  Foo f3   null  System out println f3    null      outputs true   f3  doesn t have any object reference assigned to it  System out println f3 equals null       Throws a NullPointerException  you can t dereference  f3   it doesn t refer to anything  System out println f1 equals f3       Outputs false  since  f1  is a valid instance but  f3  is null     so one of the first checks inside the  Foo equals  method will    disallow the equality because it sees that  other     null

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if you invoke  equals   on null you will get NullPointerException      So it is always advisble to check nullity before invoking method where ever it applies  if str  null  amp  amp  str equals  hi        str contains hi       Also See     difference-between-equals-and    in Java

[java] Java null check why use == instead of .equals()

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