instanceof Vs getClass

Question

I see gain in performance when using getClass   and    operator over instanceOf operator   Object  str   new Integer  2000     long starttime   System nanoTime     if str instanceof String        System out println  its string      else       if  str instanceof Integer            System out println  its integer              System out println  System nanoTime  -starttime     starttime   System nanoTime     if str getClass      String class        System out println  its string in equals      else       if str getClass      Integer class            System out println  its integer             System out println  System nanoTime  -starttime      Is there any guideline  which one to use getClass   or instanceOf   Given a scenario  I know exact classes to be matched  that is String  Integer  these are final classes   etc   Is using instanceOf operator bad practise

User · Answer

I know it has been a while since this was asked, but I learned an alternative yesterday

We all know you can do:

if(o instanceof String) {   // etc

but what if you dont know exactly what type of class it needs to be? you cannot generically do:

if(o instanceof <Class variable>.getClass()) {

as it gives a compile error.
Instead, here is an alternative - isAssignableFrom()

For example:

public static boolean isASubClass(Class classTypeWeWant, Object objectWeHave) {

    return classTypeWeWant.isAssignableFrom(objectWeHave.getClass())
}

User · Answer

getClass   has the restriction that objects are only equal to other objects of the same class  the same run time type  as illustrated in the output of below code    class ParentClass    public class SubClass extends ParentClass      public static void main String   args           ParentClass parentClassInstance   new ParentClass            SubClass subClassInstance   new SubClass            if subClassInstance instanceof ParentClass               System out println  SubClass extends ParentClass  subClassInstance is instanceof ParentClass                      if subClassInstance getClass      parentClassInstance getClass                 System out println  Different getClass   return results with subClassInstance and parentClassInstance                         Outputs   SubClass extends ParentClass  subClassInstance is instanceof ParentClass    Different getClass   return results with subClassInstance and parentClassInstance

User · Answer

The reason that the performance of instanceof and getClass          is different is that they are doing different things   instanceof tests whether the object reference on the left-hand side  LHS  is an instance of the type on the right-hand side  RHS  or some subtype   getClass           tests whether the types are identical    So the recommendation is to ignore the performance issue and use the alternative that gives you the answer that you need   Is using the instanceOf operator bad practice    Not necessarily   Overuse of either instanceOf or getClass   may be  quot design smell quot    If you are not careful  you end up with a design where the addition of new subclasses results in a significant amount of code reworking   In most situations  the preferred approach is to use polymorphism  However  there are cases where these are NOT  quot design smell quot    For example  in equals Object  you need to test the actual type of the argument  and return false if it doesn t match   This is best done using getClass     Terms like  quot best practice quot    quot bad practice quot    quot design smell quot    quot antipattern quot  and so on should be used sparingly and treated with suspicion   They encourage black-or-white thinking   It is better to make your judgements in context  rather than based purely on dogma  e g  something that someone said is  quot best practice quot    I recommend that everyone read No Best Practices if they haven t already done so

User · Answer

Do you want to match a class exactly  e g  only matching FileInputStream instead of any subclass of FileInputStream  If so  use getClass   and     I would typically do this in an equals  so that an instance of X isn t deemed equal to an instance of a subclass of X - otherwise you can get into tricky symmetry problems  On the other hand  that s more usually useful for comparing that two objects are of the same class than of one specific class   Otherwise  use instanceof  Note that with getClass   you will need to ensure you have a non-null reference to start with  or you ll get a NullPointerException  whereas instanceof will just return false if the first operand is null   Personally I d say instanceof is more idiomatic - but using either of them extensively is a design smell in most cases

[java] instanceof Vs getClass( )

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