How to check type of variable in Java

Question

How can I check to make sure my variable is an int  array  double  etc       Edit  For example  how can I check that a variable is an array  Is there some function to do this

User · Answer

I did it using  if x getClass      MyClass class

User · Answer

Well  I think checking the type of variable can be done this way    public  lt T extends Object gt  void checkType T object            if  object instanceof Integer          System out println  Integer         else if object instanceof Double          System out println  Double         else if object instanceof Float          System out println  Float           else if object instanceof List          System out println  List          else if object instanceof Set          System out println  Set          This way you need not have multiple overloaded methods  I think it is good practice to use collections over arrays due to the added benefits  Having said that  I do not know how to check for an array type  Maybe someone can improve this solution  Hope this helps    P S Yes  I know that this doesn t check for primitives as well

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I wasn t happy with any of these answers  and the one that s right has no explanation and negative votes so I searched around  found some stuff and edited it so that it is easy to understand  Have a play with it  not as straight forward as one would hope     move your variable into an Object type Object obj whatYouAreChecking  System out println obj       moving the class type into a Class variable Class cls obj getClass    System out println cls       convert that Class Variable to a neat String String answer   cls getSimpleName    System out println answer     Here is a method   public static void checkClass  Object obj        Class cls   obj getClass        System out println  The type of the object is      cls getSimpleName

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Basically   For example   public class Kerem       public static void main String   args                short x   10          short y   3          Object o   y          System out println o getClass        java lang Short

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Java is a statically typed language  so the compiler does most of this checking for you   Once you declare a variable to be a certain type  the compiler will ensure that it is only ever assigned values of that type  or values that are sub-types of that type    The examples you gave  int  array  double  these are all primitives  and there are no sub-types of them   Thus  if you declare a variable to be an int   int x    You can be sure it will only ever hold int values   If you declared a variable to be a List  however  it is possible that the variable will hold sub-types of List   Examples of these include ArrayList  LinkedList  etc   If you did have a List variable  and you needed to know if it was an ArrayList  you could do the following   List y      if  y instanceof ArrayList          its and ArrayList        However  if you find yourself thinking you need to do that  you may want to rethink your approach   In most cases  if you follow object-oriented principles  you will not need to do this   There are  of course  exceptions to every rule  though

User · Answer

You can check it easily using Java lang Class getSimpleName   Method Only if variable has non-primitive type  It doesnt work with primitive types int  long etc   reference - Here is the Oracle docs link

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You may work with Integer instead of int  Double instead of double  etc   such classes exists for all primitive types   Then you may use the operator instanceof  like if var instanceof Integer

User · Answer

Just use    getClass   getSimpleName      Example   StringBuilder randSB   new StringBuilder  just a String    System out println randSB getClass   getSimpleName       Output   StringBuilder

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None of these answers work if the variable is an uninitialized generic type  And from what I can find  it s only possible using an extremely ugly workaround  or by passing in an initialized parameter to your function  making it in-place  see here    lt T gt  T MyMethod       if T class    MyClass class          Is NOT valid because you cannot pull the type out of the T parameter directly  since it is erased at runtime time    lt T gt  void MyMethod T out        if out getClass      MyClass class          This works because the caller is responsible to instantiating the variable out before calling  This will still throw an exception if out is null when called  but compared to the linked solution  this is by far the easiest way to do this  I know this is a kind of specific application  but since this is the first result on google for finding the type of a variable with java  and given that T is a kind of variable   I feel it should be included

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The first part of your question is meaningless  There is no circumstance in which you don t know the type of a primitive variable at compile time   Re the second part  the only circumstance that you don t already know whether a variable is an array is if it is an Object  In which case object getClass   isArray   will tell you

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I hit this question as I was trying to get something similar working using Generics  Taking some of the answers and adding getClass   isArray   I get the following that seems to work  public class TypeTester  lt T extends Number gt     lt T extends Object gt  String tester T ToTest        if  ToTest instanceof Integer  return   quot Integer quot        else if ToTest instanceof Double  return   quot Double quot        else if ToTest instanceof Float  return   quot Float quot        else if ToTest instanceof String  return   quot String quot        else if ToTest getClass   isArray    return   quot Array quot        else return   quot Unsure quot         I call it with this where the myArray part was simply to get an Array into callFunction tester   to test it  public class Generics   public static void main String   args        int    myArray   new int  10        TypeTester lt Integer gt  callFunction   new TypeTester lt Integer gt         System out println callFunction tester myArray         You can swap out the myArray in the final line for say 10 2F to test Float etc

User · Answer

a getClass   getName   - will give you the datatype of the actual object referred to by a  but not the datatype that the variable a was originally declared as or subsequently cast to   boolean b   a instanceof String - will give you whether or not the actual object referred to by a is an instance of a specific class  Again  the datatype that the variable a was originally declared as or subsequently cast to has no bearing on the result of the instanceof operator   I took this information from  How do you know a variable type in java   This can happen  I m trying to parse a String into an int and I d like to know if my Integer parseInt s substring a  b   is kicking out an int or garbage before I try to sum it up   By the way  this is known as Reflection  Here s some more information on the subject  http   docs oracle com javase tutorial reflect

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public class Demo1    Object printType Object o        return o     public static void main String   args         Demo1 d new Demo1        Object o1 d printType  C        System out println o1 getClass   getSimpleName

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Actually quite easy to roll your own tester  by abusing Java s method overload ability  Though I m still curious if there is an official method in the sdk   Example   class Typetester       void printType byte x            System out println x     is an byte              void printType int x            System out println x     is an int              void printType float x            System out println x     is an float              void printType double x            System out println x     is an double              void printType char x            System out println x     is an char              then   Typetester t   new Typetester    t printType  yourVariable

[java] How to check type of variable in Java?

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