How to pass a type as a method parameter in Java

Question

In Java  how can you pass a type as a parameter  or declare as a variable      I don t want to pass an instance of the type but the type itself  eg  int  String  etc    In C   I can do this   private void foo Type t        if  t    typeof String               else if  t    typeof int              private void bar         foo typeof String        Is there a way in Java without passing an instance of type t  Or do I have to use my own int constants or enum  Or is there a better way   Edit  Here is the requirement for foo  Based on type t  it generates a different short  xml string  The code in the if else will be very small  one or two lines  and will use some private class variables

User · Answer

You should pass a Class     private void foo Class lt   gt  t       if t    String class             else if t    int class            private void bar        foo String class

User · Answer

I had a similar question  so I worked up a complete runnable answer below  What I needed to do is pass a class  C  to an object  O  of an unrelated class and have that object  O  emit new objects of class  C  back to me when I asked for them   The example below shows how this is done   There is a MagicGun class that you load with any subtype of the Projectile class  Pebble  Bullet or NuclearMissle   The interesting is you load it with subtypes of Projectile  but not actual objects of that type  The MagicGun creates the actual object when it s time to shoot   The Output  You ve annoyed the target  You ve holed the target  You ve obliterated the target  click click   The Code  import java util ArrayList  import java util List   public class PassAClass       public static void main String   args            MagicGun gun   new MagicGun            gun loadWith Pebble class           gun loadWith Bullet class           gun loadWith NuclearMissle class             gun loadWith Object class        Won t compile -- Object is not a Projectile         for int i 0  i lt 5  i                 try                   String effect   gun shoot   effectOnTarget                    System out printf  You ve  s the target  n   effect                 catch  GunIsEmptyException e                    System err printf  click n                                     class MagicGun                  projectiles holds a list of classes that extend Projectile  Because of erasure  it        can t hold be a List lt   extends Projectile gt  so we need the SuppressWarning  However        the only way to add to it is the  loadWith  method which makes it typesafe               private  SuppressWarnings  rawtypes   List lt Class gt  projectiles   new ArrayList lt Class gt                    Load the MagicGun with a new Projectile class          param projectileClass The class of the Projectile to create when it s time to shoot              public void loadWith Class lt   extends Projectile gt  projectileClass           projectiles add projectileClass                        Shoot the MagicGun with the next Projectile  Projectiles are shot First In First Out          return A newly created Projectile object          throws GunIsEmptyException             public Projectile shoot   throws GunIsEmptyException          if  projectiles isEmpty                throw new GunIsEmptyException            Projectile projectile   null             We know it must be a Projectile  so the SuppressWarnings is OK          SuppressWarnings  unchecked   Class lt   extends Projectile gt  projectileClass   projectiles get 0           projectiles remove 0           try                 http   www java2s com Code Java Language-Basics ObjectReflectioncreatenewinstance htm             projectile   projectileClass newInstance              catch  InstantiationException e                System err println e             catch  IllegalAccessException e                System err println e                     return projectile           abstract class Projectile       public abstract String effectOnTarget       class Pebble extends Projectile        Override public String effectOnTarget             return  annoyed            class Bullet extends Projectile        Override public String effectOnTarget             return  holed            class NuclearMissle extends Projectile        Override public String effectOnTarget             return  obliterated            class GunIsEmptyException extends Exception       private static final long serialVersionUID   4574971294051632635L

User · Answer

You could pass a Class lt T gt  in   private void foo Class lt   gt  cls        if  cls    String class              else if  cls    int class             private void bar         foo String class       Update  the OOP way depends on the functional requirement  Best bet would be an interface defining foo   and two concrete implementations implementing foo   and then just call foo   on the implementation you ve at hand  Another way may be a Map lt Class lt   gt   Action gt  which you could call by actions get cls   This is easily to be combined with an interface and concrete implementations  actions get cls  foo

User · Answer

If you want to pass the type  than the equivalent in Java would be   java lang Class   If you want to use a weakly typed method  then you would simply use  java lang Object   and the corresponding operator  instanceof   e g   private void foo Object o       if o instanceof String             foo   However  in Java there are primitive types  which are not classes  i e  int from your example   so you need to be careful   The real question is what you actually want to achieve here  otherwise it is difficult to answer      Or is there a better way

User · Answer

You can pass an instance of java lang Class that represents the type  i e   private void foo Class cls

User · Answer

Oh  but that s ugly  non-object-oriented code   The moment you see  if else  and  typeof   you should be thinking polymorphism   This is the wrong way to go   I think generics are your friend here   How many types do you plan to deal with     UPDATE    If you re just talking about String and int  here s one way you might do it   Start with the interface XmlGenerator  enough with  foo     package generics   public interface XmlGenerator lt T gt       String getXml T value       And the concrete implementation XmlGeneratorImpl        package generics   public class XmlGeneratorImpl lt T gt  implements XmlGenerator lt T gt        private Class lt T gt  valueType      private static final int DEFAULT CAPACITY   1024       public static void main String    args                Integer x   42          String y    foobar            XmlGenerator lt Integer gt  intXmlGenerator   new XmlGeneratorImpl lt Integer gt  Integer class           XmlGenerator lt String gt  stringXmlGenerator   new XmlGeneratorImpl lt String gt  String class            System out println  integer      intXmlGenerator getXml x            System out println  string       stringXmlGenerator getXml y               public XmlGeneratorImpl Class lt T gt  clazz                this valueType   clazz             public String getXml T value                StringBuilder builder   new StringBuilder DEFAULT CAPACITY            appendTag builder           builder append value           appendTag builder  false            return builder toString               private void appendTag StringBuilder builder    this appendTag builder  false          private void appendTag StringBuilder builder  boolean isClosing                String valueTypeName   valueType getName            builder append   lt    append valueTypeName           if  isClosing                        builder append                         builder append   gt               If I run this  I get the following result    integer   lt java lang Integer gt 42 lt java lang Integer gt  string    lt java lang String gt foobar lt java lang String gt    I don t know if this is what you had in mind

[java] How to pass a type as a method parameter in Java

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