Test if object implements interface

Question

This has probably been asked before  but a quick search only brought up the same question asked for C   See here   What I basically want to do is to check wether a given object implements a given interface   I kind of figured out a solution but this is just not comfortable enough to use it frequently in if or case statements and I was wondering wether Java does not have built-in solution   public static Boolean implementsInterface Object object  Class interf       for  Class c   object getClass   getInterfaces              if  c equals interf                 return true                      return false       EDIT  Ok  thanks for your answers  Especially to Damien Pollet and Noldorin  you made me rethink my design so I don t test for interfaces anymore

User · Answer

I had this problem tonight with android and after looking at the javadoc solutions I came up with this real working solution just for people like me that need a little more than a javadoc explanation.

Here's a working example with an actual interface using android java. It checks the activity that called implemented the AboutDialogListener interface before attempting to cast the AboutDialogListener field.

public class About extends DialogFragment implements OnClickListener,
    OnCheckedChangeListener {

public static final String FIRST_RUN_ABOUT = "com.gosylvester.bestrides.firstrunabout";


public interface AboutDialogListener {
    void onFinishEditDialog(Boolean _Checked);
}

private AboutDialogListener adl;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    Activity a = this.getActivity();
    if (AboutDialogListener.class.isInstance(a)) {
        adl = (AboutDialogListener) a;
        }
}

... Later I check if the field adl is !null before calling the interface

@Override
public void onStop() {
    super.onStop();
    sharedPref.edit().putBoolean(About.FIRST_RUN_ABOUT, _Checked).commit();
    // if there is an interface call it.
    if (adl != null) {
        adl.onFinishEditDialog(is_Checked());
    }
}

User · Answer

A variation on  AndrewKennan s answer I ended up using recently for types obtained at runtime   if  serviceType IsInstanceOfType service             service  does implement the  serviceType  type

User · Answer

In addition to testing using the  is  operator  you can decorate your methods to make sure that variables passed to it implement a particular interface  like so   public static void BubbleSort lt T gt  ref IList lt T gt  unsorted list  where T   IComparable          Some bubbly sorting     I m not sure which version of  Net this was implemented in so it may not work in your version

User · Answer

I prefer instanceof   if  obj instanceof SomeType            which is much more common and readable than SomeType isInstance obj

User · Answer

if  object is IBlah    or  IBlah myTest   originalObject as IBlah  if  myTest    null

User · Answer

For the instance   if  obj is IMyInterface       For the class   Check if typeof MyClass  GetInterfaces   contains the interface

User · Answer

This should do   public static boolean implementsInterface Object object  Class interf       return interf isInstance object       For example    java io Serializable class isInstance  a test string     evaluates to true

User · Answer

Using the is or as operators is the correct way if you know the interface type at compile time and have an instance of the type you are testing  Something that no one else seems to have mentioned is Type IsAssignableFrom   if  typeof IMyInterface  IsAssignableFrom someOtherType          I think this is much neater than looking through the array returned by GetInterfaces and has the advantage of working for classes as well

User · Answer

If you want to test for interfaces   public List lt myType gt  getElement Class lt   gt  clazz        List lt myType gt  els   new ArrayList lt myType gt         for  myType e  this elements values              if  clazz isAssignableFrom e getClass                   els add e                       return els       clazz is an Interface and myType is a Type that you defined that may implement a number of interfaces  With this code you get only the types that implement a defined interface

User · Answer

Using the is or as operators is the correct way if you know the interface type at compile time and have an instance of the type you are testing  Something that no one else seems to have mentioned is Type IsAssignableFrom   if  typeof IMyInterface  IsAssignableFrom someOtherType          I think this is much neater than looking through the array returned by GetInterfaces and has the advantage of working for classes as well

User · Answer

For the instance   if  obj is IMyInterface       For the class   Check if typeof MyClass  GetInterfaces   contains the interface

User · Answer

if  object is IBlah    or  IBlah myTest   originalObject as IBlah  if  myTest    null

User · Answer

This should work    MyInstace GetType   GetInterfaces      But nice too    if  obj is IMyInterface    Or even  not very elegant     if  obj GetType      typeof IMyInterface

User · Answer

In addition to testing using the  is  operator  you can decorate your methods to make sure that variables passed to it implement a particular interface  like so   public static void BubbleSort lt T gt  ref IList lt T gt  unsorted list  where T   IComparable          Some bubbly sorting     I m not sure which version of  Net this was implemented in so it may not work in your version

User · Answer

In addition to testing using the  is  operator  you can decorate your methods to make sure that variables passed to it implement a particular interface  like so   public static void BubbleSort lt T gt  ref IList lt T gt  unsorted list  where T   IComparable          Some bubbly sorting     I m not sure which version of  Net this was implemented in so it may not work in your version

User · Answer

Recently I tried using Andrew Kennan s answer and it didn t work for me for some reason  I used this instead and it worked  note  writing the namespace might be required    if  typeof someObject  GetInterface  MyNamespace IMyInterface      null

User · Answer

I used  Assert IsTrue myObject is ImyInterface     for a test in my unit test which tests that myObject is an object which has implemented my interface ImyInterface

User · Answer

that was easy        interf isInstance object

User · Answer

This should work    MyInstace GetType   GetInterfaces      But nice too    if  obj is IMyInterface    Or even  not very elegant     if  obj GetType      typeof IMyInterface

User · Answer

What worked for me is   Assert IsNotNull typeof  YourClass  GetInterfaces   SingleOrDefault i   gt  i    typeof  ISomeInterface

User · Answer

if  object is IBlah    or  IBlah myTest   originalObject as IBlah  if  myTest    null

User · Answer

This should work    MyInstace GetType   GetInterfaces      But nice too    if  obj is IMyInterface    Or even  not very elegant     if  obj GetType      typeof IMyInterface

User · Answer

A variation on  AndrewKennan s answer I ended up using recently for types obtained at runtime   if  serviceType IsInstanceOfType service             service  does implement the  serviceType  type

User · Answer

I had a situation where I was passing a variable to a method and wasn t sure if it was going to be an interface or an object     The goals were    If item is an interface  instantiate an object based on that interface with the interface being a parameter in the constructor call  If the item is an object  return a null since the constuctor for my calls are expecting an interface and I didn t want the code to tank    I achieved this with the following       if  typeof T  IsClass                  If your constructor needs arguments           object   args   new object     my constructor param           return  T Activator CreateInstance typeof T   args  null             else        return default T

User · Answer

The instanceof operator does the work in a NullPointerException safe way  For example    if     instanceof java io Serializable            it s true      yields true  Since    if  null instanceof AnyType            never reached      yields false  the instanceof operator is null safe  the code you posted isn t     instanceof is the built-in  compile-time safe alternative to Class isInstance Object

User · Answer

This Post is a good answer   public interface IMyInterface     public class MyType   IMyInterface      This is a simple sample     typeof IMyInterface  IsAssignableFrom typeof MyType     or  typeof MyType  GetInterfaces   Contains typeof IMyInterface

User · Answer

If you want to use the typecasted object after the check  Since C  7 0   if  obj is IMyInterface myObj    This is the same as  IMyInterface myObj   obj as IMyInterface  if  myObj    null    See  NET Docs  Pattern matching with is   Type pattern

User · Answer

Recently I tried using Andrew Kennan s answer and it didn t work for me for some reason  I used this instead and it worked  note  writing the namespace might be required    if  typeof someObject  GetInterface  MyNamespace IMyInterface      null

User · Answer

if  object is IBlah    or  IBlah myTest   originalObject as IBlah  if  myTest    null

User · Answer

I had a situation where I was passing a variable to a method and wasn t sure if it was going to be an interface or an object     The goals were    If item is an interface  instantiate an object based on that interface with the interface being a parameter in the constructor call  If the item is an object  return a null since the constuctor for my calls are expecting an interface and I didn t want the code to tank    I achieved this with the following       if  typeof T  IsClass                  If your constructor needs arguments           object   args   new object     my constructor param           return  T Activator CreateInstance typeof T   args  null             else        return default T

User · Answer

With Apache commons-lang ArrayUtils  see if the interface you require is contained in the interfaces of you object  public static Boolean implementsInterface Object object  Class interf       return ArrayUtils contains object getClass   getInterfaces    interf

User · Answer

Using the is or as operators is the correct way if you know the interface type at compile time and have an instance of the type you are testing  Something that no one else seems to have mentioned is Type IsAssignableFrom   if  typeof IMyInterface  IsAssignableFrom someOtherType          I think this is much neater than looking through the array returned by GetInterfaces and has the advantage of working for classes as well

User · Answer

For the instance   if  obj is IMyInterface       For the class   Check if typeof MyClass  GetInterfaces   contains the interface

User · Answer

This Post is a good answer   public interface IMyInterface     public class MyType   IMyInterface      This is a simple sample     typeof IMyInterface  IsAssignableFrom typeof MyType     or  typeof MyType  GetInterfaces   Contains typeof IMyInterface

User · Answer

Using the is or as operators is the correct way if you know the interface type at compile time and have an instance of the type you are testing  Something that no one else seems to have mentioned is Type IsAssignableFrom   if  typeof IMyInterface  IsAssignableFrom someOtherType          I think this is much neater than looking through the array returned by GetInterfaces and has the advantage of working for classes as well

User · Answer

I used  Assert IsTrue myObject is ImyInterface     for a test in my unit test which tests that myObject is an object which has implemented my interface ImyInterface

User · Answer

In addition to testing using the  is  operator  you can decorate your methods to make sure that variables passed to it implement a particular interface  like so   public static void BubbleSort lt T gt  ref IList lt T gt  unsorted list  where T   IComparable          Some bubbly sorting     I m not sure which version of  Net this was implemented in so it may not work in your version

User · Answer

For the instance   if  obj is IMyInterface       For the class   Check if typeof MyClass  GetInterfaces   contains the interface

User · Answer

This should work    MyInstace GetType   GetInterfaces      But nice too    if  obj is IMyInterface    Or even  not very elegant     if  obj GetType      typeof IMyInterface

User · Answer

If you want to use the typecasted object after the check  Since C  7 0   if  obj is IMyInterface myObj    This is the same as  IMyInterface myObj   obj as IMyInterface  if  myObj    null    See  NET Docs  Pattern matching with is   Type pattern

User · Answer

What worked for me is   Assert IsNotNull typeof  YourClass  GetInterfaces   SingleOrDefault i   gt  i    typeof  ISomeInterface

[java] Test if object implements interface

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