This question already has answers here:
How to determine an object's class?
(13 answers)
Closed 9 years ago.
Is there an easy way to verify that an object belongs to a given class? For example, I could do
if(a.getClass() = (new MyClass()).getClass())
{
//do something
}
but this requires instantiating a new object on the fly each time, only to discard it. Is there a better way to check that "a" belongs to the class "MyClass"?
The instanceof keyword, as described by the other answers, is usually what you would want.
Keep in mind that instanceof will return true for superclasses as well.
If you want to see if an object is a direct instance of a class, you could compare the class. You can get the class object of an instance via getClass(). And you can statically access a specific class via ClassName.class.
So for example:
if (a.getClass() == X.class) {
// do something
}
In the above example, the condition is true if a is an instance of X, but not if a is an instance of a subclass of X.
In comparison:
if (a instanceof X) {
// do something
}
In the instanceof example, the condition is true if a is an instance of X, or if a is an instance of a subclass of X.
Most of the time, instanceof is right.
If you ever need to do this dynamically, you can use the following:
boolean isInstance(Object object, Class<?> type) {
return type.isInstance(object);
}
You can get an instance of java.lang.Class by calling the instance method Object::getClass on any object (returns the Class which that object is an instance of), or you can use class literals (for example, String.class, List.class, int[].class). There are other ways as well, through the reflection API (which Class itself is the entry point for).
Use the instanceof operator:
if(a instanceof MyClass)
{
//do something
}
I agree with the use of instanceof already mentioned.
An additional benefit of using instanceof is that when used with a null reference instanceof of will return false, while a.getClass() would throw a NullPointerException.
Try operator instanceof.
The usual way would be:
if (a instanceof A)
However, there are cases when you can't do this, such as when A in a generic argument.
Due to Java's type erasure, the following won't compile:
<A> boolean someMethod(Object a) {
if (a instanceof A)
...
}
and the following won't work (and will produce an unchecked cast warning):
<A> void someMethod(Object a) {
try {
A casted = (A)a;
} catch (ClassCastException e) {
...
}
}
You can't cast to A at runtime, because at runtime, A is essentially Object.
The solutions to such cases is to use a Class instead of the generic argument:
void someMethod(Object a, Class<A> aClass) {
if (aClass.isInstance(a)) {
A casted = aClass.cast(a);
...
}
}
You can then call the method as:
someMethod(myInstance, MyClass.class);
someMethod(myInstance, OtherClass.class);
Related
This question already has answers here:
Java generics type erasure: when and what happens?
(7 answers)
Closed 7 months ago.
Given a Set<X>, I want to write a method that acts differently depending on the class X is. In actuality, I'd have 2 cases: A or B.
The method looks like:
public<X> boolean myMethod(Set<X> mySet)
{
// if X is actually the class A
...
// if X is B
...
}
I am not sure how to check this. I tried X::getClass, but it wouldn't let me use equals method.
In a general sense, this is not possible
Due to type erasure, a Java Set does not have a generic type at runtime. Generics act as compile-time checks on the objects passed into methods, but consequently the specific values are not available at runtime. A Set<String> is the same class as a Set<Map<Integer, Thread>> and the same class as a bare Set.
Potential workaround with class parameter
If you really need to know this, you could change the signature of your method to be:
public <X> boolean myMethod(Set<X> mySet, Class<X> clazz)
Now you have a Class instance available at runtime, which is guaranteed to be compatible with X. This you can introspect (e.g. String.class.isAssignableFrom(clazz)).
There are two drawbacks to this approach: firstly, you'll have to explicitly pass in the extra parameter each time the method is called. Secondly, if A and B can be subclasses of one another, this is not going to work; you could pass in e.g. B.class even when the generic parameter was A.
However, this feels like a code smell
You shouldn't be trying to do fundamentally different things based on a generic parameter. Instead of switching behaviour in your method here, do something different within the classes A and B:
public<X> boolean myMethod(Set<X> mySet)
{
boolean result = false;
for (X x : mySet) {
result &= x.doSomething();
}
return result;
}
class A implements MyDoSomethingInterface {
public boolean doSomething() {
// Here you put the logic for "if X is A"
}
}
class B implements MyDoSomethingInterface {
public boolean doSomething() {
// Here you put the logic for "if X is B"
}
}
And if your reaction is that this won't work because A and B are built-in classes, you'll need to wrap them in your own domain objects (which is the correct approach anyway because you want to associate this extra behaviour with them).
I suggest you take one object from set and check class of single object
like below, please check
public<X> boolean myMethod(Set<X> mySet)
{
Object tmpObj = null;
for(Object obj : set){
tmpObj = obj;
break;
}
if(tmpObj instanceof A){
// if X is actually the class A
}else if(tmpObj instanceof B){
// // if X is B
}
}
This question already has answers here:
How to check if a subclass is an instance of a class at runtime? [duplicate]
(7 answers)
Closed 5 years ago.
I have the following code,
public <T> T build(Object source, Class<T> destClass) {
if((Object)destClass instanceof TestDTO){
return (T) testBuilder.build((BlahDTO) source);
}
if((Object)destClass instanceof BestDTO) {
return (T) bestBuilder.build((BlahDTO) source);
}
return null;
}
I am trying to compare if the destClass if either of the type TestDTO or BestDTO and take the appropriate action. But the comparison fails even though the destClass is of the specified type. Am I missing something, also my alternate approach,
public <T> T build(Object source, Class<T> destClass) {
if(destClass.getSimpleName().equals(TestDTO.class.getSimpleName())){
return (T) testBuilder.build((BlahDTO) source);
}
if(destClass.getSimpleName().equals(BestDTO.class.getSimpleName())) {
return (T) bestBuilder.build((BlahDTO) source);
}
return null;
}
although this approach works, I find this code a bit dicey. It would be helpful if someone pointed out what I was doing wrong, or suggest an alternate approach. Thanks in advance!
instanceof checks to see if the left-hand operand is an instance of the right-hand operand. But your left-hand operand is a Class object, not an instance of that class. To use instanceof, you must have an instance of the class (something created via new TheClass).
As Thomas points out, if you really mean to be working with Class instances, you may want isAssignableFrom instead:
if (TestDTO.class.isAssignableFrom(destClass))
Side note: There's no purpose served by the (Object) cast on if((Object)destClass instanceof TestDTO). instanceof checks the object, not the kind of reference you have to it; casting is irrelevant to that check.
I have an object. I want to check to see if it is of type enum. There are two ways to do this.
object.getClass().isEnum()
or
object instanceof Enum
Is one better?
In my opinion object instanceof Enum is better for several reasons:
It is very obvious what is asked here: "is this an enum"?
It doesn't risk a NullPointerException (if object is null, it will just evaluate to false)
It's shorter.
The only reason I'd see for using isEnum() would be if I only have access to the Class object and not to a concrete instance.
You need to use the latter (object instanceof Enum) because the former may not work with enum constants with constant-specific class bodies.
For example, for this enum type:
enum MyEnum {
FOO { }
}
The expression MyEnum.FOO.getClass().isEnum() returns false.
If you want to check if an object is a enum constant without instanceof Enum, you have to use this (much more complicated) expression:
static boolean isEnum(Object obj) {
Class<?> cls = obj.getClass();
Class<?> superCls = cls.getSuperclass();
// Be careful, Object.class.getSuperclass() returns null
return cls.isEnum() || (superCls != null && superCls.isEnum());
}
Say I have 3 classes like so:
class A {}
class B extends A {}
class C extends A {}
Would it then be possible to determine whether a particular object was an instance of A, B, or C?
I thought that something like this might work:
if (myObject.getClass().isInstance(B.class)) {
// do something for B
} else (myObject.getClass().isInstance(C.class)) {
// do something for C
} else {
// do something for A
}
but after reading a little I think it would always evaluate as B since it's just testing if a cast would work and there's no substantial differences between them.
The simpler and faster code is:
if (myObject instanceof B) {
} else if (myObject instanceof C) {
} else if (myObject instanceof A) {
}
Note that the order is important: you have to have the test for A last, as that will succeed for instances of B and C as well.
However, your original code would nearly work. Class.isInstance checks whether the value really is an instance of the given class or any superclass. So if myObject is an instance of C, then B.class.isInstance(myObject) will return false. All you've got wrong is that you're calling getClass() on myObject unnecessarily, instead of using B.class etc.
This is the approach you would take if you didn't know which classes you were interested in at compile time - the instanceof operator only works when you can specify the type statically in code.
Now if you want to find out whether myObject is an instance of exactly B (and not a subclass) then just use:
if (myObject.getClass() == B.class)
(This will blow up if myObject is a null reference, of course.)
Do this:
if (myObject instanceof B) {
// do something for B
} else (myObject instanceof C) {
// do something for C
} else {
// do something for A
}
You also might want to look at the double dispatch idiom which is an OO way of changing behaviour based on the type of an argument in languages which don't support multi-methods.
There is the instanceof operator that does what you want, as others have answered.
But beware that if you need something like this, it's a sign that there might be a flaw in the design of your program. The better, more object oriented way to do this is by using polymorphism: put a method in the superclass A, override that method in B and C, and call the method on myObject (which should have type A):
A myObject = ...; // wherever you get this from
// Override someMethod in class B and C to do the things that are
// specific to those classes
myObject.someMethod();
instanceof is ugly and should be avoided as much as possible, just like casting is ugly, potentially unsafe and should be avoided.
you could do this
if (myObject.getClass() == B.class) {
// do something for B (not subclasses of b!!!)
} else if(myObject.getClass() == C.class) {
// do something for C (not subclasses of c!!!)
} else if(myobject.getClass() == A.class) {
// do something for A (not subclasses of A!!)
} else if(myobjects instanceof A){
//all other subclasses of A
}
I want to know how to identify subclasses at run time in java. In my program I am calling a method that returns a superclass object. The object can be an instance of any one of its sub-classes. I want to know the object is instance of which sub-class at run-time so that I can cast it to that subclass and access subclass methods. Can anyone help me in this?
Thanks
You can just use instanceof to check if an object is an instance of a particular class. eg:
if (animal instanceof Cat) {
Cat cat = (Cat) animal;
cat.meow();
} else if (animal instanceof Dog) {
Dog dog = (Dog) animal;
dog.bark();
}
However, overuse of instanceof (or down-casting, for that matter) is generally considered to be a sign of poor design. It's better to instead make use of polymorphism. For example, have a (possibly abstract) "speak" method in Animal, and then each subclass would have a different implementation. The above code would then be replaced with a call to speak:
animal.speak();
Call getClass() on the object to access a Class object that tells you the object's actual type. You can then compare this to the ".class" static member of any class.
if (obj.getClass().equals(Foo.class)) { ... }
However many people would say what you're suggesting is bad design. Make sure it's necessary and consider alternatives. It's often necessary in implementing methods like equals().
Use instanceof operator. Like this
Superclass aSup = ...;
if(aSup instanceof Subclass) {
Subclass aSub = (Subclass) aSup;
aSub.subclass_method(...);
}
Note, instanceof is true if the object is a type assignable to the
class specified. The following (as described above) would work
if (o instanceof Cat.class) {
// ..
} else if (o instanceof Dog.class) {
// ..
} else {
throw IllegalArgumentException("Unexpected type");
}
However if you introduce a new subclass of cat, e.g. Tiger then the
first clause above would be triggered unless you updated all the code
that did this.
You may want to search for Double Dispatch for a potential way of
having to do the above, of course without knowing teh problem you are
trying to solve this may not be applicable.
You can use 'instanceof' operator and direct Class objects comparison as was mentioned above.
However, if you want to modify the code in order to avoid explicit checks I see two ways to go:
GoF Visitor pattern;
encapsulate class-specific processing logic, i.e. create something like this:
interface Handler<T> {
void handle(T object);
}
Map<Class<?>, Handler<?>> HANDLERS = /* init handlers mappings */;
...
public void process(Object obj) {
Handler<?> handler = HANDLERS.get(obj.getClass());
if (handler == null) {
throw new IllegalStateException("bla-bla-bla, no handler is registered for class " + obj.getClass());
}
handler.handle(obj);
}
You can use the instanceof operator to test the type an object and then cast.