I have a java method that should check through an ArrayList and check if it contains an instance of a given class. I need to pass the method the type of class to check for as a parameter, and if the List contains an object of the given type, then return it.
Is this achievable?
public static <T> T find(Collection<?> arrayList, Class<T> clazz)
{
for(Object o : arrayList)
{
if (o != null && o.getClass() == clazz)
{
return clazz.cast(o);
}
}
return null;
}
and call
String match = find(myArrayList, String.class);
Class.isAssignableFrom to handle subclasses, also the method should take Collection instead of ArrayList (ie, program to the interface). Also consider using Java's recommended coding standard of making method names start with lowercase.
Class.isInstancepublic static <T> T getFirstElementOfTypeIn( List<?> list, Class<T> clazz )
{
for ( Object o : list )
{
if ( clazz.isAssignableFrom( o.getClass() ) )
{
return clazz.cast( o );
}
}
return null;
}
clazz.isAssignableFrom( o.getClass() ) can be written as clazz.isInstance( o )If you're using Java 8 you can do:
public <T> Optional<T> getInstanceOf(Class<T> clazz, Collection<?> collection) {
return (Optional<T>) collection.stream()
.filter(e -> clazz.isInstance(e.getClass()))
.findFirst();
}
Check the documentation of Optional if you never used it before. Actually this is better practice than returning null.
You can iterator over your list and test each element
Class<?> zz = String.class;
for (Object obj : list) {
if (zz.isInstance(obj)) {
System.out.println("Yes it is a string");
}
}
Note that isInstance also captures subclasses. Otherwise see Bela`s answer.
check one of the the item instance in the list
if (object instanceof ArrayList<?>) {
if (((ArrayList<?>) object).get(0) instanceof ClassA) {
classAList= (ArrayList<ClassA>) object;
} else if (((ArrayList<?>) object).get(0) instanceof ClassB) {
classBList= (ArrayList<classB>) object;
} else if (((ArrayList<?>) object).get(0) instanceof ClassC) {
classCList= (ArrayList<ClassC>) object;
}
}
