Check if an object belongs to a class in Java

Use the instanceof operator:

if(a instanceof MyClass)
{
    //do something
}

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).

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.