In Java, we cannot assign a parent class reference object to the child class, but if we perform downcasting, we will not get any compile-time error. In Upcasting, we assign a parent class reference object to the child class. Upcasting is another type of object typecasting. Upcasting is also known as Generalization and Widening. We access only some specified variables and methods of the child class. Here, we don't access all the variables and the method. By using the Upcasting, we can easily access the variables and methods of the parent class to the child class. Upcasting is a type of object typecasting in which a child object is typecasted to a parent class object. Let's dive into deep of both these type of object casting: 1) Upcasting We can perform Upcasting implicitly or explicitly, but downcasting cannot be implicitly possible. In Upcasting and Downcasting, we typecast a child object to a parent object and a parent object to a child object simultaneously. Typecasting is used to ensure whether variables are correctly processed by a function or not. So, there are two types of typecasting possible for an object, i.e., Parent to Child and Child to Parent or can say Upcasting and Downcasting. Parent and Child objects are two types of objects. In Java, the object can also be typecasted like the datatypes. Using Generics to make the List parameterized restricts the types of objects the list can hold to valid instances of Integer, which in turn makes any attempt to insert any other, incompatible type in the list detectable at compile-time, as shown in the revised example below.Next → ← prev Upcasting and Downcasting in JavaĪ process of converting one data type to another is known as Typecasting and Upcasting and Downcasting is the type of object typecasting.
To better understand ClassCastException, consider the following Java class hierarchy: class X Exception in thread "main" : class cannot be cast to class Īt $main$0(ClassCastExceptionGenerics.java:15)Īt java.base/(ArrayList.java:1511)Īt (ClassCastExceptionGenerics.java:15) To help achieve type safety and catch these issues at compile time, two builtin Java mechanisms are available:
To prevent the ClassCastException exception, one should be careful when casting objects to a specific class or interface and ensure that the target type is a child of the source type, and that the actual object is an instance of that type. IFace i = (IFace) s // compilation error (the String class is final) The only scenario where the compiler is able to detect invalid type casts of this kind is when the source type is a final class and it neither extends nor implements the target type, because it is known in advance that the final class does not have any subtypes, i.e., it cannot be subclassed. IFace i = (IFace) parent // Is parent an instance of a subclass that implements IFace? Parent parent = new Child() Ĭhild c = (Child) parent // is parent actually an instance of Child? Consequently, if either of these scenarios is encountered at runtime, Java will throw the ClassCastException exception. This exception can not be checked at compile-time because the compiler has no way of knowing whether the object is actually an instance of the target subclass, or if it is an instance of a subclass that implements the target interface. This relates to explicit type casting and the reason the cast fails can be traced to an attempt at downcasting an object to a class of which it is not an instance, or to an interface which it does not implement.ĬlassCastException is a subclass of the RuntimeException class which means it is an unchecked, runtime exception. What is ClassCastException and When does it Happen?Īs its name implies, ClassCastException is an exception that happens when the JVM tries to cast an object to a class (or in some instances, an interface) and fails. Despite being less serious and critical than the unchecked runtime errors, these exceptions can still be very problematic and cause unexpected issues at runtime, especially if necessary precautions aren’t taken and relevant exception handling mechanisms aren’t put in place. In Java, there are a myriad of classes derived from the RuntimeException class, all of which represent unchecked exceptions that need to be carefully considered and managed. Runtime exceptions are exceptions which can not be checked at compile time.
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