Lets say for this example I have two subclasses Dog and. transforming an object of type A to type B. I am trying to understand Javas polymorphism, and I have one question about downcasting an object. What you are asking about is down-casting which is making an object be of a more specific type, i.e. When you use an object of type B in place of A you actually do a up-cast, meaning you make the object of type B look like an object of type A (a less specific class type). If you have a class hierarchy where class B extends class A (B is a specialization of A) you can use B in the place of A since B supports all the operations that B does. In the working example, above, the dogwood references a Dogma, so you can downcast the Dogma. You also cannot downcast a Dog to a Dogma. You cannot upcast a Dogmatic to a Dog and then downcast it to a Dogma. Upcasting can be done but for downcasting, we need to check the types or else we. 1.7K Training / Learning / Certificationĭog dogwood = new Dogma() // a Dogma object is upcast to a Dogĭogma bush = (Dogma) dogwood // a Dogma object is downcast from a Dog Upcasting (Generalization or Widening) is casting to a parent type in simple words casting individual type to one common type is called upcasting while downcasting (specialization or narrowing) is casting to a child type or casting common type to individual type.

of Number user> (cast Number 1) 1 user> (type (cast Number 1)) java.lang. 165.3K Java EE (Java Enterprise Edition) Cast doesnt change the type of the input - it just throws an exception if.7.9K Oracle Database Express Edition (XE).of type cast is called a downcast) With this explicit type cast, the Java. There are times when this doesn't always work, however. The following Java class, FormattedInteger, stores a single integer value. In general, if an object is known to be a Cat, one should use a variable of type Cat, rather than a variable of type Animal, to refer to it. 3.8K Java and JavaScript in the Database The time when downcasting should be regarded as most dubious is when the object being cast is 'known' to be of the proper type.The key question is determining when one can sensibly avoid downcasting, and avoiding it when sensibly possible. In cases where downcasting is legitimately unavoidable, one should use it without qualm. Thus, even if Cat's playWith(Animal other) method would only work when other was a Cat, the Zoo would need to be able to pass it an element of an Animal, so its parameter type would have to be Animal rather than Cat. In such a case, the objects in each pair would still have to accept a non-specific parameter type such that they could, syntactically, be passed the objects from any other pair. Downcasting is performed to access the methods and. We can only explicitly downcast an object. For example, a Zoo collection might hold pairs of objects in even/odd array slots, with the expectation that the objects in each pair will be able to act upon each other, even if they cannot act upon the objects in other pairs. Downcasting is the process of casting superclass to subclass. There are times when this doesn't always work, however. In general, if an object is known to be a Cat, one should use a variable of type Cat, rather than a variable of type Animal, to refer to it. downcasting in javadown casting javajava upcasting downcastingtypecasting javaupcasting and downcasting in javacasting javajava casting methodupcasting vs. In this approach, downcasting prevents the compiler from detecting a possible error and instead causes a run-time error. The time when downcasting should be regarded as most dubious is when the object being cast is 'known' to be of the proper type. Often you have an Object object that you downcast to your own custom class. If a String is asked whether it equals a Dog, however, it cannot access the length of the Dog, but it shouldn't have to instead, if the object to which a String is supposed to compare itself isn't a String, the comparison should use a default behavior (reporting that the other object isn't equal). Is downcasting possible in Java Not only is downcasting possible, but you often do that. For one String to equal another String, it must examine the length and backing character array of the other string.

As a simple example, suppose a String is asked whether it is equal to some other arbitrary object. In particular, it's often appropriate in cases where one has objects that may or may not have some ability, and one wishes to use that ability when it exists while handling objects without that ability in some default fashion. Downcasting is sometimes necessary and appropriate. explicitly downcast b) Java downcasts automatically, but you must explicitly upcast c) Java expects the user to explicitly upcast and downcast d) Java.