Upcast vs downcast1/27/2024 Java compiler will do the implicit casting. If we do not use the cast operator in the above case, even we will not get any error message. This kind of conversion is called upcasting or widening in java. Here, sub class object type has converted into super class type. It can be done like this: One o = (One) new Two() // Converting class Two's type into class One. We will convert object’s data type into class O using cast operator. In this statement, on the left side, the data type of reference o is One but on the right side, we got object whose data type is Two. But when a reference of class refers to different class’s object, we need casting.įor example: One o = new Two() // class One's reference o is pointing to class Two's object. Therefore, casting is not required in this case. Data type of the class object is also One. So, data type of o is One.Īt right side of assignment operator, we have class One’s object. This is because on the left side and at the right side of assignment operator (=), we have the same data type.įor example, in the first statement, o is a reference of class One. In the above two statements, we do not need casting. In this statement, class Two’s reference t is pointing to Two’s object. Similarly, sub class reference is used to point to sub class object. In the above statement, class One’s reference o is pointing or referring to One’s object.Ģ. Super class reference is used to refer to superclass object. There is a more elegant way: virtual methods.1. This is not ideal and leads to hard-to-read code. For example, if we had an array of 'Animal' objects, our loop would be nested with 'if' statements performing 'instanceof' checks and subsequent type casts. While we were able to resolve the correct 'render' method using casts, this is not the most elegant way to resolve the method. If you compile the code now, you will notice the first cat ("Kitty") has its name shown on mouse over, but the second cat ("Kat") does not have the same behavior. Notice that I changed the type of 'cat2' back to 'Cat' in order to illustrate upcasting. Calling the 'render' method on this cat will mean it uses the 'Animal' class render() method which will not include the name: To illustrate upcasting, we can also cast one of our cats to the 'Animal' type. If you compile and run the code now, you should see that the cats and dogs once again show their names when you hover your mouse over their icon. ![]() See the JS++ operator precedence table for more details. The reason there are extra parentheses in our revised main.jspp code is because we want the type cast to take precedence so that the cast occurs before the 'render' method is called. The syntax for a type cast is: (type) expression One way to remedy this is with a downcast: Since we are now equipped with an understanding of compile-time versus runtime polymorphism, and why and how the compiler resolved the 'render' method when the data type was specified as 'Animal', let's restore our main.jspp code:Īs you'll recall, this is the code that rendered all our animals, but we no longer got the animal names on mouse over. However, you cannot cast data of either type to 'int' because there is no type relationship between 'int' and 'Animal' or 'Dog'. Conversely, if you have data of type 'Dog', you can "upcast" it to its supertype 'Animal'. In other words, if you have data of type 'Animal', you can "downcast" it to its subtype 'Dog'. ![]() ![]() Upcasting and downcasting is based on type relationships. Now that we understand both subtyping and static versus dynamic polymorphism, we can learn about upcasting and downcasting. Upcasting and Downcasting with Variance.Static Members and "Application-Global" Data.Naming our Animals with Fields and Methods.Converting the "Untyped" JavaScript into "Typed" JS++.JavaScript Forced Conversions ("Type Coercion").Forgiving "Silent" Errors in JavaScript. ![]() Nested Modules and Partial Qualification.
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