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Converting between user-defined types: downcasting

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• Downcasting: the inverse of upcasting

  • Downcasting:

    Downcasting is the inverse of upcasting Downcasting = casting a reference of the base class to a derived class

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  • Example:

    ClassA x = new ClassA( ... ); // ClassA is the base class of ClassB ClassB y; ClassB z; x = y; // **** this is "upcasting" (discussed prviously) z = (ClassB) x; // **** Downcasting: casting a reference of // the base class (x of type ClassA) to // a derived class (y of type ClassB)

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  • Warnings:

    The downcasting operation z = (ClassB) x; // z is a ClassA variable is a potentially dangerous operation and we must specify the "casting" operator (ClassB) Example: z = x; // Illegal !!! will result in compile error !!! Java will check for correctness when it executes the downcasting operation: When Java execute a downcasting operation: z = (ClassB) x; the Java run time system will make sure that the object assigned to variable z is indeed a ClassB type object. If x is not referencing a ClassB object, the Java run time system will report an error

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  • Example of a legal downcasting operation:

    public class Downcasting1 { public static void main(String[] args) { BankAccount x; // x is a Base class variable CheckingAccount y; // y and z are Derived class variables CheckingAccount z; /* ------------------------------------------------------- Remember that: A derived class may have MORE variables/methods than the base class ! ------------------------------------------------------- */ /* -------------------------------- A legal downcasting scenario -------------------------------- */ y = new CheckingAccount(343, "John", 2000.0); x = y; // Upcasting, allowed (discussed before) // x references a CheckingAccount object z = (CheckingAccount) x; // Downcasting, allowed because: // x references a CheckingAccount object // z is a CheckingAccount reference var. // ⇒ compatible !!! } }

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac Downcasting1.java To run:          java Downcasting1 Result: No errors

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  • Example of a illegal downcasting operation:

    public class Downcasting2 { public static void main(String[] args) { BankAccount x; // x is a Base class variable CheckingAccount y; // y and z are Derived class variables CheckingAccount z; /* ------------------------------------------------------- Remember that: A derived class may have MORE variables/methods than the base class ! ------------------------------------------------------- */ /* -------------------------------- An ILLEGAL downcasting scenario -------------------------------- */ x = new BankAccount(343, "John", 2000.0); // x references a BankAccount object z = (CheckingAccount) x; // Downcasting, NOT allowed because: // x references a BankAccount object // z is a CheckingAccount reference var. // // Reason: // A CheckingAccount object has // MORE variables/methods than // a BankAccount object. // Some operations are NOT possible ! } }

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac Downcasting2.java To run:          java Downcasting2 Result: Execution error >> java Downcasting2 Exception in thread "main" java.lang.ClassCastException: BankAccount at Downcasting2.main(Downcasting2.java:25)

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    Illustrating the reason why not allowed:

    Content of the BankAccount and CheckingAccount objects: Content of a BankAccount object Content of a CheckingAccount object public class CheckingAccount { private int accNum; private String name; private double balance; public double getBalance() { ... } public void deposit( double amount ) { ... } public void withdraw( double amount ) { ... } public String toString( ) { ... } } public class CheckingAccount { private int accNum; // These variable are present private String name; // But they are not private double balance; // directly accessible public double getBalance() { ... } public void deposit( double amount ) { ... } public void withdraw( double amount ) { ... } public String toString( ) { ... } public void transfer( BankAccount toAcc, double amt ) { ... } } When we define a CheckingAccount object (reference) variable z as: CheckingAccount z; We can (= are able to) use the variable z to invoke transfer() method: z.transfer( ... ); Therefore: if Java allows this downcast operation: BankAccount x; // x is a Base class variable CheckingAccount z; // z is a Derived class variable /* ------------------------------------------------------- Remember that: A derived class may have MORE variables/methods than the base class ! ------------------------------------------------------- */ /* -------------------------------- An ILLEGAL downcasting scenario -------------------------------- */ x = new BankAccount(343, "John", 2000.0); // x references a BankAccount object z = (CheckingAccount) x; // Downcasting, NOT allowed because: // x references a BankAccount object // z is a CheckingAccount reference var. // // A CheckingAccount object has // MORE variables/methods than // a BankAccount object. // Some operations are NOT possible ! Therefore: If Java allows z to reference a BankAccount object, it can lead to executing a non-existing method: z.transfer( ... ); (Because the method transfer does not exist inside a BankAccount object !!!)

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• Summary on upcasting and downcasting
  • Upcasting:

    is the act of assigning a reference of a derived class to a reference variable of the base class Example: BankAccount a; CheckingAccount b = new CheckingAccount( .... ); a = b; // Upcasting

  • Upcasting is a very useful operation because:

    Allows programs that manipulate variables of the based class to manipulate variables of any derived class without having to change the code of the programs. See the example about SelectionSort() on an array of BankAccount objects - it can also sort an array of CheckingAccount objects.

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  • Becasuse upcasting is done, there must be an operation to undo upcasting

    This undo upcasting operation is called: upcasting Example: BankAccount a; CheckingAccount b = new CheckingAccount( .... ); a = b; // Upcasting CheckingAccount c; c = (CheckingAccount) a; // ***** Downcasting

    In other words:

    The reason that Java provides the downcasting operation is mainly to undo upcasting

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  • You must specify the casting operator in the downcasting operation (because it is potentially dangerous - can cause errors)

    BankAccount a; CheckingAccount b = new CheckingAccount( .... ); a = b; // Upcasting CheckingAccount c; c = a; // *** results in compile error c = (CheckingAccount) a; // Must use casting operator