Overriding vs Overloading in Java

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Method overriding should not be confused with method overloading (see Section 33, p 47) Method overriding always requires the same method signature (name and parameter types) and the same or covariant return types Overloading occurs when the method names are the same, but the parameter lists differ Therefore, to overload methods, the parameters must differ either in type, order, or number As the return type is not a part of the method signature, just having different return types is not enough to overload methods
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72: OVERRIDING METHODS
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Only non-final instance methods in the superclass that are directly accessible from the subclass can be overridden Both instance and static methods can be overloaded in the class they are defined in or in a subclass of their class Invoking an overridden method in the superclass from a subclass requires a special syntax (eg, the keyword super) This is not necessary for invoking an overloaded method in the superclass from a subclass If the right kinds of arguments are passed in the method call occurring in the subclass, the overloaded method in the superclass will be invoked In Example 72, the method getBill() at (2) in class Light is overridden in class TubeLight at (6) and overloaded at (7) When invoked at (25), the definition at (7) is executed For overloaded methods, which method implementation will be executed at runtime is determined at compile time (see Section 710, p 324), but for overridden methods, the method implementation to be executed is determined at runtime (see Section 712, p 340) Table 71 provides a comparison between overriding and overloading
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Table 71
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Overriding vs Overloading Comparison Criteria Method name Argument list Return type Overriding Must be the same Must be the same Can be the same type or a covariant type Must not throw new checked exceptions Can narrow exceptions thrown Can make it less restrictive, but not more restrictive A method can only be overridden in a subclass Overloading Must be the same Must be different Can be different
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A method can be overloaded in the same class or in a subclass At compile time, the declared type of the reference is used to determine which method will be executed at runtime
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The runtime type of the reference, ie, the type of the object referenced at runtime, determines which method is selected for execution
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CHAPTER 7: OBJECT-ORIENTED PROGRAMMING
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73 Hiding Members
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A subclass cannot override fields of the superclass, but it can hide them The subclass can define fields with the same name as in the superclass If this is the case, the fields in the superclass cannot be accessed in the subclass by their simple names; therefore, they are not inherited by the subclass Code in the subclass can use the keyword super to access such members, including hidden fields A client can use a reference of the superclass to access members that are hidden in the subclass, as explained below Of course, if the hidden field is static, it can also be accessed by the superclass name The following distinction between invoking instance methods on an object and accessing fields of an object must be noted When an instance method is invoked on an object using a reference, it is the class of the current object denoted by the reference, not the type of reference, that determines which method implementation will be executed In Example 72 at (14), (15), and (16), this is evident from invoking the overridden method getBill(): the method from the class corresponding to the current object is executed, regardless of the reference type When a field of an object is accessed using a reference, it is the type of the reference, not the class of the current object denoted by the reference, that determines which field will actually be accessed In Example 72 at (19), (20), and (21), this is evident from accessing the hidden field billType: the field accessed is declared in the class corresponding to the reference type, regardless of the object denoted by the reference In contrast to method overriding, where an instance method cannot override a static method, there are no such restrictions on the hiding of fields The field billType is static in the subclass, but not in the superclass The type of the fields need not be the same either, it is only the field name that matters in the hiding of fields
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