Inheritance

1
Inheritance

• Lakshmish Ramaswamy

2
Method Overriding

• A mechanism through which a subclass replace a
  method existing in its superclass
• New method defn. in subclass must have same
  method signature (method name parameters) and
  same return
• New defn. cannot narrow the accessibility, but
  can widen
• New method defn. can only specify all or a subset
  of the exception classes specified by the throws
  clause of the overridden method
• Cannot override final methods

3
Partial Inheritance and super

• Subclass method may need to access super class
  method that is overridden
• Doing a little extra from what the base class
  method does
• Cant cut and paste the code due to accessibility
  restrictions
• super provides a way of invoking the overridden
  base class method
• Example toString method in Person and Student
  classes

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Running Example
5
Naïve Implementation
6
Better Implementation with super
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Constructors and super

• Need to initialize all fields of subclass (even
  those declared in superclass)
• What happens if superclass members are private?
• Cannot be accessed by the subclass constructor
• super provides a way of invoking superclass
  constructor from subclass constructor

8
Use of super in Constructor
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Dynamic Dispatch Polymorhpism

• If reference type and object being referenced
  have different implementation which one would be
  used
• Reference type may be assigned to any subclass
  objects - Polymorphism
• Actual object assignment may not be known until
  runtime
• Dynamic dispatch Implementation of the dynamic
  type will be used