Reusable Classes

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• Reusable Classes

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Reusable Classes

• Motivation Write less code!

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Reusable Classes

• A reusable class is one that can be used in
  multiple programs
• Not all classes should be reusable
• Decide before you design it which category it
  fits into

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Reusable Classes

• We have already heard of the STO principle

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Reusable Classes

• We have already heard of the STO principle
• Design a class that implements a single
  well-defined task.
• Do not overburden the class with multiple tasks

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Reusable Classes

• 4 main reusable object types
• User interface object
• Controller object
• Application logic object
• Storage object

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Reusable Classes

• Dont forget other
• Not everything fits into these categories, but
  most will

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Reusable Classes

• Overriding and overloaded methods
• Overriding redefining a method previously
  defined in the extends hierarchy
• Ex. toString( )
• Overloading defining multiple methods with the
  same name and purpose (different parameters)
• Ex. - multiple constructors

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Reusable Classes

• Overriding
• public class MarriedWoman extends Woman
• public String toString()
• String name super.toString()
• name name husbandName

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Reusable Classes

• Overloading
• A method signature is determined by the method
  name and number and data type of parameters
• If methods have the same name but different
  signatures, they are overloaded
• println() is a good example
• public void drawCircle(int x, int y, int radius)
• public void drawCircle(Circle c)

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Reusable Classes

• Overloading
• public class Person
• public String name
• public Person()
• this(UNKNOWN)
• public (String myName)
• name myName

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Reusable Classes

• Overloading
• public class Person
• public String name
• public Person()
• this(UNKNOWN)
• public (String myName)
• name myName

this calls this constructor
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Reusable Classes

• This gives users of your class more flexibility
  in how they call the methods.
• It makes your classes more easily usable
• Hence, more reusable.
• Review
• Inhertance can help you make a number of objects
  similar in nature from a base class
• You can override methods from the base class as
  needed in the sub classes
• Within every class you can overload methods to
  make your class easier to use.

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Reusable Classes

• Abstract Classes can help define inheritance
• Abstract classes are defined to show general
  behavior that must be implemented by sub classes
• Abstract classes are not instantiable
• abstract class Student
• abstract public void computeGrade()

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Reusable Classes

• abstract class Student
• abstract public void computeGrade()
• Abstract methods must be implemented by a sub
  class
• This can be very helpful in defining
  functionality
• Not all methods in an abstract class are abstract
• More on lesson 8

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Reusable Classes

• Questions??

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Reusable Classes

• What is the purpose of defining multiple
  constructors?

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Reusable Classes

• What is the purpose of defining multiple
  constructors?
• To increase the ease of use. By having multiple
  constructors, the programmer will have a
  flexibility in creating the instances.

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Reusable Classes

• Which of the following method declarations have
  the same signature?
• public void one ( int x, int y) / 1 /
• ...
• private void one ( int x, int y) / 2 /
• ...
• public int one ( int x, int y) / 3 /
• ...
• private void one ( int x, float y) / 4 /
• ...

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Reusable Classes

• Which of the following method declarations have
  the same signature?
• public void one ( int x, int y) / 1 /
• ...
• private void one ( int x, int y) / 2 /
• ...
• public int one ( int x, int y) / 3 /
• ...
• private void one ( int x, float y) / 4 /
• ...

The method signature does not include the access
modifier and the return type, so the methods 1,
2, and 3 have the same signature. Method 4 has a
different signature because the data type of the
second parameter is float.
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Reusable Classes

• Which is the subclass and which is the superclass
  in the following declaration?
• class X extends Y ...

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Reusable Classes

• Which is the subclass and which is the superclass
  in the following declaration?
• class X extends Y ...
• X is the subclass, and Y is the superclass.

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Reusable Classes

• Suppose Truck and Motorcycle are subclasses of
  Vehicle. Which of the following declarations are
  invalid?
• Truck t new Vehicle()
• Vehicle v new Truck()
• Motorcycle m1 new Vehicle()
• Motorcycle m2 new Truck()

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Reusable Classes

• Suppose Truck and Motorcycle are subclasses of
  Vehicle. Which of the following declarations are
  invalid?
• Truck t new Vehicle()
• Vehicle v new Truck()
• Motorcycle m1 new Vehicle()
• Motorcycle m2 new Truck()
• The declarations for t, m1, and m2 are invalid.

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Reusable Classes

• Suppose Truck and Motorcycle are subclasses of
  Vehicle. Which of the following declarations are
  invalid?
• Truck t new Vehicle()
• Vehicle v new Truck()
• Motorcycle m1 new Vehicle()
• Motorcycle m2 new Truck()
• The declarations for t, m1, and m2 are invalid.