Lecture 11

Fall 2018

Narges Norouzi

More survey feedback on Tuesday

access specifiers

access specifiers in inheritance 

• Subclass has access to public members of parent class

  • Public methods and properties are accessible in subclass

• Private members are not accessible in subclass
• What if you want to let subclasses access a member, but not other classes?

Protected member

• Intermediate level of protection between public and private

• Protected members accessible by

  • ​subclass members

  • Classes in the same package

access levels

Access modification in inheritance 

• You cannot reduce accessibility of methods in subclasses

• Meaning of extends

  • Parent-granted privileges cannot be revoked/reduced by child

Static members

writing ebay for cars

static (1)

• When you create a class
  • You describe how objects of that class look
  • And how they will behave
• You don’t actually get an object until you create one, using new
• At that point storage is allocated and methods become available.

static (2)

• There are two situations in which this approach is not sufficient.

1. Class data or static property

2. Class method or static method

class data

• You want to have only a single piece of storage for a particular field

• Regardless of how many objects of that class are created

  • Or even if no objects are created

• Example:

  • Altima.designYear

• Class data = static properties

class method

• If you need a method that isn’t associated with any particular object of this class.

• You need a method that you can call even if no objects are created

• Example:

  • Counting how many objects we created so far

• Class methods = static methods

static Fields

• Static properties are shared among all the objects
• Static properties are properties of classes, not objects
• Example:
  • Player.NumberOfObjects

static methods

• Static methods can access only static properties

• Static methods are actually class operations

• If a method uses only static fields, make it static!

• Static methods are accessible via classes

  • double d = Double.parseDouble("12");

  • String s = String.valueOf(12);

  • public static void main(String[] args)

• Static properties and methods are invoked on class name

example

static initialization

• Static properties are class data, not object data

• Constructors are created for initializing object data

• How to initialize class data?

• Two ways:

  • Inline values

  • Static block

• Static initialization is done when Class Loader loads the class

Inline initialization

static block

Initialization block

order of initialization

• Once per class

  • Static variable declaration

  • Static block

• Once per object
  • variable declaration
  • Initialization block
  • Constructor

• Zybooks chapters we covered

• section 20.6: static member