Pages 75-84

Pages 75-84

1. Class Members in UML Class Diagrams

   • UML allows you to include field types and method signatures
     • Field and its type is separated by a colon
     • A methods argument list is presented in parentheses and its return comes after a colon

2. Creating Objects

   • Object = instance
     • Construct = create
       • Instantiating a class = creating an instance of the class
   • Create an Object
     • Use keyword new followed by the constructor of the class to be instantiated
       • new Employee(); is how to make a new Employee object
         • Manipulating the object later
           • Assign the object to an object variable (reference variable)
             • Declare an object reference with the same type as the object
               • Employee employee = new Employee()
           • Once an object is made you can call its methods and access its fields
             • Use the object reference that was assigned to the object using a .
               • ObjectReference.methodName
                 • Employee employee = new Employee(); Employee.age = 24; Employee.salary = 50000

3. The Null Keyword

   • Reference variable refer to an object but those objects may not have variables = null
     • Class level reference variables are initialized upon instance creation and do not need to have null assigned. But local references do need null assigned
       • Accessing the field or method of a null value raises an error. You can test this by using ==
         • if (book == null) { book = new Book();}

4. Objects in Memory

   • When variables are declared in the class or method level memory space is used for them
     • declaring an int costs 4 bytes while declaring a long uses 8 bytes. These are primitive values
       • Stored in Javas stack
     • Objects data is stored in Javas heap
     • Reference variable have data stored in Javas stack but the data stored is only the address of the object in the heap
   • Stack data holds reference points that request data while Heap data holds the information

5. Java Packages

   • Allow you to group related classes or classes with similar functionality
     • Java.io package = classes for performing input and output operations
   • Java packages that start with JAVA are reserved for the core libraries
   • Packages starting with javax are meant for extension libraries
   • Classes with the same name can be used if they belong to different packages
   • To group a class to a package use the keyword package followed by the package name. package com.brainysoftware.common
   • fully qualified name = class name that carries with it its package name

6. Encapsulation and Access Control

   • This is a mechanism that protects parts of an object that need to be secure. It also only exposes the parts that are safe to be exposed.
     • 4 access control modifiers in Java: PUBLIC, PROTECTED, PRIVATE, and DEFAULT ACCESS LEVEL
       • CLASS ACCESS CONTROL MODIFIERS
         • Access control modifier at the beginning of the class declaration allows you to control from which packages your class can be seen
           • Make a class public by using the public access control modifier
           • A class whose declaration bears no access control modifier has default access