Java Interfaces

Interfaces in Java

• Interfaces in Java are unique to the language and serve a similar function to multiple inheritance, even though Java does not directly support multiple inheritance
• An interface in Java is a special kind of class that contains methods and variables
• Interfaces are always completely abstract, unlike classes

Interface Definition

• Use the keyword interface instead of class
• Interfaces can contain member elements, but these elements are implicitly static and final
• Methods defined in an interface lack implementation code

Inheritance

• A class can inherit from only one superclass
• A class can implement multiple interfaces
• Classes implement interfaces using a specific syntax after definition

Implementing Interfaces

• A class implements interfaces to use their functionalities
• Example uses the CV interface implemented by Teacher and Student
• Teacher class extends Employee and implements CV
• Methods declared in an interface must be defined in implementing classes
• Interfaces allows for polymorphism

Interface Methods

• Interface methods are abstract by default
• Interface methods cannot be final or static
• Interfaces cannot implement other interfaces and do not have constructors
• Interfaces cannot implement classes
• Interfaces can inherit from multiple other interfaces

Interface Inheritance

• Interfaces can be derived from other interfaces, forming a chain of inheritance

I/O Streams

• Programs need to access external data from an input source
• Programs send results to an output destination
• Abstractions are key for managing different I/O types

Java I/O Package

• Comes with various streams, such as objects, datatypes, characters and files
• Java has three default streams:

System.in

• Standard input stream
• For reading from the keyboard or other input device

System.out

• Standard output stream
• For producing the results of a program

System.err

• Standard error stream
• For outputting error data

Types of Streams

• Input streams are used to read data
• Output streams are used to write data

Stream Classification

• Byte streams handle data byte by byte (8 bits)
• FileInputStream is used to read from the source
• FileOutputStream is used to write to the destination -Character streams read and write data character by character, using Unicode
• FileReader and FileWriter are character streams utilized for reading and writing

Multithreading

• Multithreading divides a program into multiple processes that can run in parallel and means multiple flow of control
• Programs with multiple threads improve system resource utilization
• Multithreading can solve a range of problems

Java Multithreading

• Java is multi-threaded, allowing concurrent execution of processes within a single program
• Threads in Java are lightweight and share the same memory space, they have high efficiency in inter-thread communication

Multitasking

• Process-based multitasking allocates separate memory areas, leading to high communication costs and slower switching
• Thread-based multitasking offers lightweight threads sharing the same address space, reducing communication costs

Thread Creation Methods

• Extending the Thread class
• Implementing the Runnable Interface

Thread Class

• Extends the Thread class
• The class overrides the run() method
• Starts the execution of a thread using object.start()

Runnable Interface

• Implements the java.lang.Runnable interface
• Overrides the run() method
• Creates a Thread object and calls start

Thread Lifecycle

• Threads transition through New, Active, Waiting/Blocked, Timed Waiting, and Terminated states

New State

• A thread is in the New state, code has not yet been run and process is uninitiated

Active State

• A thread transitions to the Active state upon invoking the start() method
• The sub-states are Runnable and Running

Runnable State

• Ready to run
• The thread is ready to run at all times

Running State

• Receives CPU resources
• The thread goes from the Runnable state

Additional States

• Waiting/Blocked means the thread is temporarily inactive
• Timed Waiting has an avoidance of the starvation situation, invoking thread has a sleep() method and resumes after expiration
• Terminated achieved once the thread finishes tasks, or because of unusual events

App Development in Java

• Java can develop applications such as enterprise, network, desktop, web, games, and Android apps
• It can also develop applets

Java App vs Java applet

• Java Application program runs on an operating system with support of virtual machine, whereas the Applet has been embedded into a web page and works on the client-side
• JRE is needed to execute
• Applications have full access and are executed from file/system
• Applications need to be installed locally on the computer

Abstract Window Toolkit (AWT)

• AWT creates GUIs in Java
• AWT uses GUI components as objects
• AWT component resources are OS-dependent
• AWT classes use the Java.awt package

AWT Components

• Components are parts of an AWT, such as buttons, text fields, and scroll bars, and are added to containers
• Containers hold the components on screen; examples are the Window, Frame, Dialog, and Panel classes

Common Containers

• Panel containers do not contain other bars or menu bars
• Panels are generic containers used for buttons and text fields
• Windows have no borders or menu bars
• Frames contain title bars, borders, and menu bars, and are most used in apps

Java Swing

• Is part of Java Foundation Classes (JFC)
• Swing creates application windows using the Abstract Window Toolkit (AWT) API
• Java provides plattform-independent components
• Swing components are lightweight, and has javax.swing package with JButton, JTextField, JTextArea, JRadioButton, JCheckbox, JMenu, JColorChooser
• Uses MVC

Java vs Swing

• AWT components are platform-dependent and heavy weight
• Swing provides more powerful compontents

Java Database Connectivity (JDBC)

• API connects and executes the request with the database, communicating with different data, using database standards
• To store application information, JDBC technology can interact with databases
• Interacting an efficient database used the ODBC (open database database connectivity) driver
• SQL server database, MS Access ,Oracle

JDBC Elements

• The application is a program that communicates data
• Execute SQL statements and take information from an API, such as Driver Manager or JDBC drivers
• The DriverManager connects enterprise apps to databases, and that JDBC drivers communicates with data source