Notes, summaries, assignments, exams, and problems for Computers

Buttons in Java Applets

Introduction

Buttons are essential UI elements in Java applets, allowing user interaction and triggering specific actions. This guide explores various button types and their implementation.

Basic Buttons

Buttons can be created using the Button class. They are displayed on the screen and trigger events when clicked. Each button has an identifier used for event handling.

Example:

Button button;button = new Button("Button");

Button Events

When a user clicks a button, an event is generated. These events can be captured using the action() method:

public boolean action(Event evt, Object obj) {    if (evt.target instanceof Button) {        System.out.println((String) obj);    } else {        System.out.println("Event No-Button");

Core Database Concepts

Registration

The set of fields that describe an object.

Field

A feature of an object that stores a value.

Database

A collection of records or data that contain relevant information.

Database Applications

Databases are fundamental across various sectors, including:

• Banks
• Airlines
• Universities
• Credit Card Transactions
• Telecommunications
• Finance
• Sales
• Production
• Human Resources

Levels of Data Abstraction

Physical Level

This is the lowest level of abstraction, describing how data is actually stored.

Conceptual Level

This is the next higher level of abstraction, describing what data is stored in the database and the relationships between data.

External Level (View Level)

This is the highest abstraction level, describing only a part of the database

E1 Primary Access and Multiplexing

Multiplexing and Quantization

True or False: Multiframes are sent 500 per second in E1 primary access.

Plesiochronous multiplexers cause a loss of original formatting information.

CMI code requires greater bandwidth.

Question: What is the step size of a uniform quantizer with 2048 levels and a dynamic signal range of ±1V?

Answer: 0.977 mV

Question: How many quantization levels are in the second segment of a non-uniform quantizer using the A-law approach?

Answer: 16 levels

E1 Primary Access Characteristics

False Statement: In E1 primary access, the word alignment multiframe is transmitted every 15 frames.

True Statement: In E1 primary access, the word alignment multiframe is transmitted every 2 ms.

NFAS: Offers a speed... Continue reading "E1 Primary Access and Multiplexing in PDH and SDH Networks" »

MySQL Essentials: Database & Table Management

Connecting to MySQL

To connect to a MySQL server, use the following command:

mysql -u [username] -p [password]

Database Operations

Viewing Databases

To see all available databases:

SHOW DATABASES;

Creating a Database

To create a new database:

CREATE DATABASE [database_name];

Selecting a Database

To select a database to work with:

USE [database_name];

Table Operations

Viewing Tables

To list all tables within the currently selected database:

SHOW TABLES;

Viewing Table Structure

To display the structure (columns, data types, etc.) of a specific table:

DESCRIBE [table_name];

Creating a Table

To create a new table within the selected database:

CREATE TABLE [table_name] (
    [field_name_1] [data_type_1] [field_definitions_

OpenSSL Encryption and Decryption Commands

RC4 Symmetric Encryption

To encrypt the file "fichero.txt" using the RC4 symmetric encryption algorithm:

OpenSSL> rc4 -in c:\cifrado\fichero.txt -out c:\cifrado\cifrado.rc4
enter rc4 encryption password:
Verifying - enter rc4 encryption password:

To decrypt the previously encrypted file:

OpenSSL> rc4 -d -in c:\cifrado\cifrado.rc4 -out c:\cifrado\descifrado.txt
enter rc4 decryption password:

DES Encryption

Encrypting a file:

OpenSSL> des -salt -in c:\csb.log -out c:\csb.bf.txt
enter des-cbc encryption password:
Verifying - enter des-cbc encryption password:

Decrypting a file:

OpenSSL> des -d -in c:\csb.bf.txt -out c:\csb.txt
enter des-cbc decryption password:

Triple DES (DES3) Algorithm

Encrypting... Continue reading "OpenSSL Encryption and Decryption Techniques" »

Input/Output (I/O) Units

I/O units connect the system to the outside world, enabling the introduction and extraction of information. These units typically consist of registers operated by control buses, storing address information from the data bus. Their outputs are externally accessible for connecting to various devices.

Another crucial function of I/O units is to adapt:

• The format of signals from outside to the format of internal signals.
• Working speeds.
• Voltage levels.
• Binary codes.

System Buses

All elements of a programmable microsystem are interconnected by information channels, commonly known as buses. These buses are composed of multiple lines, each designed to transmit information of a specific type. The number of lines forming a bus determines... Continue reading "Computer System Components: I/O, Buses & Peripherals Explained" »

Understanding DNS (Domain Name System)

DNS (Domain Name System) is a protocol that manages and maintains a distributed database of IP addresses and domain names.

Uses of DNS

• Resolution: Converts domain names to IP addresses using a host.
• Inverse Resolution: Converts IP addresses to hostnames.
• Mail Routing: Determines the mail server for a given domain.

Domain Name Space

The domain name space is a distributed database stored across multiple machines. It stores the correspondence between domain names and IP addresses in a hierarchical structure.

TLDs (Top Level Domains)

Top Level Domains are the first level of the DNS organization, forming an inverted tree structure. They store the descendants of the root DNS domain. Examples include generic TLDs (.com,... Continue reading "Understanding DNS: Domain Name System Explained" »

What is a Database?

A database is a structured data support system that allows us to store and process data quickly, safely, and efficiently.

A database consists of fields, records, and files. We define:

• Field: Each of the elements that make up a record, such as name, address, town, and telephone.
• Record: A set of interrelated fields, such as personal data belonging to a shareholder of a video store.
• File: Consists of the union of multiple records.

What is a Relational Database?

Using relational databases is useful when multiple records of a database are related to the same record in another database. A simple and intuitive way of structuring data is using tables, i.e., arranged in rows and columns. The relational model uses the table structure for... Continue reading "Understanding Databases: Relational Models and Access" »

Electronic Communication includes email, electronic newsletters, and video conferencing systems.

Interpersonal Network involves informal discussions with team members and external individuals who may offer valuable insights or visions to aid the team.

Decomposition of the Problem

Decomposition of the problem, also known as partitioning or elaboration, is an activity based on core software requirements analysis. The level of decomposition is not intended to be complete. Instead, it is applied in two main areas: the functionality to be delivered and the process to be used for delivery.

PROCESS: The generic stages characterizing the software process—definition, development, and support—are applicable to all software.

Software Process Models

• Linear

Java Programming Fundamentals

Variables and Data Types

• Which of the following options is not a variable? Literal
• A block of related data elements is called a table.
• According to convention, the dollar sign character is never used to start a variable name.
• An object is a software bundle related to its state.
• Using 16-bit Unicode.
• You can use the short data type to save memory in large tables.
• You do not need to assign a value when a field is declared.
• Each item in a table is called an element.
• Parameters are always variables.
• The length of a table is set when the table is created.

Operators and Expressions

• The operator with the highest priority is evaluated before operators with lower priority.
• Operators on the same line have equal priority.
• If the name of