Category: Programming Concepts

SQL Cheat Sheet

March 31st, 2023
When working on something I don’t work on a lot and I need a reference source I write a cheat sheet.

Here’s a simple SQL query language cheat sheet:

SELECT:

The SELECT statement is used to retrieve data from one or more tables.

Example:
```
SELECT column1, column2 FROM table_name;
```
FROM:

The FROM clause is used to specify the table(s) from which data is to be retrieved.

Example:
```
SELECT column1, column2 FROM table_name WHERE condition;
```
WHERE:

The WHERE clause is used to filter the results of a query based on a specified condition.

Example:
```
SELECT column1, column2 FROM table_name WHERE column3 = 'value';
```
ORDER BY:

The ORDER BY clause is used to sort the results of a query based on one or more columns.

Example:
```
SELECT column1, column2 FROM table_name ORDER BY column1 ASC;
```
GROUP BY:

The GROUP BY clause is used to group the results of a query by one or more columns.

Example:
```
SELECT column1, COUNT(*) FROM table_name GROUP BY column1;
```
HAVING:

The HAVING clause is used to filter the results of a GROUP BY query based on a specified condition.

Example:
```
SELECT column1, COUNT(*) FROM table_name GROUP BY column1 HAVING COUNT(*) > 1;
```
JOIN:

The JOIN clause is used to combine data from two or more tables based on a common column.

Example:
```
SELECT column1, column2 FROM table1 JOIN table2 ON table1.column3 = table2.column4;
```
INNER JOIN:

The INNER JOIN clause is used to combine data from two or more tables based on a common column, returning only the rows that have a match in both tables.

Example:
```
SELECT column1, column2 FROM table1 INNER JOIN table2 ON table1.column3 = table2.column4;
```
LEFT JOIN:

The LEFT JOIN clause is used to combine data from two or more tables based on a common column, returning all rows from the left table and matching rows from the right table.

Example:
```
SELECT column1, column2 FROM table1 LEFT JOIN table2 ON table1.column3 = table2.column4;
```
RIGHT JOIN:

The RIGHT JOIN clause is used to combine data from two or more tables based on a common column, returning all rows from the right table and matching rows from the left table.

Example:
```
SELECT column1, column2 FROM table1 RIGHT JOIN table2 ON table1.column3 = table2.column4;
```
UNION:

The UNION clause is used to combine the results of two or more SELECT statements into a single result set.

Example:
```
SELECT column1 FROM table1 UNION SELECT column1 FROM table2;
```
LIKE:

The LIKE operator is used to search for a specified pattern in a column.

Example:
```
SELECT column1 FROM table_name WHERE column2 LIKE '%value%';
```
IN:

The IN operator is used to match a value against a list of values.

Example:
```
SELECT column1 FROM table_name WHERE column2 IN ('value1', 'value2', 'value3');
```
BETWEEN:

The BETWEEN operator is used to match a value against a range of values.

Example:
```
SELECT column1 FROM table_name WHERE column2 BETWEEN 'value1' AND 'value2';
```
DISTINCT:

The DISTINCT keyword is used to return only unique values in a column.

Example:
```
SELECT DISTINCT column1 FROM table_name;
```
These are some of the most commonly used SQL query language statements and operators. There are many more advanced features and functions available, but this cheat sheet

COUNT:

The COUNT function is used to count the number of rows in a table that meet a specified condition.

Example:
```
SELECT COUNT(*) FROM table_name WHERE column1 = 'value';
```
AVG:

The AVG function is used to calculate the average value of a column.

Example:
```
SELECT AVG(column1) FROM table_name;
```
SUM:

The SUM function is used to calculate the sum of a column.

Example:
```
SELECT SUM(column1) FROM table_name;
```
MAX:

The MAX function is used to find the maximum value in a column.

Example:
```
SELECT MAX(column1) FROM table_name;
```
MIN:

The MIN function is used to find the minimum value in a column.

Example:
```
SELECT MIN(column1) FROM table_name;
```
NULL:

The NULL value represents an unknown or missing value in a column.

Example:
```
SELECT column1 FROM table_name WHERE column2 IS NULL;
```
NOT NULL:

The NOT NULL constraint is used to specify that a column must not contain NULL values.

Example:
```
CREATE TABLE table_name (

    column1 INT NOT NULL,

    column2 VARCHAR(255) NOT NULL

);
```
PRIMARY KEY:

The PRIMARY KEY constraint is used to specify a column or set of columns as the primary key for a table.

Example:
```
CREATE TABLE table_name (

    column1 INT PRIMARY KEY,

    column2 VARCHAR(255)

);
```
FOREIGN KEY:

The FOREIGN KEY constraint is used to specify a relationship between two tables, based on a key column in each table.

Example:
```
CREATE TABLE table1 (

    column1 INT PRIMARY KEY,

    column2 VARCHAR(255)

);

CREATE TABLE table2 (

    column3 INT PRIMARY KEY,

    column4 VARCHAR(255),

    column5 INT,

    FOREIGN KEY (column5) REFERENCES table1(column1)

);
```
INDEX:

The INDEX statement is used to create an index on one or more columns in a table, to improve query performance.

Example:
```
CREATE INDEX index_name ON table_name (column1);
```
These are some of the most commonly used SQL statements and keywords. With this cheat sheet, you should be able to write basic SQL queries and understand the results. However, SQL is a vast and powerful language with many advanced features and functions. I encourage you to continue exploring and learning more about SQL as you work with databases.
The Class in Java

March 28th, 2023
Object-oriented programming (OOP) is a programming paradigm that uses objects to represent real-world entities and their interactions. In Java, a class is a blueprint or a template that defines the properties and behavior of an object. In this blog post, we’ll discuss the OOP class in Java in detail, including its syntax, properties, and methods, with examples.

Syntax of a Java class

The syntax of a Java class is as follows:
```
[access_modifier] class ClassName [extends SuperClassName] [implements InterfaceName]
{
  // Fields
  // Constructors
  // Methods
}
```
Access modifier specifies the level of access to the class, and it can be public, protected, or private. ClassName is the name of the class, and it should start with a capital letter. The extends keyword is used to inherit properties and behavior from a parent class or a superclass. The implements keyword is used to implement the methods of an interface.

Properties of a Java class

A Java class can have the following types of properties:
1. Instance variables: These are non-static variables that belong to an object. Each object of the class has its own set of instance variables.
2. Static variables: These are class-level variables that belong to the class rather than an object. They are shared by all objects of the class.
3. Final variables: These are constants that cannot be changed once initialized.
4. Transient variables: These are variables that are not serialized when an object is written to a file.
5. Volatile variables: These are variables that are accessed by multiple threads, and their value is always read from main memory.
Methods of a Java class

A Java class can have the following types of methods:
1. Constructors: These are special methods that are used to create objects of the class. They have the same name as the class and do not have a return type.
2. Instance methods: These are non-static methods that belong to an object. They can access instance variables and can modify their values.
3. Static methods: These are class-level methods that belong to the class rather than an object. They cannot access instance variables and can only access static variables.
4. Getter and setter methods: These are methods used to get and set the values of instance variables.
Example of a Java class

Here’s an example of a Java class that defines a Circle object:
```
public class Circle {
  private double radius;

  public Circle(double radius) {
    this.radius = radius;
  }

  public double getRadius() {
    return radius;
  }

  public void setRadius(double radius) {
    this.radius = radius;
  }

  public double getArea() {
    return Math.PI * radius * radius;
  }

  public double getCircumference() {
    return 2 * Math.PI * radius;
  }
}
```
In this example, the Circle class has one instance variable, radius, which is a double value. The class has a constructor that takes a radius parameter and initializes the radius instance variable. The class also has getter and setter methods for the radius instance variable. Finally, the class has two methods, getArea() and getCircumference(), that return the area and circumference of the circle, respectively.

In conclusion, a class in Java is a blueprint that defines the properties and behavior of an object. It can have instance variables, static variables, final variables, transient variables, and volatile variables, and it can have constructors, instance methods, static methods, getter and setter methods, and other types of methods. Understanding classes is an essential part of object-oriented programming in Java, and it’s important to use them effectively to create well-structured and maintainable code.
What is an Object in Java

March 27th, 2023
In object-oriented programming (OOP), an object is an instance of a class that represents a real-world entity and its interactions. In Java, objects are created from classes, which define their properties and behavior. In this blog post, we’ll discuss objects in Java in detail, including their creation, properties, and methods, with examples.

Creating objects in Java

To create an object in Java, you need to follow these steps:
1. Declare a variable of the class type. For example, to create an object of the Person class, you would declare a variable as follows:
```
Person person;
```
1. Allocate memory for the object using the new keyword. For example:
```
person = new Person();
```
1. Initialize the object’s properties using its constructor or setter methods. For example:
```
person.setName("John");
person.setAge(30);
```
Properties of objects in Java

An object in Java has two types of properties: instance variables and methods. Instance variables are non-static variables that belong to an object, and each object has its own set of instance variables. Methods are functions that define the behavior of the object.

Instance variables can be accessed using dot notation, as shown in the following example:
```
Person person = new Person();
person.setName("John");
System.out.println(person.getName());
```
In this example, we create a new Person object, set its name to “John,” and then print out the value of its name instance variable using the getName() method.

Methods of objects in Java

Methods are used to define the behavior of an object in Java. There are two types of methods: instance methods and static methods. Instance methods are non-static methods that belong to an object and can access its instance variables. Static methods are class-level methods that belong to the class rather than an object and cannot access instance variables.

Here’s an example of an instance method in Java:
```
public class Person {
  private String name;
  private int age;

  public void setName(String name) {
    this.name = name;
  }

  public void setAge(int age) {
    this.age = age;
  }

  public void sayHello() {
    System.out.println("Hello, my name is " + name + " and I am " + age + " years old.");
  }
}
```
In this example, we define a Person class with two instance variables, name and age, and an instance method, sayHello(), that prints out a greeting using the object’s name and age instance variables.

To use this method, we first create a new Person object and set its name and age properties, like this:
```
Person person = new Person();
person.setName("John");
person.setAge(30);
```
Then, we can call the sayHello() method on the object, like this:
```
person.sayHello();
```
This will output the following message:
```
Hello, my name is John and I am 30 years old.
```
In conclusion, objects in Java are instances of classes that represent real-world entities and their interactions. Objects have instance variables and methods, which define their properties and behavior, respectively. To create an object in Java, you need to declare a variable of the class type, allocate memory for the object using the new keyword, and initialize its properties using its constructor or setter methods. Understanding objects is an essential part of object-oriented programming in Java, and it’s important to use them effectively to create well-structured and maintainable code.
A look at the args[] array in Java’s main class.

March 27th, 2023
Part 1

In Java, the main method serves as the entry point for a Java program. This method takes an argument called args, which is an array of strings. In this blog, we will explore how the args array works in the main class in Java.

The main method is defined as follows:
```
public static void main(String[] args) {

    // Code to be executed

}
```
The args parameter is an array of strings that holds any command-line arguments passed to the program. Command-line arguments are values that are passed to the program when it is executed on the command line.

For example, if we have a Java program called MyProgram.java, and we want to pass two arguments to it, we would execute the program as follows:
```
java MyProgram arg1 arg2
```
In this case, arg1 and arg2 would be passed to the program as elements of the args array. The args array would contain two elements, args[0] would be “arg1” and args[1] would be “arg2”.

It is important to note that the args array is optional. If we do not need to pass any command-line arguments to the program, we can simply omit the args parameter from the main method definition, like this:
```
public static void main() {

    // Code to be executed

}
```
If we do include the args parameter, we can access the individual elements of the args array using standard array notation. For example, if we want to print out all the elements of the args array, we could do so as follows:
```
public static void main(String[] args) {

    for (int i = 0; i < args.length; i++) {

        System.out.println(args[i]);

    }

}
```
This would print out each element of the args array on a separate line.

In summary, the args array in the main class in Java is an array of strings that holds any command-line arguments passed to the program. We can access the individual elements of the args array using standard array notation. The args parameter is optional, and if we do not need to pass any command-line arguments to the program, we can simply omit it from the main method definition.
A History of Java

March 25th, 2023
Java is a high-level, object-oriented programming language that was created by James Gosling, Mike Sheridan, and Patrick Naughton at Sun Microsystems (now owned by Oracle Corporation) in 1995. It is one of the most widely used programming languages in the world, particularly for developing enterprise applications, mobile apps, and games. Java is known for its simplicity, portability, and security features, which have made it a popular choice for developers across various industries. In this blog, we will take a closer look at the history and milestones of Java programming language.

History of Java:

The history of Java dates back to the early 1990s when James Gosling, Mike Sheridan, and Patrick Naughton, who were working at Sun Microsystems at the time, began developing a programming language that could be used to create software for consumer electronics devices. They named this language “Oak,” after a tree outside their office window. However, they soon realized that the name was already taken, and they renamed the language “Java” after a type of coffee that was popular among the developers.

The first version of Java was released to the public in 1995, along with the slogan “Write once, run anywhere,” which highlighted the language’s ability to run on multiple platforms. This was a significant breakthrough in the world of programming, as it allowed developers to write code once and deploy it on different operating systems without any changes. Java quickly gained popularity among developers, and in 1996, Sun Microsystems released the first version of the Java Development Kit (JDK), which included a compiler, debugger, and other tools for developing Java applications.

Milestones of Java:

Since its release, Java has undergone several milestones that have contributed to its widespread adoption and success. Here are some of the major milestones in the history of Java:
1. Java 1.0: The first version of Java was released in 1995, which included the core features of the language, such as the Java Virtual Machine (JVM), class libraries, and the Java Development Kit (JDK).
2. Java 1.1: In 1997, Sun Microsystems released Java 1.1, which added new features, such as inner classes, JavaBeans, and JDBC (Java Database Connectivity) for database connectivity.
3. Java 2: In 1998, Sun Microsystems released Java 2, which included significant improvements and new features, such as the Swing GUI toolkit, the Collections framework, and the Java Naming and Directory Interface (JNDI).
4. Java 5: In 2004, Sun Microsystems released Java 5, which added new features, such as generics, annotations, autoboxing, and enhanced for loops.
5. Java 6: In 2006, Sun Microsystems released Java 6, which included new features, such as scripting support, JDBC 4.0, and the Java Compiler API.
6. Java 7: In 2011, Oracle Corporation released Java 7, which included new features, such as the try-with-resources statement, multi-catch exceptions, and the diamond operator.
7. Java 8: In 2014, Oracle Corporation released Java 8, which included new features, such as lambdas, streams, and the new date and time API.
8. Java 9: In 2017, Oracle Corporation released Java 9, which included new features, such as the Java Platform Module System (JPMS), JShell (an interactive Java REPL), and the Process API.
9. Java 10: In 2018, Oracle Corporation released Java 10, which included new features, such as local variable type inference, improved container awareness, and the introduction of the Application Class-Data Sharing (CDS
1. Java 11: In 2018, Oracle Corporation released Java 11, which included new features, such as the HTTP Client API, Flight Recorder, and the launch of the LTS (Long-Term Support) release cycle.
2. Java 12: In 2019, Oracle Corporation released Java 12, which included new features, such as switch expressions, new GC algorithms, and enhancements to the JVM.
3. Java 13: In 2019, Oracle Corporation released Java 13, which included new features, such as text blocks, improvements to the ZGC (Z Garbage Collector), and the introduction of the Switch Expressions (Preview) feature.
4. Java 14: In 2020, Oracle Corporation released Java 14, which included new features, such as pattern matching for instanceof, records (Preview), and text blocks (Preview).
5. Java 15: In 2020, Oracle Corporation released Java 15, which included new features, such as sealed classes (Preview), hidden classes, and the ZGC (Z Garbage Collector) as a production feature.
6. Java 16: In 2021, Oracle Corporation released Java 16, which included new features, such as records, pattern matching for instanceof, and the introduction of the Packaging Tool (jpackage).
Java has come a long way since its inception in 1995. Over the years, it has evolved and improved significantly, adding new features and capabilities that have made it a popular choice among developers worldwide. Today, Java is widely used for developing enterprise applications, mobile apps, games, and other software applications. As the language continues to evolve, we can expect to see even more exciting developments and advancements in the years to come.
Java 1.8 Cheat Sheet

March 24th, 2023
Lambda expressions:

Lambda expressions are a concise way to represent a single method interface using an expression. It is a shorthand syntax for implementing interfaces that have only one method.
```
(parameters) -> expression
```
Example:
```
Arrays.asList("a", "b", "c").forEach(e -> System.out.println(e));
```
Stream API:

Stream API is a new feature introduced in Java 1.8 for processing data in a more efficient and concise way. It allows for filtering, sorting, mapping, and reducing data.

Example:
```
List<String> list = Arrays.asList("apple", "banana", "orange", "pear");

long count = list.stream().filter(e -> e.contains("a")).count();
```
Optional:

Optional is a class introduced in Java 1.8 that can be used to represent null values. It forces the programmer to handle null values in a more explicit way.

Example:
```
Optional<String> optional = Optional.ofNullable(null);

String value = optional.orElse("default");
```
Default methods in interfaces:

Default methods in interfaces are methods that have an implementation in the interface itself. They were introduced in Java 1.8 to support the addition of new methods to interfaces without breaking existing code.

Example:
```
public interface MyInterface {

    default void myMethod() {

        System.out.println("MyInterface");

    }

}

public class MyClass implements MyInterface {

    // ...

}
```
Method references:

Method references are a shorthand syntax for writing lambda expressions that call a single method. They provide a way to reuse existing methods as lambda expressions.

Example:
```
List<String> list = Arrays.asList("apple", "banana", "orange", "pear");

list.forEach(System.out::println);
```
Date and Time API:

Java 1.8 introduced a new Date and Time API to replace the old java.util.Date and java.util.Calendar classes. The new API provides a more flexible and concise way to work with dates and times.

Example:
```
LocalDate date = LocalDate.now();

LocalTime time = LocalTime.now();

System.out.println(date);

System.out.println(time);
```
Base64 encoding and decoding:

Java 1.8 introduced a new class called Base64 for encoding and decoding data in Base64 format. It provides a simple way to encode and decode data in Base64 format.

Example:
```
String text = "hello world";

String encoded = Base64.getEncoder().encodeToString(text.getBytes());

String decoded = new String(Base64.getDecoder().decode(encoded));

System.out.println(encoded);

System.out.println(decoded);
```
Type annotations:

Java 1.8 introduced a new feature called type annotations that allow annotations to be applied to any type use. This includes type declarations, type casts, and instanceof expressions.

Example:
```
List<@NonNull String> list = Arrays.asList("apple", "banana", "orange", "pear");
```
CompletableFuture:

CompletableFuture is a new class introduced in Java 1.8 that provides a way to perform asynchronous computations. It allows you to chain multiple asynchronous operations together and handle errors and exceptions.

Example:
```
CompletableFuture.supplyAsync(() -> "hello")

        .thenApplyAsync(s -> s + " world")

        .thenAccept(System.out::println);
```
Parallel Streams:

Parallel Streams are a feature in Java 1.8 that allow for parallel processing of streams. They provide a way to split the stream into multiple parts and process them in parallel for improved performance.

Example:
```
List<String> list = Arrays.asList("apple", "banana", "orange", "pear");

long count = list.parallelStream().filter(e -> e.contains("a")).count();
```
11. JavaFX:

JavaFX is a Java library for building rich client applications. It provides a set of graphics and media APIs that can be used to create GUI applications.

Example:
```
import javafx.application.Application;

import javafx.scene.Scene;

import javafx.scene.control.Label;

import javafx.scene.layout.StackPane;

import javafx.stage.Stage;

public class HelloWorld extends Application {

scss

Copy code

public void start(Stage primaryStage) {

    Label label = new Label("Hello, World!");

    StackPane root = new StackPane();

    root.getChildren().add(label);

    Scene scene = new Scene(root, 300, 250);

    primaryStage.setScene(scene);

    primaryStage.show();

}

public static void main(String[] args) {

    launch(args);

}

}
```
12. Javadoc:

Javadoc is a tool that comes with the JDK for generating documentation from source code comments. It can be used to generate API documentation for classes, interfaces, and methods.

Example:
```
/**

This is a sample class that demonstrates the use of Javadoc comments.

*/

public class SampleClass {

/**

This method adds two integers and returns the result.

@param a the first integer

@param b the second integer

@return the sum of a and b

*/

public int add(int a, int b) {

return a + b;

}

}
```
This cheat sheet provides a quick overview of some of the key features introduced in Java 1.8. While there are many more features and APIs in Java, these are some of the most commonly used ones.
Object Oriented Programming (OOP) Concepts

March 23rd, 2023

Object-oriented programming (OOP) is a programming paradigm that is used to design and build software systems by creating objects that interact with each other. OOP is based on a set of fundamental concepts that help to make code more modular, flexible, and reusable. In this blog, we will cover the key OOP concepts that you need to understand to become a proficient OOP developer.

Class

A class is a blueprint or a template for creating objects. It defines the properties and methods that an object of that class will have. A class can be thought of as a user-defined data type.

Object

An object is an instance of a class. It has its own set of properties and methods, which are defined by the class. Objects are created using the “new” keyword in Java.

Inheritance

Inheritance is a mechanism that allows a class to inherit properties and methods from another class. The class that inherits from another class is called the subclass, and the class that is being inherited from is called the superclass. Inheritance is used to promote code reuse and to create a hierarchy of related classes.

Polymorphism

Polymorphism is the ability of an object to take on different forms. In Java, polymorphism is achieved through method overloading and method overriding. Method overloading allows a class to have multiple methods with the same name but with different parameters. Method overriding allows a subclass to provide its own implementation of a method that is already defined in the superclass.

Encapsulation

Encapsulation is the practice of hiding the internal details of an object from the outside world. This is done by making the data members of the object private and providing public methods to access and modify those data members. Encapsulation helps to improve code maintainability and reduces the likelihood of bugs.

Abstraction

Abstraction is the process of simplifying complex systems by breaking them down into smaller, more manageable parts. In Java, abstraction is achieved through abstract classes and interfaces. An abstract class is a class that cannot be instantiated and is used as a base class for other classes. An interface is a collection of abstract methods that are implemented by classes that implement the interface.

In conclusion, understanding OOP concepts is essential for becoming a proficient software developer. By mastering these fundamental concepts, you will be able to write more modular, flexible, and reusable code. To become proficient in OOP, it is important to practice these concepts through coding exercises and projects. By doing so, you will become more comfortable with these concepts and will be able to apply them to real-world software development projects.

Category: Programming Concepts

History of Java:

Milestones of Java: