package org.kodejava.util;
import java.util.ArrayList;
import java.util.LinkedList;
public class CollectionToArrayList {
public static void main(String[] args) {
// We create LinkedList collection type at put some values
// in it. Here we put A, B, C and D letter into it.
LinkedList<String> linkedList = new LinkedList<>();
linkedList.push("A");
linkedList.push("B");
linkedList.push("C");
linkedList.push("D");
// Let say you want to convert it to other type of collection,
// for instance here we convert it into ArrayList. To do it
// we can pass the collection created above as a parameter to
// ArrayList constructor.
ArrayList<String> arrayList = new ArrayList<>(linkedList);
// Now we have converted the collection into ArrayList and
// printed what is inside.
for (String s : arrayList) {
System.out.println("s = " + s);
}
}
}
How do I get number of days in a month?
Let say you want to know the number of days in a month, or we can say it as the last date of a month. The example below shows you how to obtain the number of days or the date.
package org.kodejava.util;
import java.util.Calendar;
public class MonthDaysExample {
public static void main(String[] args) {
// First get an instance of calendar object.
Calendar calendar = Calendar.getInstance();
// We'll set the date of the calendar to the following
// date. We can use constant variable in the calendar
// for months value (JANUARY - DECEMBER). Be informed that
// month in Java started from 0 instead of 1.
int year = 2021;
int month = Calendar.FEBRUARY;
int date = 1;
// We have a new date of 2021-02-01
calendar.set(year, month, date);
// Here we get the maximum days for the date specified
// in the calendar. In this case we want to get the number
// of days for february 2021
int maxDay = calendar.getActualMaximum(Calendar.DAY_OF_MONTH);
System.out.println("Max Day: " + maxDay);
// Here we want to see what is the days for february on
// a leap year.
calendar.set(2020, Calendar.FEBRUARY, 1);
maxDay = calendar.getActualMaximum(Calendar.DAY_OF_MONTH);
System.out.println("Max Day: " + maxDay);
}
}
The result of the code snippet above:
Max Day: 28
Max Day: 29
In JDK 8 you can use the new Date Time API to get the number of days in a month. Here an example that show you how to do it: How do I get the length of month represented by a date object?.
How do I create random number?
The java.lang.Math.random() method returns random number between 0.0 and 1.0 including 0.0 but not including 1.0. By multiplying Math.random() result with a number, for example 10 will give us a range of random number between 0.0 and 10.0.
To get a random number between two numbers (n and m) we can use the formula of: n + (Math.random() * (m - n)). Where n is the lowest number (inclusive) and m is the highest number (exclusive).
package org.kodejava.lang;
public class RandomNumberExample {
public static void main(String[] args) {
// The Math.random() returns a random number between 0.0 and 1.0
// including 0.0 but not including 1.0.
double number = Math.random();
System.out.println("Generated number: " + number);
// By multiplying Math.random() result with a number will give
// us a range of random number between, for instance 0.0 to 10.0 as
// shown in the example below.
number = Math.random() * 10;
System.out.println("Generated number: " + number);
// To get a random number between n and m we can use the formula:
// n + (Math.random() * (m - n)). The example below creates random
// number between 100.0 and 200.0.
int n = 100;
int m = 200;
number = n + (Math.random() * (m - n));
System.out.println("Generated number: " + number);
// Creates an integer random number
int random = 100 + (int) (Math.random() * 100);
System.out.println("Generated number: " + random);
}
}
Here is an example result of our program.
Generated number: 0.024974902600698234
Generated number: 2.271051510086771
Generated number: 147.542543014888
Generated number: 112
How do I query records from a table?
package org.kodejava.jdbc;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;
public class JdbcQueryExample {
// Database connection information
private static final String URL = "jdbc:mysql://localhost/kodejava";
private static final String USERNAME = "kodejava";
private static final String PASSWORD = "s3cr*t";
public static void main(String[] args) {
// Get a connection to database.
try (Connection connection =
DriverManager.getConnection(URL, USERNAME, PASSWORD)) {
// Create a statement object.
Statement statement = connection.createStatement();
// Executes a query command to select isbn and the book title
// from books table. The execute query returns a ResultSet
// object which is the result of our query execution.
String query = "SELECT isbn, title, published_year FROM book";
ResultSet books = statement.executeQuery(query);
// To get the value returned by the statement.executeQuery we
// need to iterate the books object until the last items.
while (books.next()) {
// To get the value from the ResultSet object we can call
// a method that correspond to the data type of the column
// in database table. In the example below we call
// books.getString("isbn") to get the book's ISBN
// information.
System.out.println(books.getString("isbn") + ", " +
books.getString("title") + ", " +
books.getInt("published_year"));
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
Maven Dependencies
<dependency>
<groupId>com.mysql</groupId>
<artifactId>mysql-connector-j</artifactId>
<version>8.4.0</version>
</dependency>
How do I get my screen size?
package org.kodejava.awt;
import java.awt.*;
public class ScreenSizeExample {
public static void main(String[] args) {
Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
System.out.println("Screen Width: " + screenSize.getWidth());
System.out.println("Screen Height: " + screenSize.getHeight());
}
}
The output of the code snippet above:
Screen Width: 2560.0
Screen Height: 1080.0