When to use LinkedHashMap.removeEldestEntry method

removeEldestEntry method exists only on subclasses of LinkedHashMap
removeEldestEntry always invoked after an element is inserted. Based on what this method returns following action will happen if method returns true based on some condition, then the oldest entry will be removed. if method always returns true then basically list will be empty. if method return false, then nothing will be deleted and map will behave like any other LinkedHashMap. after every put or putAll insertion, the eldest element will be removed, no matter what. The JavaDoc shows a very sensible example on how to use it:
Lets see an example of removeEldestEntry.
MapRemoveEntry Example
import java.util.LinkedHashMap;
import java.util.Map;

public class MapRemoveEntry {

	public static void main(String argv[]) {

		LinkedHashMap<Integer, String> map = new LinkedHashMap<Integer, String>() {
			private static final long serialVersionUID = 1L;

			protected boolean removeEldestEntry(Map.Entry<Integer,String> eldest) {
				return size() > 4;
			}
		};;
		

		map.put(0, "A");
		map.put(1, "B");
		map.put(2, "C");
		map.put(3, "D");
		map.put(4, "E");
		
		
		
		map.forEach((k,v) -> { System.out.println("key = " + k + " value = " + v);});
	}
}
If we run the above example we will get following output.
Terminal
Console
key = 1 value = B
key = 2 value = C
key = 3 value = D
key = 4 value = E
Reason is, method removeEldestEntry returns true when the map size is >4. That means when the map size is greater than 4 => the oldest entry will be removed. In this case oldest is the very first entry with (key=0, value=A).

Summary

Using removeEldestEntry we can control when to remove the most oldest entry from the map. This can be used for creating a cache.

About sealed classes and interfaces

Starting Java 17 we can now allow additional class modifiers like sealed, non-sealed etc. A sealed class or interface can be extended or implemented only by those classes and interfaces permitted to do so.
Earlier we had only two options, allow subclasses to inherit the parent class or interface or not at all. In the later case using Final class modifier. Using sealed we can take a more controlled middle path where the Parent Class or Interface can dictate the Sub classes which can inherit from the Class.
Following are the keywords related to sealed classes. sealed : only permitted classes can extend a sealed class/interface permits: used along with sealed and mentions the names of the child classes. non-sealed: if we want a specific class (inherited from a sealed class) to be inheritable we can open it up by marking it non-sealed.

Defining sealed class

Let's define a sealed class Tesla with 3 known subclasses that it can have.

sealed Tesla

Tesla
package java17.sealed;

public abstract sealed class Tesla permits Model3, ModelS, TeslaSUV{
	public abstract Integer maxRange();
	public String basePrice() {
		return "25000 USD";
	}
}

Now start implementing the subclasses possible. We implement Model3 and mark it final which will make the inheritance chance closed at this level.
Model3
//Subclass 1
public final class Model3 extends Tesla {
	@Override
	public Integer maxRange() {
		return 200;
	}
}
ModelS

//Subclass 2
public final class ModelS extends Tesla {
	@Override
	public Integer maxRange() {
		return 400;
	}
}
In this third case we want the TeslaSUV to be extendable so that it can be extended. That's why we will mark it with non-sealed.
TeslaSUV
//Subclass3 (non-sealed)
public non-sealed class TeslaSUV extends Tesla {
	@Override
	public Integer maxRange() {
		// TODO Auto-generated method stub
		return null;
	}
}

Style Two: Using member classes

We can also directly implement possible child classes that we want to allow as member classes without using permit. In the following example we are creating a sealed class BMWCars and three member classes.
BMWCars
package java17.sealed;

public sealed class BMWCars {

	public final class BMW3 extends BMWCars implements ElectricVehicle{

	}
	
	public final class BMWI extends BMWCars implements Vehicle {

	}
	
	public non-sealed class BMWJV extends BMWCars implements Vehicle {

	}

}

Rules for Defining Sealed Classes or Interfaces

Both Class and Interface can be marked with sealed. Inherited child classes can be marked with either final, sealed and non-sealed. Every permitted subclass must directly extend the sealed class. A permitted subclass may be non-sealed so that it is open for extension by unknown subclasses.

Summary

With introduction of sealed classes we can define the known subclasses of an abstract class. In other words now we can enforce extension of classes that is known in compile time. It allows us to have greater control of class proliferation and improve security.

Java record in details with example

With Java 14 we have a new way to represent data intensive objects with less boilerplate codes.
Consider the following object representing Address and containing 5 fields. Address:{ street: city: zip: state: country: }
following record AddressRecord will be able to represent above address information.
AddressRecord
package Java14;

public record AddressRecord(String street, String city, Integer zip, String state, String country) {

}
We can instantiate AddressRecord record instance by passing the parameter values. Also system will generate the getter methods. The JVM will generate the getter methods for us. like, street() city() zip() state() country()
AddressRecordDemo
package Java14;

public class AddressRecordDemo {

	public static void main (String arg[]) {
		
		AddressRecord address1 = new AddressRecord("1044 Main Street", "Livermore", 94550, "CA", "USA");
		
		System.out.println(address1.street());
		System.out.println(address1.city());
		System.out.println(address1.state());
	}
}
Record can be very useful when we want to write DTO objects which is to carry or represent information. Following are some of the properties/rules that record have.
Rules for record

    How to check if a Java object is final

    final modifier can be applied at class level. If a Class is marked final it cannot be extended.
    Sample final Class
    
    final class MyCustomClass {
    	
    }

    Using java.lang.reflect.Modifier

    Using java.lang.reflect.Modifier,, We can check if a class is final though java.lang.reflect.Modifier.isFinal() method. MyCustomClass.class.getModifiers() will provide an integer number representing the class modifier. For example for the above class it prints 16. Now the value returned from the getModifiers() can be decoded with method Modifier.isFinal();
    JavaReflectionModifiers
    import java.lang.reflect.Modifier;
    
    public class JavaReflectionModifiers {
    
    	public static void main(String[] args) {
    		
                     //prints 16
    		System.out.println(MyCustomClass.class.getModifiers());
    		
                     //returns true
    		Modifier.isFinal(MyCustomClass.class.getModifiers());
    
    	}
    
    }
    
    
    final class MyCustomClass {
    	
    }

    More Examples

    Examples
    package Java14;
    
    import java.lang.reflect.Modifier;
    
    public class JavaReflectionModifiers {
    
    	public static void main(String[] args) {
    		
    		System.out.println(MyCustomClass.class.getModifiers());
    		
    		//prints true
    		System.out.println(Modifier.isFinal(MyCustomClass.class.getModifiers()));
    		
    		//prints true
    		System.out.println(Modifier.isFinal(String.class.getModifiers()));
    		
    		//prints false
    		System.out.println(Modifier.isFinal(Number.class.getModifiers()));
    
    	}
    
    }
    
    
    final class MyCustomClass {
    	
    }

    Can Java record be extended?

    Java record introduced in java 14. It provides a way to reduce boiler plate code while defining data classes. record is final implicitly and cannot be extended.
    For example in the following record MobilePhone, we cannot extend it.
    record MobilePhone
    record MobilePhone(String brand, String modelName, Number osVersion, boolean canFlip) {
    
    }
    Java records are implicitly final it can be confirmed by the following code. Which checks if the java.lang.Class for the object have final modifier or not.
    RecordFinalTest
    package Java14;
    
    import java.lang.reflect.Modifier;
    
    public class RecordFinalTest {
    
    	public static void main(String[] args) {
    		
    		MobilePhone phone1 = new MobilePhone("Samsung", "Galaxy1", 1, false);
    
    		//false
    		System.out.println(Modifier.isFinal(phone1.getClass().getModifiers()));
    		
    		//false
    		System.out.println(Modifier.isFinal(MobilePhone.class.getModifiers()));
    
    	}
    
    }
    
    For more details about record in Java please check the following references.

    References

    How to create double linked list in Java

    In Double Linked List we have each node pointing to the previous and next node. Here we will see how to create a custom Double Linked List in java.
    Some properties of linked list is that, it has a node that can contain some value and then each node is linked to next node and also previous node. That's why we can traverse the linked list sequentially given we know the starting (head) node or last (tail) node.
    Custom Linked List
    class MyLinkedList<E> {
    
    	/* first points to head of the list */
    	public Node<E> first = null;
    
    	/* last points to tail of the list */
    	public Node<E> last = null;
    
    	/**
    	 * Add item to tail (end) of the List
    	 * 
    	 * @param item
    	 * @return
    	 */
    	public boolean add(E item) {
    		Node<E> newNode = new Node<E>(last, item, null);
    
    		if (last == null) {
    			// last points to the new node created
    			first = newNode;
    		} else {
    			last.next = newNode;
    		}
    		// update last so that it points to the new node
    		last = newNode;
    		return true;
    	}
    
    	static class Node<E> {
    		public E value;
    		public Node<E> next;
    		public Node<E> prev;
    
    		Node(Node<E> prev, E element, Node<E> next) {
    			this.value = element;
    			this.next = next;
    			this.prev = prev;
    		}
    	}
    
    }
    In the above implementation we have defined the main class MyLinkedList and inside it we defined a generic static class Node. Each Node object can hold a value and has links to previous (prev) and next nodes.
    Create Instance of MyLinkedList
    //create instance
    MyLinkedList<Integer> list = new MyLinkedList<Integer>();
    
    //add values 1,2,3,4,5
    list.add(1);
    list.add(2);
    list.add(3);
    list.add(4);
    list.add(5);
    Print the content of the node
    //Get the first node (head node) and then print it by traversing all nodes
    MyLinkedList.Node<Integer> node = list.first;
    while (node != null) {
    	System.out.println("Content of Node: " + node.value);
    	node = node.next;
    }
    Terminal
    Output
    Content of Node: 1
    Content of Node: 2
    Content of Node: 3
    Content of Node: 4
    Content of Node: 5

    Syntax Highlighting on Webpage including Blogger.com

    Syntax Highlighting on Webpage including Blogger.com
    Prism is a lightweight syntax highlighter, very easy to use and works very well.

    Prism Source

    Like most Java scripts prism can be downloaded and used in any web application or it can also be served from CDN Networks. Some of the CDN Networks are as follows as of writing. https://www.jsdelivr.com/package/npm/prismjs https://cdnjs.com/libraries/prism https://unpkg.com/browse/prismjs@1.29.0/

    Adding Prism JS

    Include JS and CSS for Prism in the Head section of page
    <head>
    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/prismjs@1.29.0/themes/prism.min.css" integrity="sha256-ko4j5rn874LF8dHwW29/xabhh8YBleWfvxb8nQce4Fc=" crossorigin="anonymous">
    <script src="https://cdn.jsdelivr.net/npm/prismjs@1.29.0/prism.min.js"></script>
    <script src="https://cdn.jsdelivr.net/npm/prismjs@1.29.0/plugins/autoloader/prism-autoloader.min.js"></script>
    </head>
    

    Use respective language css class in page

    Example for language java.
    <pre>
    <code class="language-java">
       Java Code here
    </code>
    </pre>

    Example with Java Script

    Example for language javascript.
    <pre>
    <code class="language-javascript">
       Javascript Code here
    </code>
    </pre>
    Basically for the supported language we need to use respective css class (language-LANGUAGE}, for instance in above case we used language-java and language-javascript as css class. Prism support a lot of different languages, the full list can be found in the official prism site.

    About sealed classes and interfaces

    Starting Java 17 we can now allow additional class modifiers like sealed, non-sealed etc. A sealed class or interface can be extended or implemented only by those classes and interfaces permitted to do so. Following are the keywords related to sealed classes.
    New Keywords
      Earlier we had only two options, allow subclasses to inherit the parent class or interface or not at all. In the later case using Final class modifier. Using sealed we can take a more controlled middle path where the Parent Class or Interface can dictate the Sub classes which can inherit from the Class.

      Defining sealed class (Two Styles)

      Style One: Using permit keyword

      using the class modifiers sealed and permits we can create sealed classes. In the example bellow we are Defining abstract sealed class Tesla which permits three known subclasses Model3, ModelS and TeslaSUV.
      Tesla
      package java17.sealed;
      
      public abstract sealed class Tesla permits Model3, ModelS, TeslaSUV{
      	public abstract Integer maxRange();
      	public String basePrice() {
      		return "25000 USD";
      	}
      }
      
      //Subclass 1
      public final class Model3 extends Tesla {
      	@Override
      	public Integer maxRange() {
      		return 200;
      	}
      }
      
      //Subclass 2
      public final class ModelS extends Tesla {
      	@Override
      	public Integer maxRange() {
      		return 400;
      	}
      }
      
      //Subclass3 (non-sealed)
      public non-sealed class TeslaSUV extends Tesla {
      	@Override
      	public Integer maxRange() {
      		// TODO Auto-generated method stub
      		return null;
      	}
      }
      
      

      Style Two: Using member classes

      In the following example we are creating a sealed class BMWCars and three member classes.
      BMWCars
      package java17.sealed;
      
      public sealed class BMWCars {
      
      	public final class BMW3 extends BMWCars implements ElectricVehicle{
      
      	}
      	
      	public final class BMWI extends BMWCars implements Vehicle {
      
      	}
      	
      	public non-sealed class BMWJV extends BMWCars implements Vehicle {
      
      	}
      
      }
      

      Rules for Defining Sealed Classes or Interfaces

      Both Class and Interface can be marked with sealed. Inherited child classes can be marked with either final, sealed and non-sealed. Every permitted subclass must directly extend the sealed class. A permitted subclass may be non-sealed so that it is open for extension by unknown subclasses.

      Summary

      With introduction of sealed classes we can define the known subclasses of an abstract class. In other words now we can enforce extension of classes that is known in compile time. It allows us to have greater control of class proliferation and improve security.

      This article shows how to install Maven (3.6.3) on macOS Monterey(version 12.2.1) with M1 processor.

      Download Maven

      Goto https://maven.apache.org/download.cgi and download the version for instance 
      apache-maven-3.8.7-bin.tar.gz



      Install Maven

      Go to the download location
      Normally /Users/${username}/Downloads and execute the following command

      Terminal
      tar -xvzf apache-maven-4.0.0-alpha-3-bin.tar.gz
      x apache-maven-4.0.0-alpha-3/bin/mvn.cmd
      x apache-maven-4.0.0-alpha-3/bin/mvn
      x apache-maven-4.0.0-alpha-3/README.txt
      x apache-maven-4.0.0-alpha-3/LICENSE
      x apache-maven-4.0.0-alpha-3/NOTICE
      x apache-maven-4.0.0-alpha-3/lib/
      x apache-maven-4.0.0-alpha-3/lib/aopalliance.license
      x apache-maven-4.0.0-alpha-3/lib/commons-cli.license
      x apache-maven-4.0.0-alpha-3/lib/commons-codec.license
      x apache-maven-4.0.0-alpha-3/lib/commons-lang3.license
      x apache-maven-4.0.0-alpha-3/lib/failureaccess.license
      x apache-maven-4.0.0-alpha-3/lib/guava.license
      x apache-maven-4.0.0-alpha-3/lib/guice.license
      x apache-maven-4.0.0-alpha-3/lib/httpclient.license
      x apache-maven-4.0.0-alpha-3/lib/httpcore.license
      
      It will create a folder apache-maven-4.0.0-alpha-3 which will contain unzipped contains from the downloaded file. Now Move the folder apache-maven-4.0.0-alpha-3 to final destination. In my case i moved to a folder DevRuntime which contains other runtimes for my development.
      ~/.zshenv
      export MAVEN_HOME=~/DevRuntime/apache-maven-4.0.0-alpha-3 export PATH=$PATH:$MAVEN_HOME/bin
      Now source the zshenv file
      Terminal
      source ~/.zshenv

      Install Maven

      Now Check that Maven is working
        
      Terminal
      mvn --verison
      Apache Maven 4.0.0-alpha-3 (2ccf57baa5191468f9911fe85fd99672ac3bacb9)
      Maven home: /Users/SiddB/DevRuntime/apache-maven-4.0.0-alpha-3
      Java version: 18.0.1.1, vendor: Oracle Corporation, runtime: /Library/Java/JavaVirtualMachines/jdk-18.0.1.1.jdk/Contents/Home
      Default locale: en_US, platform encoding: UTF-8
      OS name: "mac os x", version: "12.2.1", arch: "aarch64", family: "mac"
      

      How to get Bash version number in Mac OS

      Following command can be used to know the bash version in Mac OS.
      Execute the following command to know the bash version.
      bash --version
      When I execute the above command I get the followings.
      
      GNU bash, version 3.2.57(1)-release (arm64-apple-darwin21)
      Copyright (C) 2007 Free Software Foundation, Inc.
            

      Image showing the command and it's output.



      JUnit 5 Example : Custom Display Name

      In JUnit 5, we can use @DisplayName to declare custom display names for test classes and test methods.
      The custom display name will be visible while debugging or while generating any reports from test execution.

      First Let's see an example of Test Class without using Display Name.

      Example 1 without Display name

      package com.bootng.display;
      
      import org.junit.jupiter.api.DisplayName;
      import org.junit.jupiter.api.Test;
      
      public class DisplayCustomNameTest {
      
      	@Test
      	public void test_if_it_will_rain() {
      	}
      
      	@Test
      	public void test_if_it_will_be_cloudy() {
      	}
      
      	@Test
      	public void test_if_it_will_be_sunny() {
      	}
      
      }
      
      

      Output




      You can see in the above example, once the Test is executed it shows the exact test method name. Now using DisplayName annotation we will be able to override the display name without changing the test method name.

      Display CustomName Example

      package com.bootng.display;
      @DisplayName("Vacation Weather Test")
      public class DisplayCustNameTest {
      	@DisplayName("🌧")
      	@Test
      	public void test_if_it_will_rain() {
      	}
      
      	@DisplayName("🌨")
      	@Test
      	public void test_if_it_will_be_cloudy() {
      	}
      
      	@DisplayName("🌞")
      	@Test
      	public void test_if_it_will_be_sunny() {
      	}
      }
      
      Output


      We can also use a Custom Name generator to generate the test names. 

      As of Version 5.4 it comes with the following generators out of the box.

      Custom Name Generator Example


      DisplayNameGenerator.IndicativeSentences
      DisplayNameGenerator.ReplaceUnderscores
      DisplayNameGenerator.Simple
      DisplayNameGenerator.Standard

      Each of the above generators have different implementations of how to generate test name.  In this article we will see how to use the ReplaceUnderscores. 

      In the Following example we are using ReplaceUnderscores.  As a result the test names will be generated by replacing the "_" from respective names.
      package com.bootng.display;
      
      import org.junit.jupiter.api.DisplayName;
      import org.junit.jupiter.api.Test;
      
      @DisplayName("Weather Test DisplayExample ")
      @DisplayNameGeneration(DisplayNameGenerator.ReplaceUnderscores.class)
      public class DisplayGeneratorExampleTest {
      
      	@Test
      	public void test_if_it_will_rain() {
      	}
      
      	@Test
      	public void test_if_it_will_be_cloudy() {
      	}
      
      	@Test
      	public void test_if_it_will_be_sunny() {
      	}
      }
      

      Output