HashMap computeIfPresent() method in Java with Examples

The computeIfPresent(Key, BiFunction) method of HashMap class is used to update value in a key-value pair in the map. If key does not exist then it does not do any thing.

Syntax

undefined
V computeIfPresent(K key,
            BiFunction<? super K, ? super V, ? extends V> remappingFunction) 

Example

Following is an example of computeIfAbsent invocation and its results.
ComputeIfAbsentExample
package corejava.map;

import java.util.HashMap;
import java.util.Map;

public class ComputeIfPresentExample {

	public static void main (String argv[]) {
		
		Map<String, Integer> map = new HashMap<String, Integer>();

		map.put("Sunglass", 105);
		map.put("Watch", 1501);
		
		
		newline("Original Map");
		map.forEach((a, b) -> {
			System.out.println(a + " -> " + b);
		});
		
		
		map.computeIfPresent("Watch", (k,v) -> {return v + v;});
		
		newline("After calling computeIfAbsent Map");
		map.forEach((a, b) -> {
			System.out.println(a + " -> " + b);
		});
	}
	
	static void newline(String s) {
		System.out.println("\n" + s);
	};
}
Output of above program.
Original Map
Watch -> 1501
Sunglass -> 105

After calling computeIfAbsent Map
Watch -> 3002
Sunglass -> 105

Summary

computeIfPresent helps in writing more compact code and also make code more readable.

HashMap computeIfAbsent() method in Java with Examples

The computeIfAbsent(Key, Function) method of HashMap class is used to enter a new key-value pair to map if the key does not exist. If key exists then it does not do any thing.

Syntax

undefined
public V computeIfAbsent(K key, Function<? super K, ? extends V> remappingFunction)

Example

Following is an example of computeIfAbsent invocation and its results.
ComputeIfAbsentExample
package corejava.map;

import java.util.HashMap;
import java.util.Map;

public class ComputeIfAbsentExample {

	public static void main (String argv[]) {
		
		Map<Integer, String> map = new HashMap<Integer, String>();

		map.put(1, "one");
		map.put(2, "two");
		map.put(3, "three");
		map.put(4, "four");
		map.put(5, "five");
		
		newline("Original Map");
		map.forEach((a, b) -> {
			System.out.println(a + " -> " + b);
		});
		
		map.computeIfAbsent(6, k-> { return "six";});
		map.computeIfAbsent(7, k -> "seven");
		
		newline("After calling computeIfAbsent Map");
		map.forEach((a, b) -> {
			System.out.println(a + " -> " + b);
		});
	}
	
	static void newline(String s) {
		System.out.println("\n" + s);
	};
}
Output of above program.
Original Map
1 -> one
2 -> two
3 -> three
4 -> four
5 -> five

After calling computeIfAbsent Map
1 -> one
2 -> two
3 -> three
4 -> four
5 -> five
6 -> six
7 -> seven

Summary

computeIfAbsent helps in writing more compact code and also make code more readable.

Remove all entries in map by value

Java Map is a data structure that holds key->value pairs. In this article we will see how to delete all entries in the map by map values.

Using removeAll

removeAll() takes one argument a collection. And entries matching the passed collection will be removed from the original map
Using removeAll
Map<Integer, String> map = new HashMap<Integer, String>();
map.put(1, "one");
map.put(2, "two");
map.put(3, "three");
map.put(4, "four");
map.put(5, "five");
map.put(6, "five");

newline("Original Map");
map.values().removeAll(Collections.singleton("five"));

newline("After removing values = five");
map.forEach( (a,b) -> {
	System.out.println(a + " -> " + b);
});
Output of above program.
Original Map
1 -> one
2 -> two
3 -> three
4 -> four
5 -> five
6 -> five
7 -> five

After removing values = five
1 -> one
2 -> two
3 -> three
4 -> four

Using removeIf

removeIf
Map<Integer, String> map = new HashMap<Integer, String>();
map.put(1, "one");
map.put(2, "two");
map.put(3, "three");
map.put(4, "four");
map.put(5, "five");
map.put(6, "five");

newline("Original Map");

//remove entries where value = five
map.values().removeIf("five"::equals);

newline("After removing values = five");
map.forEach( (a,b) -> {
	System.out.println(a + " -> " + b);
});
Output of above program.
Original Map
1 -> one
2 -> two
3 -> three
4 -> four
5 -> five
6 -> five
7 -> five

After removing values = five
1 -> one
2 -> two
3 -> three
4 -> four

Remove all entries from java map

Java Map is a data structure that holds key->value pairs. In this article we will see how to delete all entries in the map in java easily and efficetively.

Method clear()

clear() method is method that does not return any thing (void return type) and is part of the Map interface. Each implementation subclasses provides an implementation. clear () methods deleted all entries and turn them map into empty map.
Clear method
static void clearALLItemsInMap() {
		Map<Integer, String> map = new HashMap<Integer, String>();
		map.put(1, "one");
		map.put(2, "two");
		map.put(3, "three");
		map.put(4, "four");
		map.put(5, "five");
		map.put(6, "five");
		
		newline("Original Map");
		map.forEach( (a,b) -> {
			System.out.println( a + " -> " + b);
		});
		
                 //clear the map
		map.clear();
		
		newline("After classing clear() Map");
		map.forEach( (a,b) -> {
			System.out.println(a + " -> " + b);
		});
	}
Output of above program.
Original Map
1 -> one
2 -> two
3 -> three
4 -> four
5 -> five
6 -> five

After classing clear() Map
empty

Summary

For removing all the entries from java map, clear() method is the best option. Also please note that the clear() method is implemented in each of its implementation classes like, HashMap, TreeMap, LinkedHashMap etc.

When to use Java BigInteger

Java BigInteger class is used to perform arithmetic operation or holding Numbers large enough that Integer cannot hold. In fact BigInteger does not have any limit on the numerical value that it can represent.

java.math.BigInteger

Integer Max and Min value

In Java Integer can hold between 2147483647 and -2147483648. This is denoted by Integer.MAX_VALUE and Integer.MIN_VALUE

Print Value of Integer Max and Min

Print max and min value
System.out.println(Integer.MAX_VALUE); //2147483647
System.out.println(Integer.MIN_VALUE);  //-2147483648
When we have numbers that cannot fit in the Integer range we can use BigInteger class. Also this Class provides a lot of methods to carry out different mathematical operations such as add, subtract, divide, multiply and many more
BigInteger Example
BigInteger a = BigInteger.valueOf(15);
BigInteger b = BigInteger.ONE;

BigInteger result = a.add(b);
System.out.println("Addition of a,b = " + result);

result = a.subtract(b);
System.out.println("Subtration of a,b = " + result);

result = a.divide(b);
System.out.println("Division of a,b = " + result);

result = a.multiply(b);
System.out.println("Multipllication of a,b = " + result);

Generate random number

Following java code will generate a random number between a given range. For instance if we want to generate a random number between 80, 100 we can use the following codes. In java to generate random number we have two options, using class Math or Random. We will see both examples

Using Class java.lang.Math

Math.random() generates a fraction(double) number between 0.0 and 1.0. We then add the lower bound to the product of (upperbound-lowerbound)*random_between0_1.
GenerateRandom1
private static int generateRandomInteger(int rangeStart, int rangeEnd) {
		// generate double between 0.0 and 1.0
		double r = Math.random();
		Integer result = (int) (rangeStart + r * (rangeEnd - rangeStart));
		return result;
	}

Using Class java.util.Random

GenerateRandom2
private static int generateRandomIntegerII(int rangeStart, int rangeEnd) {
		// generate double between 0.0 and 1.0
		Random random = new Random();
		double r = random.nextDouble();
		Integer result = (int) (rangeStart + r * (rangeEnd - rangeStart));
		return result;
}

Full Example

GenerateRandomNumber
import java.util.Random;
public class GenerateRandomNumber {

	public static void main(String[] args) {

		System.out.println(GenerateRandomNumber.generateRandomInteger(35, 40));
		System.out.println(GenerateRandomNumber.generateRandomIntegerII(35, 40));

	}

	private static int generateRandomInteger(int rangeStart, int rangeEnd) {
		// generate double between 0.0 and 1.0
		double r = Math.random();
		Integer result = (int) (rangeStart + r * (rangeEnd - rangeStart));
		return result;
	}

	private static int generateRandomIntegerII(int rangeStart, int rangeEnd) {
		// generate double between 0.0 and 1.0
		Random random = new Random();
		double r = random.nextDouble();
		Integer result = (int) (rangeStart + r * (rangeEnd - rangeStart));
		return result;
	}

}

JDK Module listing

Go to Terminal and execute following command (java --list-modules). It will list all the modules available in the JDK.
Terminal
$java --list-modules

java.base@18.0.1.1
java.compiler@18.0.1.1
java.datatransfer@18.0.1.1
java.desktop@18.0.1.1
java.instrument@18.0.1.1
java.logging@18.0.1.1
java.management@18.0.1.1
java.management.rmi@18.0.1.1
java.naming@18.0.1.1
java.net.http@18.0.1.1
java.prefs@18.0.1.1
java.rmi@18.0.1.1
java.scripting@18.0.1.1
java.se@18.0.1.1
java.security.jgss@18.0.1.1
java.security.sasl@18.0.1.1
java.smartcardio@18.0.1.1
java.sql@18.0.1.1
java.sql.rowset@18.0.1.1
java.transaction.xa@18.0.1.1
java.xml@18.0.1.1
java.xml.crypto@18.0.1.1
jdk.accessibility@18.0.1.1
jdk.attach@18.0.1.1
jdk.charsets@18.0.1.1
jdk.compiler@18.0.1.1
jdk.crypto.cryptoki@18.0.1.1
jdk.crypto.ec@18.0.1.1
jdk.dynalink@18.0.1.1
jdk.editpad@18.0.1.1
jdk.hotspot.agent@18.0.1.1
jdk.httpserver@18.0.1.1
jdk.incubator.foreign@18.0.1.1
jdk.incubator.vector@18.0.1.1
jdk.internal.ed@18.0.1.1
jdk.internal.jvmstat@18.0.1.1
jdk.internal.le@18.0.1.1
jdk.internal.opt@18.0.1.1
jdk.internal.vm.ci@18.0.1.1
jdk.internal.vm.compiler@18.0.1.1
jdk.internal.vm.compiler.management@18.0.1.1
jdk.jartool@18.0.1.1
jdk.javadoc@18.0.1.1
jdk.jcmd@18.0.1.1
jdk.jconsole@18.0.1.1
jdk.jdeps@18.0.1.1
jdk.jdi@18.0.1.1
jdk.jdwp.agent@18.0.1.1
jdk.jfr@18.0.1.1
jdk.jlink@18.0.1.1
jdk.jpackage@18.0.1.1
jdk.jshell@18.0.1.1
jdk.jsobject@18.0.1.1
jdk.jstatd@18.0.1.1
jdk.localedata@18.0.1.1
jdk.management@18.0.1.1
jdk.management.agent@18.0.1.1
jdk.management.jfr@18.0.1.1
jdk.naming.dns@18.0.1.1
jdk.naming.rmi@18.0.1.1
jdk.net@18.0.1.1
jdk.nio.mapmode@18.0.1.1
jdk.random@18.0.1.1
jdk.sctp@18.0.1.1
jdk.security.auth@18.0.1.1
jdk.security.jgss@18.0.1.1
jdk.unsupported@18.0.1.1
jdk.unsupported.desktop@18.0.1.1
jdk.xml.dom@18.0.1.1
jdk.zipfs@18.0.1.1
As we can see in the above command output, we have around 70 modules. All JDK Modules starts with "jdk.*" All Java SE Specifications Modules starts with "java.*"

What is package-info.java used for in Java

package-info.java is a special file which is used for generating package level documentations as well as keeping all package level annotations. For each package we can have one package-info file.
Here will see two different examples of package-info.java.

Package Documentation

For instance in a project, I have the following package-info.java for package javaexp.corejava.collection.
package-info.java
/**
 * Tutorials and Samples form Java  Collection framework.
 */

package javaexp.corejava.collection;
If we see the generated Javadocs we will notice the package level comments as well.





Package Level Annotation

We have the following entry, to mark the whole package deprecated.
package-info.java
@java.lang.Deprecated
package javaexp.corejava.oldcollection;
If we see the generated Javadoc we will see the package is marked as deprecated.



Summary

All though package-info.java is not must for java project, but it do add a lot of value specially if the code base is large and we want to generate package level documentations or say apply Annotations to all the classes in the packge.

Java @Deprecated Annotation

@Deprecated is an annotation to mark method class or package as deprecated. We mark something deprecated to let developers know that the deprecated item can be removed in future.

Where @Deprecated can be used

  • Method can be marked @Deprecated.
  • Class can be marked with @Deprecated.
  • Package can be also marked deprecated with @Deprecated.

Effect of marking with @Deprecated

When we mark a method or class say @Deprecated then the IDE will mark the method with special style to alert about the deprecations. Also the generated Javadocs will covers deprecated items.



Example

OldCalculator with @Deprecated
/**
 * Sample Calculator which can add two numbers
 * 
 */

public class OldCalculator {

	/**
	 * Add two number and return the value as Number
	 * 
	 * @param a : First number
	 * @param b : Second number
	 * @return result of addition as Number
	 * @see Number
	 */
	public Number add(Number a, Number b) {
		return a.doubleValue() + b.doubleValue();
	}

	@Deprecated(forRemoval = false, since = "1.1")
	public Number add(CalParameter a, CalParameter b) {
		return a.val.doubleValue() + b.val.doubleValue();
	}

}

@Deprecated
class CalParameter {
	Integer val;
}
In the above example class CalParameter is marked as @Deprecated. Where as method add is marked @Deprecated.

Documentation of @Deprecated

Following image shows Javadocs generated for a project, where some method, class and package are marked @Deprecated


When to use @Deprecated

When we have a better implementation of a method and want to let other developers use the new method rather then old method, we can mark old method @Deprecated. While refactoring code and marking @Deprecated to yet to be refactored code makes refactoring effective and easier.

Create Java project using maven

Here we will create a project using maven template and import the project in eclipse.
Technology used
  • Java 18.0.1.1 2022-04-22
  • Apache Maven 4.0.0-alpha-3
  • OS: MacOS Monterey (M1)
  • Eclipse IDE

Create project

Using maven we will generate the project using archetype = maven-archetype-quickstart. archetype is predefined templates. And then we will generate the eclipse related config files so that the generate project can be imported in eclipse.
Console : Command
mvn archetype:generate -DgroupId=com.project -DartifactId=java-module-project -DarchetypeArtifactId=maven-archetype-quickstart -DinteractiveMode=false  
Console : Output
$ mvn archetype:generate -DgroupId=com.project -DartifactId=java-module-project -DarchetypeArtifactId=maven-archetype-quickstart -DinteractiveMode=false
[INFO] Scanning for projects...
[INFO] 
[INFO] -----------------------------------------< org.apache.maven:standalone-pom >-----------------------------------------
[INFO] Building Maven Stub Project (No POM) 1
[INFO] -------------------------------------------------------[ pom ]-------------------------------------------------------
[INFO] 
[INFO] --- archetype:3.2.1:generate (default-cli) @ standalone-pom ---
[INFO] Generating project in Batch mode
[INFO] ----------------------------------------------------------------------------
[INFO] Using following parameters for creating project from Old (1.x) Archetype: maven-archetype-quickstart:1.0
[INFO] ----------------------------------------------------------------------------
[INFO] Parameter: basedir, Value: /Users/sbhome
[INFO] Parameter: package, Value: com.project
[INFO] Parameter: groupId, Value: com.project
[INFO] Parameter: artifactId, Value: java-module-project
[INFO] Parameter: packageName, Value: com.project
[INFO] Parameter: version, Value: 1.0-SNAPSHOT
[INFO] project created from Old (1.x) Archetype in dir: /Users/sbhome/java-module-project
[INFO] ---------------------------------------------------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ---------------------------------------------------------------------------------------------------------------------
[INFO] Total time:  6.667 s
[INFO] Finished at: 2023-02-07T22:46:00-08:00
[INFO] ---------------------------------------------------------------------------------------------------------------------
Terminal:: $ 
After the maven command execution is complete it will create the project folder java-module-project. Next move inside the folder (cd java-module-project)
Console : Command
cd java-module-project

Create Eclipse Project

Console : Command
mvn eclipse:eclipse
Console : Output
[INFO] Scanning for projects...
[INFO] 
[INFO] -----------------------------------------< com.project:java-module-project >-----------------------------------------
[INFO] Building java-module-project 1.0-SNAPSHOT
[INFO]   from pom.xml
[INFO] -------------------------------------------------------[ jar ]-------------------------------------------------------
[INFO] 
[INFO] --- eclipse:2.10:eclipse (default-cli) @ java-module-project ---
[WARNING] The POM for org.apache.maven.wagon:wagon-http-lightweight:jar:1.0-beta-6 is invalid, transitive dependencies (if any) will not be available, enable verbose output (-X) for more details
[WARNING] The POM for org.apache.maven.wagon:wagon-http:jar:1.0-beta-6 is invalid, transitive dependencies (if any) will not be available, enable verbose output (-X) for more details
[WARNING] The POM for org.apache.maven.wagon:wagon-webdav-jackrabbit:jar:1.0-beta-6 is invalid, transitive dependencies (if any) will not be available, enable verbose output (-X) for more details
[WARNING] The POM for org.apache.maven.wagon:wagon-http-lightweight:jar:1.0-beta-2 is invalid, transitive dependencies (if any) will not be available, enable verbose output (-X) for more details
[INFO] Using Eclipse Workspace: null
[INFO] Adding default classpath container: org.eclipse.jdt.launching.JRE_CONTAINER
[INFO] Not writing settings - defaults suffice
[INFO] Wrote Eclipse project for "java-module-project" to /Users/sbhome/java-module-project.
[INFO] 
[INFO] ---------------------------------------------------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ---------------------------------------------------------------------------------------------------------------------
[INFO] Total time:  0.494 s
[INFO] Finished at: 2023-02-07T22:51:03-08:00
[INFO] ---------------------------------------------------------------------------------------------------------------------

Import Project in Eclipse



Project in Eclipse




POM file


pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.project</groupId>
  <artifactId>java-module-project</artifactId>
  <packaging>jar</packaging>
  <version>1.0-SNAPSHOT</version>
  <name>java-module-project</name>
  <url>http://maven.apache.org</url>
  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>3.8.1</version>
      <scope>test</scope>
    </dependency>
  </dependencies>
</project>

Build Project

Console : Command
mvn install
Console : Output
[INFO]  T E S T S
[INFO] -------------------------------------------------------
[INFO] Running com.project.AppTest
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.006 s - in com.project.AppTest
[INFO] 
[INFO] Results:
[INFO] 
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
[INFO] 
[INFO] 
[INFO] --- jar:3.2.0:jar (default-jar) @ java-module-project ---
[INFO] Building jar: java-module-project-1.0-SNAPSHOT.jar
[INFO] 
[INFO] --- install:3.0.0-M1:install (default-install) @ java-module-project ---
[INFO] 
[INFO] ---------------------------------------------------------------------------------------------------------------------
[INFO] BUILD SUCCESS