Typically, we use Arrays.asList or the new Java 9 List.of to create a List. However, both methods return a fixed size or immutable List, it means we can’t modify it, else it throws UnsupportedOperationException. JavaListExample.java package com.mkyong; import java.util.Arrays; import java.util.List; public class JavaListExample { public static void main(String[] args) { // immutable list, cant […]
The java.util.Arrays$ArrayList is a nested class inside the Arrays class. It is a fixed size or immutable list backed by an array. Arrays.java public static <T> List<T> asList(T… a) { return new ArrayList<>(a); } /** * @serial include */ private static class ArrayList<E> extends AbstractList<E> implements RandomAccess, java.io.Serializable { private static final long serialVersionUID = […]
This article shows you three algorithms to find duplicate elements in a Stream. Set.add() Collectors.groupingBy Collections.frequency At the end of the article, we use the JMH benchmark to test which one is the fastest algorithm. 1. Filter & Set.add() The Set.add() returns false if the element was already in the set; let see the benchmark […]
In Java 8, we can use reduce or skip to get the last element of a Stream. 1. Stream.reduce Java8Example1.java package com.mkyong; import java.util.Arrays; import java.util.List; public class Java8Example1 { public static void main(String[] args) { List<String> list = Arrays.asList("node", "java", "c++", "react", "javascript"); String result = list.stream().reduce((first, second) -> second).orElse("no last element"); System.out.println(result); } […]
Review the Comparator class; it has two abstract methods, why it can be a function interface? Comparator.java package java.util; @FunctionalInterface public interface Comparator<T> { // abstract method int compare(T o1, T o2); // abstract method boolean equals(Object obj); // few default and static methods } Definition of function interface Conceptually, a functional interface has exactly […]
This example shows you how to display a percentage % in String.format. JavaStringFormat1.java package com.mkyong; public class JavaStringFormat1 { public static void main(String[] args) { String result = String.format("%d%", 100); System.out.println(result); } } Output Exception in thread "main" java.util.UnknownFormatConversionException: Conversion = ‘%’ at java.base/java.util.Formatter.checkText(Formatter.java:2732) at java.base/java.util.Formatter.parse(Formatter.java:2718) at java.base/java.util.Formatter.format(Formatter.java:2655) at java.base/java.util.Formatter.format(Formatter.java:2609) at java.base/java.lang.String.format(String.java:2897) Solution To display […]
This article shows you how to use the JDK1.5 String.format() and Apache Common Lang to left or right pad a String in Java. 1. String.format By default, the String.format() fills extra with spaces \u0020. Normally, we use replace() to pad with other chars, but it will replace the spaces in between the given string. JavaPadString1.java […]
This article shows you how to format a string in Java, via String.format(). Here is the summary. Conversion Category Description %b, %B general true of false %h, %H general hash code value of the object %s, %S general string %c, %C character unicode character %d integral decimal integer %o integral octal integer, base 8 %x, […]
Both remainder and modulo are two similar operations; they act the same when the numbers are positive but much differently when the numbers are negative. In Java, we can use Math.floorMod() to describe a modulo (or modulus) operation and % operator for the remainder operation. See the result: | rem & +divisor| rem & -divisor […]
In Java 8, the double colon (::) operator is called method references. Refer to the following examples: Anonymous class to print a list. List<String> list = Arrays.asList("node", "java", "python", "ruby"); list.forEach(new Consumer<String>() { // anonymous class @Override public void accept(String str) { System.out.println(str); } }); Anonymous class -> Lambda expressions. List<String> list = Arrays.asList("node", "java", […]
On Ubuntu, we can add JAVA_HOME environment variable in /etc/environment file. Note /etc/environment system-wide environment variable settings, which means all users use it. It is not a script file, but rather consists of assignment expressions, one per line. We need admin or sudo to modify this it. Further Reading Ubuntu – EnvironmentVariables 1. JAVA_HOME 1.1 […]
In Java, byte is an 8-bit signed (positive and negative) data type, values from -128 (-2^7) to 127 (2^7-1). For unsigned byte, the allowed values are from 0 to 255. Java doesn’t have unsigned bytes (0 to 255). To make an unsigned byte, we can cast the byte into an int and mask (bitwise and) […]
In Java 8, Supplier is a functional interface; it takes no arguments and returns a result. Supplier.java @FunctionalInterface public interface Supplier<T> { T get(); } 1. Supplier 1.1 This example uses Supplier to return a current date-time. Java8Supplier1.java package com.mkyong; import java.time.LocalDateTime; import java.time.format.DateTimeFormatter; import java.util.function.Supplier; public class Java8Supplier1 { private static final DateTimeFormatter dtf […]
In Java 8, UnaryOperator is a functional interface and it extends Function. The UnaryOperator takes one argument, and returns a result of the same type of its arguments. UnaryOperator.java @FunctionalInterface public interface UnaryOperator<T> extends Function<T, T> { } The Function takes one argument of any type and returns a result of any type. Function.java @FunctionalInterface […]
This article shows how to install Java JDK on macOS, Homebrew package manager, manual installation, and switch between different JDK versions. Tested with macOS 11 Big Sur Homebrew 2.7.4 JDK 8, 14, 16, 16 (AdoptOpenJDK and OpenJDK) Topics Homebrew install latest Java (OpenJDK) on macOS Homebrew install Java 8 (OpenJDK) on macOS Homebrew install a […]
In Java 8, BinaryOperator is a functional interface and it extends BiFunction. The BinaryOperator takes two arguments of the same type and returns a result of the same type of its arguments. BinaryOperator.java @FunctionalInterface public interface BinaryOperator<T> extends BiFunction<T,T,T> { } The BiFunction takes two arguments of any type, and returns a result of any […]
Few Java 8 examples to get a primitive int from an IntStream. 1. IntStream -> int Java8Example1.java package com.mkyong; import java.util.Arrays; import java.util.OptionalInt; import java.util.stream.IntStream; public class Java8Example1 { public static void main(String[] args) { int[] num = {1, 2, 3, 4, 5}; //1. int[] -> IntStream IntStream stream = Arrays.stream(num); // 2. OptionalInt OptionalInt […]
In Java 8, BiFunction is a functional interface; it takes two arguments and returns an object. BiFunction.java @FunctionalInterface public interface BiFunction<T, U, R> { R apply(T t, U u); } T – Type of the first argument to the function. U – Type of the second argument to the function. R – Type of the […]
In Java 8, Function is a functional interface; it takes an argument (object of type T) and returns an object (object of type R). The argument and output can be a different type. Function.java @FunctionalInterface public interface Function<T, R> { R apply(T t); } T – Type of the input to the function. R – […]
This article shows Java 8 and legacy date-time APIs example to check if a date is 30 days or 6 months older than the current date. 1. Java 8 isBefore() 2. Java 8 ChronoUnit.{UNIT}.between() 3. Java 8 Period.between() 4. Legacy Calendar and Date 5. References 1. Java 8 isBefore() First minus the current date and […]
In Java 8, we can use the Stream.reduce() to sum a list of BigDecimal. 1. Stream.reduce() Java example to sum a list of BigDecimal values, using a normal for loop and a stream.reduce(). JavaBigDecimal.java package com.mkyong; import java.math.BigDecimal; import java.util.LinkedList; import java.util.List; public class JavaBigDecimal { public static void main(String[] args) { List<BigDecimal> invoices = […]
In Java 8, the Stream.reduce() combine elements of a stream and produces a single value. A simple sum operation using a for loop. int[] numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; int sum = 0; for (int i : numbers) { sum += i; } System.out.println("sum : " + sum); […]
In Java 8, we can use .map(Object::toString) to convert an Optional<String> to a String. String result = list.stream() .filter(x -> x.length() == 1) .findFirst() // returns Optional .map(Object::toString) .orElse(""); Samples A standard Optional way to get a value. Java8Example1.java package com.mkyong; import java.util.Arrays; import java.util.List; import java.util.Optional; public class Java8Example1 { public static void main(String[] […]
In Docker, we cannot remove a running container, stop it first. Stop all running containers. Terminal $ sudo docker stop $(sudo docker ps -aq) Remove all stopped containers. Terminal $ sudo docker rm $(sudo docker ps -aq) 1. Stop Container 1.1 List all containers sudo docker ps Terminal $ sudo docker ps CONTAINER ID IMAGE […]
This article shows how to parse JSON Array using Gson. Table of contents: 1. Download Google Gson 2. Parse JSON Array using Gson 3. Parse JSON Array of Object Array using Gson 4. JSON array contains array of object 5. Unusual JSON array contains array of string 6. Download Source Code 7. References P.S Tested […]
In this tutorial, we will show you how to enable SSL (HTTPS) support for a Spring Boot web application (mvc + thymeleaf). Tested with Maven 3 Java 8 Spring Boot 2.2.4.RELEASE Spring Boot default embedded Tomcat 9 Self-signed certificate (PKCS12) To enable SSL or HTTPS for Spring Boot web application, puts the certificate file .p12 […]
For the SSL certificate, Java doesn’t understand PEM format, and it supports JKS or PKCS#12. This article shows you how to use OpenSSL to convert the existing pem file and its private key into a single PKCS#12 or .p12 file. Solution Convert cert.pem and private key key.pem into a single cert.p12 file, key in the […]
In Spring Boot 2.x, we can create a ServletWebServerFactory to redirect a port from HTTP 8080 to HTTPS 8443. Access localhost:8080, it will redirect to localhost:8443 1. ServletWebServerFactory For Spring Boot 2.x, try this: pom.xml <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-web</artifactId> </dependency> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-thymeleaf</artifactId> </dependency> StartApplication.java package com.mkyong; import org.apache.catalina.Context; import org.apache.catalina.connector.Connector; import org.apache.tomcat.util.descriptor.web.SecurityCollection; import org.apache.tomcat.util.descriptor.web.SecurityConstraint; import […]
In this tutorial, we will show you how to Dockerize a Spring Boot web application (mvc + thymeleaf). Tested with Docker 19.03 Ubuntu 19 Java 8 or Java 11 Spring Boot 2.2.4.RELEASE Maven At the end of the article, we will create a Spring Boot MVC web application and run inside a docker container. P.S […]
In Java, we can use LinkedHashMap to keep the insertion order. P.S HashMap does not guarantee insertion order. 1. HashMap Generate a HashMap, UUID as key, index 0, 1, 2, 3, 4… as value. JavaHashMap.java package com.mkyong.samples; import java.util.HashMap; import java.util.Map; import java.util.UUID; import java.util.stream.IntStream; public class JavaHashMap { public static void main(String[] args) { […]
A simple Java 8 tip to print the Array or List with index in the front. 1. Array with Index Generate the index with IntStream.range. JavaListWithIndex.java package com.mkyong.java8; import java.util.List; import java.util.stream.Collectors; import java.util.stream.IntStream; public class JavaArrayWithIndex { public static void main(String[] args) { String[] names = {"Java", "Node", "JavaScript", "Rust", "Go"}; List<String> collect = […]
In Java 8, BiConsumer is a functional interface; it takes two arguments and returns nothing. @FunctionalInterface public interface BiConsumer<T, U> { void accept(T t, U u); } Further Reading – Java 8 Consumer Examples 1. BiConsumer JavaBiConsumer1.java package com.mkyong.java8; import java.util.function.Consumer; public class JavaBiConsumer1 { public static void main(String[] args) { BiConsumer<Integer, Integer> addTwo = […]
In Java 8, Consumer is a functional interface; it takes an argument and returns nothing. @FunctionalInterface public interface Consumer<T> { void accept(T t); } 1. Consumer Java8Consumer1.java package com.mkyong.java8; import java.util.function.Consumer; public class Java8Consumer1 { public static void main(String[] args) { Consumer<String> print = x -> System.out.println(x); print.accept("java"); // java } } Output java 2. […]
In Java 8, we can use ChronoUnit.DAYS.between(from, to) to calculate days between two dates. 1. LocalDate JavaBetweenDays1.java package com.mkyong.java8; import java.time.LocalDate; import java.time.temporal.ChronoUnit; public class JavaBetweenDays1 { public static void main(String[] args) { LocalDate from = LocalDate.now(); LocalDate to = from.plusDays(10); long result = ChronoUnit.DAYS.between(from, to); System.out.println(result); // 10 } } Output 10 2. LocalDateTime […]
The below program is a typical Java trap, and also a popular Java interview question. It has no compiler error or warning but running very slow. Can you spot the problem? JavaSum.java package com.mkyong; import java.time.Duration; import java.time.Instant; public class JavaSum { public static void main(String[] args) { Instant start = Instant.now(); Long total = […]
In Java 8, we can use Period, Duration or ChronoUnit to calculate the difference between two LocalDate or LocaldateTime. Period to calculate the difference between two LocalDate. Duration to calculate the difference between two LocalDateTime. ChronoUnit for everything. 1. Period JavaLocalDate.java package com.mkyong.java8; import java.time.LocalDate; import java.time.Period; public class JavaLocalDate { public static void main(String[] […]
In Java 8, BiPredicate is a functional interface, which accepts two arguments and returns a boolean, basically this BiPredicate is same with the Predicate, instead, it takes 2 arguments for the test. @FunctionalInterface public interface BiPredicate<T, U> { boolean test(T t, U u); } Further Reading Java 8 Predicate Examples 1. BiPredicate Hello World. If […]
In Java 8, we can use Instant.ofEpochMilli().atZone() to convert the epoch time in milliseconds back to LocalDate or LocalDateTime Epoch time to LocalDate LocalDate ld = Instant.ofEpochMilli(epoch).atZone(ZoneId.systemDefault()).toLocalDate(); Epoch time to LocalDateTime LocalDateTime ldt = Instant.ofEpochMilli(epoch).atZone(ZoneId.systemDefault()).toLocalDateTime(); P.S Epoch time is the number of seconds that have elapsed since 0:00:00 UTC on 1 January 1970 1. Epoch […]
In Java 8, Predicate is a functional interface, which accepts an argument and returns a boolean. Usually, it used to apply in a filter for a collection of objects. @FunctionalInterface public interface Predicate<T> { boolean test(T t); } Further Reading Java 8 BiPredicate Examples 1. Predicate in filter() filter() accepts predicate as argument. Java8Predicate.java package […]
In Java 8 Stream, the findFirst() returns the first element from a Stream, while findAny() returns any element from a Stream. 1. findFirst() 1.1 Find the first element from a Stream of Integers. Java8FindFirstExample1.java package com.mkyong.java8; import java.util.Arrays; import java.util.List; import java.util.Optional; public class Java8FindFirstExample1 { public static void main(String[] args) { List<Integer> list = […]
