java 8

How to convert String to Integer in Java

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In Java, we can use Integer.valueOf(String) to convert a String to an Integer object; For unparsable String, it throws NumberFormatException. Integer.valueOf("1"); // ok Integer.valueOf("+1"); // ok, result = 1 Integer.valueOf("-1"); // ok, result = -1 Integer.valueOf("100"); // ok Integer.valueOf(" 1"); // NumberFormatException (contains space) Integer.valueOf("1 "); // NumberFormatException (contains space) Integer.valueOf("2147483648"); // NumberFormatException (Integer max …

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Java – How to remove items from a List while iterating?

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In Java, if we remove items from a List while iterating it, it will throw java.util.ConcurrentModificationException. This article shows a few ways to solve it. Table of contents 1. java.util.ConcurrentModificationException 2. Java 8 Collection#removeIf 2.1 removeIf examples 2.2 removeIf uses Iterator 3. ListIterator example 4. Filter and Collect example 5. References P.S Tested with Java …

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Mocking Spring Data DateTimeProvider

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In this article, we will demonstrate an integration test where we have to persist the entities with mocked auditing date fields when JPA Auditing is enabled. Technologies used: Spring Boot 2.4.0 Spring 5.3.1 JUnit Jupiter 5.7.0 Tomcat embed 9.0.39 Java 8 1. Project Dependencies pom.xml <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-data-jpa</artifactId> </dependency> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-web</artifactId> </dependency> <dependency> <groupId>com.h2database</groupId> …

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Where is the java.security file?

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In Java, we can find the java.security file at the following location: $JAVA_HOME/jre/lib/security/java.security $JAVA_HOME/conf/security For Java 8, and early version, we can find the java.security file at $JAVA_HOME/jre/lib/security/java.security. Terminal $ /usr/lib/jvm/java-8-openjdk-amd64/jre/lib/security$ ls -lsah total 12K 4.0K drwxr-xr-x 3 root root 4.0K Mei 12 11:53 . 4.0K drwxr-xr-x 8 root root 4.0K Mei 12 11:53 .. …

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Java – How to convert a byte to a binary string

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In Java, we can use Integer.toBinaryString(int) to convert a byte to a binary string representative. Review the Integer.toBinaryString(int) method signature, it takes an integer as argument and returns a String. Integer.java public final class Integer extends Number implements Comparable<Integer>, Constable, ConstantDesc { public static String toBinaryString(int i) { return toUnsignedString0(i, 1); } //… } If …

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Java – Convert negative binary to Integer

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Review the following Java example to convert a negative integer in binary string back to an integer type. String binary = Integer.toBinaryString(-1); // convert -1 to binary // 11111111 11111111 11111111 11111111 (two’s complement) int number = Integer.parseInt(binary, 2); // convert negative binary back to integer System.out.println(number); // output ?? The result is NumberFormatException! Exception …

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Java 8 – How to convert Iterator to Stream

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In Java 8, we can use StreamSupport.stream to convert an Iterator into a Stream. // Iterator -> Spliterators -> Stream Stream<String> stream = StreamSupport.stream( Spliterators.spliteratorUnknownSize( iterator, Spliterator.ORDERED) , false); Review the StreamSupport.stream method signature, it accepts a Spliterator. StreamSupport.java public static <T> Stream<T> stream(Spliterator<T> spliterator, boolean parallel) { Objects.requireNonNull(spliterator); return new ReferencePipeline.Head<>(spliterator, StreamOpFlag.fromCharacteristics(spliterator), parallel); } …

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Java 8 – Find duplicate elements in a Stream

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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 …

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Java 8 – Get the last element of a Stream?

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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); } …

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Is Comparator a function interface, but it has two abstract methods?

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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 …

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Java 8 method references, double colon (::) operator

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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", …

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Java 8 Supplier Examples

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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 …

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Java 8 UnaryOperator Examples

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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 …

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Java 8 BinaryOperator Examples

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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 …

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Java 8 – How to convert IntStream to int or int[]

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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 …

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Java – Check if the date is older than 30 days or 6 months

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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 …

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Java 8 – How to Sum BigDecimal using Stream?

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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 = …

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Java 8 – Convert Optional<String> to String

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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[] …

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Java 8 forEach print with Index

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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 = …

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Java 8 BiConsumer Examples

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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 = …

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Java 8 Consumer Examples

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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. …

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Java 8 – How to calculate days between two dates?

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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 …

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Java 8 – Difference between two LocalDate or LocalDateTime

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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[] …

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Java 8 BiPredicate Examples

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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 …

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Java 8 – Convert Epoch time milliseconds to LocalDate or LocalDateTime

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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 …

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Java 8 Predicate Examples

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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 …

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Java 8 Stream findFirst() and findAny()

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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 = …

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Java 8 – Convert LocalDate and LocalDateTime to Date

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A Java example to convert Java 8 java.time.LocalDate and java.time.LocalDateTime back to the classic java.uti.Date. JavaDateExample.java package com.mkyong.time; import java.time.LocalDate; import java.time.LocalDateTime; import java.time.ZoneId; import java.time.ZonedDateTime; import java.util.Date; public class JavaDateExample { public static void main(String[] args) { // LocalDate -> Date LocalDate localDate = LocalDate.of(2020, 2, 20); Date date = Date.from(localDate.atStartOfDay(ZoneId.systemDefault()).toInstant()); // LocalDateTime -> …

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Java 8 – How to parse date with "dd MMM" (02 Jan), without year?

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This example shows you how to parse a date (02 Jan) without a year specified. JavaDateExample.java package com.mkyong.time; import java.time.LocalDate; import java.time.format.DateTimeFormatter; import java.util.Locale; public class JavaDateExample { public static void main(String[] args) { DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd MMM", Locale.US); String date = "02 Jan"; LocalDate localDate = LocalDate.parse(date, formatter); System.out.println(localDate); System.out.println(formatter.format(localDate)); } } Output …

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Java 8 – Unable to obtain LocalDateTime from TemporalAccessor

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An example of converting a String to LocalDateTime, but it prompts the following errors: Java8Example.java package com.mkyong.demo; import java.time.LocalDateTime; import java.time.format.DateTimeFormatter; import java.util.Locale; public class Java8Example { public static void main(String[] args) { String str = "31-Aug-2020"; DateTimeFormatter dtf = DateTimeFormatter.ofPattern("dd-MMM-yyyy", Locale.US); LocalDateTime localDateTime = LocalDateTime.parse(str, dtf); System.out.println(localDateTime); } } Output Exception in thread "main" …

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Java – How to print a name 10 times?

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This article shows you different ways to print a name ten times. 1. Looping 1.1 For loop JavaSample1.java package com.mkyong.samples; public class JavaSample1 { public static void main(String[] args) { for (int i = 0; i < 10; i++) { System.out.println("Java "); } } } Output Java Java Java Java Java Java Java Java Java …

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Java – How to search a string in a List?

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In Java, we can combine a normal loop and .contains(), .startsWith() or .matches() to search for a string in ArrayList. JavaExample1.java package com.mkyong.test; import java.util.ArrayList; import java.util.List; public class JavaExample1 { public static void main(String[] args) { List<String> list = new ArrayList<>(); list.add("Java"); list.add("Kotlin"); list.add("Clojure"); list.add("Groovy"); list.add("Scala"); List<String> result = new ArrayList<>(); for (String s …

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Java – How to get keys and values from Map

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In Java, we can get the keys and values via map.entrySet() Map<String, String> map = new HashMap<>(); // Get keys and values for (Map.Entry<String, String> entry : map.entrySet()) { String k = entry.getKey(); String v = entry.getValue(); System.out.println("Key: " + k + ", Value: " + v); } // Java 8 map.forEach((k, v) -> { …

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