FastJson provides easily APIs to convert Java objects to / from JSON JSON.toJSONString – Java objects to JSON JSON.parseObject – JSON to Java objects JSON.parseArray – JSON array to List of Java objects Note You may have interest to read this How to parse JSON with Jackson Overall, the FastJson is really simple and easy […]
Here’s the Java password generator to generate a secure password that consists of two lowercase chars, two uppercase chars, two digits, two special chars, and pad the rest with random chars until it reaches the length of 20 characters. Secure password requirements: Password must contain at least two digits [0-9]. Password must contain at least […]
Few Java examples to show you how to generate a random alphanumeric String, with a fixed length. 1. Random [a-ZA-Z0-9] 1.1 Generate a random alphanumeric String [a-ZA-Z0-9], with a length of 8. RandomExample.java package com.mkyong; import java.security.SecureRandom; public class RandomExample { private static final String CHAR_LOWER = "abcdefghijklmnopqrstuvwxyz"; private static final String CHAR_UPPER = CHAR_LOWER.toUpperCase(); […]
In Java, we can use Double.parseDouble() to convert a String to double 1. Double.parseDouble() Example to convert a String 3.142 to an primitive double. String str = "3.142"; double pi = Double.parseDouble(str); System.out.println(pi); Output 3.142 2. Double.valueOf() Example to convert a String 3.142 to an Double object. String str = "3.142"; Double pi = Double.valueOf(str); […]
In Java, we can use new ArrayList(Arrays.asList(array)) to convert an Array into an ArrayList ArrayExample1.java package com.mkyong; import java.util.ArrayList; import java.util.Arrays; import java.util.List; public class ArrayExample1 { public static void main(String[] args) { String[] str = {"A", "B", "C"}; List<String> list = new ArrayList<>(Arrays.asList(str)); list.add("D"); list.forEach(x -> System.out.println(x)); } } Output A B C D […]
In this tutorial, we will show you how to parse JSON with JSON.simple Note You may have interest to read – How to parse JSON with Jackson or Gson JSON.simple short history This project was formerly JSON.simple 1.x from a Google code project by Yidong, now maintaining by Clifton Labs, read this JSON.simple history P.S […]
Few Java examples to show you how to check if a String is numeric. 1. Character.isDigit() Convert a String into a char array and check it with Character.isDigit() NumericExample.java package com.mkyong; public class NumericExample { public static void main(String[] args) { System.out.println(isNumeric("")); // false System.out.println(isNumeric(" ")); // false System.out.println(isNumeric(null)); // false System.out.println(isNumeric("1,200")); // false System.out.println(isNumeric("1")); […]
In Java, we can use (str != null && !str.isEmpty()) to make sure the String is not empty or null. StringNotEmpty.java package com.mkyong; public class StringNotEmpty { public static void main(String[] args) { System.out.println(notEmpty("")); // false System.out.println(notEmpty(null)); // false System.out.println(notEmpty("hello")); // true System.out.println(notEmpty(" ")); // true, a space… } private static boolean notEmpty(String str) { […]
In the old days, we can use list.toArray(new String[0]) to convert a ArrayList<String> into a String[] StringArrayExample.java package com.mkyong; import java.util.ArrayList; import java.util.List; public class StringArrayExample { public static void main(String[] args) { List<String> list = new ArrayList<>(); list.add("A"); list.add("B"); list.add("C"); // default, returns Object[], not what we want, // Object[] objects = list.toArray(); // […]
Start a Java class and hits this java.lang.UnsupportedClassVersionError, what is class file version 52 56? Error: A JNI error has occurred, please check your installation and try again Exception in thread "main" java.lang.UnsupportedClassVersionError: TestHello has been compiled by a more recent version of the Java Runtime (class file version 56.0), this version of the Java […]
In Java, we can use java.util.Scanner to get user input from console. 1. Scanner 1.1 Read a line. UserInputExample1.java package com.mkyong; import java.util.Scanner; public class UserInputExample1 { public static void main(String[] args) { // auto close scanner try (Scanner scanner = new Scanner(System.in)) { System.out.print("Enter something : "); String input = scanner.nextLine(); // Read user […]
A Java 2d array example. ArrayExample.java package com.mkyong; public class ArrayExample { public static void main(String[] args) { int[][] num2d = new int[2][3]; num2d[0][0] = 1; num2d[0][1] = 2; num2d[0][2] = 3; num2d[1][0] = 10; num2d[1][1] = 20; num2d[1][2] = 30; //or init 2d array like this : int[][] num2dInit = { {1, 2, 3}, […]
A simple List.add() and hits the following java.lang.UnsupportedOperationException ListExample.java package com.mkyong; import java.util.Arrays; import java.util.List; public class ListExample { public static void main(String[] args) { List<String> str = Arrays.asList("A", "B", "C"); str.add("D"); System.out.println(str); } } Output Exception in thread "main" java.lang.UnsupportedOperationException at java.base/java.util.AbstractList.add(AbstractList.java:153) at java.base/java.util.AbstractList.add(AbstractList.java:111) at com.mkyong.ListExample.main(ListExample.java:14) Solution The Arrays.asList returns a fixed-size list, modify […]
In Java, for primitive types, parameters are pass-by-value; For object types, object reference is pass-by-value, however, Java is allowed to modify object’s fields via object reference. 1. Primitive Type No doubt, for primitive type, the parameters are pass by value. PrimitiveExample.java package com.mkyong; public class PrimitiveExample { public static void main(String[] args) { int value […]
A popular common error for new Java users. Terminal C:\Users\mkyong>javac ‘javac’ is not recognized as an internal or external command, operable program or batch file. C:\Users\mkyong>java -version ‘java’ is not recognized as an internal or external command, operable program or batch file. 1. Solution To fix it, update the system variable PATH to JDK_folder\bin 1. […]
A popular error message for new Java users. Error: Could not find or load main class ClassName.class Caused by: java.lang.ClassNotFoundException: ClassName.class 1. No Package 1.1 Reviews a simple Java Hello World, no package. C:\projects\HelloWorld.java public class HelloWorld { public static void main(String[] args) { System.out.println("Hello World"); } } Terminal # Compile Java source code C:\projects> […]
In Java, we can use Files.readAllBytes to read a file. byte[] content = Files.readAllBytes(Paths.get("app.log")); System.out.println(new String(content)); 1. Text File A Java example to write and read a normal text file. FileExample1.java package com.mkyong.calculator; import java.io.IOException; import java.nio.charset.Charset; import java.nio.charset.StandardCharsets; import java.nio.file.Files; import java.nio.file.Paths; import java.util.Arrays; import java.util.List; public class FileExample1 { public static void main(String[] […]
In Java, there are 3 ways to loop or iterate a HashMap 1. If possible, always uses the Java 8 forEach. Map<String, String> map = new HashMap<>(); map.forEach((key, value) -> System.out.println("[Key] : " + key + " [Value] : " + value)); 2. Normal for loop in entrySet() Map<String, String> map = new HashMap<>(); for […]
Java JDBC PreparedStatement example to create a SQL IN condition. 1. PreparedStatement + Array In JDBC, we can use createArrayOf to create a PreparedStatement IN query. @Override public List<Integer> getPostIdByTagId(List<Integer> tagIds) { List<Integer> result = new ArrayList<>(); String sql = "SELECT tr.object_id as post_id FROM wp_term_relationships tr " + " JOIN wp_term_taxonomy tt JOIN wp_terms […]
Few Java examples to declare, initialize and manipulate Array in Java 1. Declares Array 1.1 For primitive types. ArrayExample1.java package com.mkyong; import java.util.Arrays; public class ArrayExample1 { public static void main(String[] args) { //declares an array of integers int[] num1 = new int[5]; int[] num2 = {1, 2, 3, 4, 5}; int[] num3 = new […]
In Java, you can use String.valueOf() to convert an Integer to String. 1. String.valueOf 1.1 Example to convert an Integer or int 10 to a String. Integer num = 10; //int num = 10; String numInString = String.valueOf(num); System.out.println(numInString); Output 10 2. toString() It works on Integer object only. Integer num = 10; String numInString […]
This java.lang.ArrayIndexOutOfBoundsException is thrown when we are accessing an array with an index which is greater than the size of an array. P.S Array index starts with 0 ArrayExample.java package com.mkyong; public class ArrayExample { public static void main(String[] args) { // array of 3 int[] num = new int[3]; num[0] = 1; num[1] = […]
In Java, we can use ProcessBuilder to run a Windows batch file like this : ProcessBuilder processBuilder = new ProcessBuilder("C:\\Users\\mkyong\\hello.bat"); //or ProcessBuilder processBuilder = new ProcessBuilder(); processBuilder.command("cmd", "/c", "hello.bat"); File dir = new File("C:\\Users\\mkyong\\"); processBuilder.directory(dir); Alternatively, Runtime.getRuntime().exec like this : Process process = Runtime.getRuntime().exec( "cmd /c hello.bat", null, new File("C:\\Users\\mkyong\\")); 1. Java Example 1.1 A […]
Two Java examples to show you how to list files in a directory : For Java 8, Files.walk Before Java 8, create a recursive loop to list all files. 1. Files.walk 1.1 List all files. try (Stream<Path> walk = Files.walk(Paths.get("C:\\projects"))) { List<String> result = walk.filter(Files::isRegularFile) .map(x -> x.toString()).collect(Collectors.toList()); result.forEach(System.out::println); } catch (IOException e) { e.printStackTrace(); […]
In Java, we can use ProcessBuilder to call external commands easily : ProcessBuilder processBuilder = new ProcessBuilder(); // — Linux — // Run a shell command processBuilder.command("bash", "-c", "ls /home/mkyong/"); // Run a shell script processBuilder.command("path/to/hello.sh"); // — Windows — // Run a command processBuilder.command("cmd.exe", "/c", "dir C:\\Users\\mkyong"); // Run a bat file processBuilder.command("C:\\Users\\mkyong\\hello.bat"); Process […]
In Java, there are few ways to read a file line by line into a List 1. Java 8 stream List<String> result; try (Stream<String> lines = Files.lines(Paths.get(fileName))) { result = lines.collect(Collectors.toList()); } 2. Java 7 Files.readAllLines(new File(fileName).toPath(), Charset.defaultCharset()); 3. Classic BufferedReader example. List<String> result = new ArrayList<>(); BufferedReader br = null; try { br = […]
In Java, we can use BlockingQueue to create a queue which shared by both producer and the consumer. Producer – Generate data and put it into the queue. Consumer – Remove the data from the queue. Note Read this article to understand what is producer and consumer. The BlockingQueue implementations are thread-safe, safely be used […]
In Java, we can use String.contains() to check if a String contains a substring. 1. String.contains() – Case Sensitive JavaExample1.java package com.mkyong; public class JavaExample1 { public static void main(String[] args) { String name = "mkyong is learning Java 123"; System.out.println(name.contains("Java")); // true System.out.println(name.contains("java")); // false System.out.println(name.contains("MKYONG")); // false System.out.println(name.contains("mkyong")); // true if (name.contains("Java")) { […]
In Java, we can use Semaphore to limit the number of threads to access a certain resource. 1. What is Semaphore? In short, a semaphore maintains a set of permits (tickets), each acquire() will take a permit (ticket) from semaphore, each release() will return back the permit (ticket) back to the semaphore. If permits (tickets) […]
An example to show you how to create a thread safe sequence generator. 1. SequenceGenerator SequenceGenerator.java package com.mkyong.concurrency.examples.sequence.generator; public interface SequenceGenerator { long getNext(); } 1.1 First try, read, add, write the value directly. Below method is not thread safe, multiple threads may get the same value at the same time. UnSafeSequenceGenerator.java package com.mkyong.concurrency.examples.sequence.generator; public […]
In Java, we can use ExecutorService to create a thread pool, and tracks the progress of the asynchronous tasks with Future. The ExecutorService accept both Runnable and Callable tasks. Runnable – Return void, nothing. Callable – Return a Future. 1. ExecutorService 1.1 A classic ExecutorService example to create a thread pool with 5 threads, submit […]
In Java, we can use ScheduledExecutorService to run a task periodically or once time after a predefined delay by TimeUnit. 1. Run a Task Once The ScheduledExecutorService accepts both Runnable and Callable tasks. 1.1 Run a Runnable task after 5 seconds initial delay. ScheduledExecutorExample.java package com.mkyong.concurrency.executor.scheduler; import java.util.concurrent.Executors; import java.util.concurrent.ScheduledExecutorService; import java.util.concurrent.TimeUnit; public class ScheduledExecutorRunnable […]
Fibonacci number – Every number after the first two is the sum of the two preceding. Few Java examples to find the Fibonacci numbers. 1. Java 8 stream 1.1 In Java 8, we can use Stream.iterate to generate Fibonacci numbers like this : Stream.iterate(new int[]{0, 1}, t -> new int[]{t[1], t[0] + t[1]}) .limit(10) .forEach(x […]
Benchmark (N) Mode Cnt Score Error Units BenchmarkLoop.loopFor 10000000 avgt 10 61.673 ± 1.251 ms/op BenchmarkLoop.loopForEach 10000000 avgt 10 67.582 ± 1.034 ms/op BenchmarkLoop.loopIterator 10000000 avgt 10 66.087 ± 1.534 ms/op BenchmarkLoop.loopWhile 10000000 avgt 10 60.660 ± 0.279 ms/op In Java, we can use JMH (Java Microbenchmark Harness) framework to measure the performance of a […]
In Java, there is no global keyword, but we can use public static variable to referring a global variable. For example : MagicUtils.java package com.mkyong.example; public class MagicUtils { public static final String NAME = "mkyong"; // global public static final int LUCKY_NUMBER = 7; // global } JavaExample.java package com.mkyong.example; public class JavaExample { […]
In Java, we can use Set.iterator().next() to get the first item from a java.util.Set JavaExample.java package com.mkyong; import java.util.HashSet; import java.util.Set; public class JavaExample { public static void main(String[] args) { Set<String> examples = new HashSet<>(); examples.add("1"); examples.add("2"); examples.add("3"); examples.add("4"); examples.add("5"); System.out.println(examples.iterator().next()); // java 8 System.out.println(examples.stream().findFirst().get()); } } Output 1 1 References Oracle doc – […]
In Java, we can use BigInteger.valueOf(int) to convert int to a BigInteger JavaExample.java package com.mkyong; import java.math.BigInteger; public class JavaExample { public static void main(String[] args) { int n = 100; System.out.println(n); // convert int to Integer Integer integer = Integer.valueOf(n); System.out.println(integer); // convert int to BigInteger BigInteger bigInteger = BigInteger.valueOf(n); System.out.println(bigInteger); // convert Integer […]
In Java, we can convert the BigInteger to a String object and get the length of the String. JavaExample.java package com.mkyong; import java.math.BigInteger; public class JavaExample { public static void main(String[] args) { BigInteger bigInteger = BigInteger.valueOf(100000000000000000L); //18 System.out.println(bigInteger.toString().length()); } } Output 18 References BigInteger JavaDoc Quora – What are the limits of String Size […]
We can just divide the nanoTime by 1_000_000_000, or use the TimeUnit.SECONDS.convert to convert it. Note 1 second = 1_000_000_000 nanosecond JavaExample.java package com.mkyong; import java.util.concurrent.TimeUnit; public class JavaExample { public static void main(String[] args) throws InterruptedException { long start = System.nanoTime(); Thread.sleep(5000); long end = System.nanoTime(); long elapsedTime = end – start; System.out.println(elapsedTime); // […]
In Java, we can use List.size() to count the number of items in a List package com.mkyong.example; import java.util.Arrays; import java.util.List; public class JavaExample { public static void main(String[] args) { List<String> list = Arrays.asList("a", "b", "c"); System.out.println(list.size()); } } Output 3 References Java doc – List
