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 Jackson, we can use the annotation @JsonView to control which fields of the same resource are displayed for different users. Table of contents: 1. Download Jackson 2. Define View Classes 3. Annotate model class with @JsonView 4. Testing the Jackson @JasonView 5. Download Source Code 6. References P.S Tested with Jackson 2.17.0 1. Download […]
This article uses the Jackson framework to parse JSON strings in Java. Table of contents: 1. Download Jackson 2. Parse JSON String with Jackson 3. Parse JSON Array with Jackson 4. Convert Java Object to JSON String 5. Convert JSON String to Java Object 6. Download Source Code 7. References P.S Tested with Jackson 2.17.0 […]
In Jackson, we can use @JsonInclude(JsonInclude.Include.NON_NULL) to ignore the null fields during serialization. Table of contents: 1. Jackson default include null fields 2. Ignore all null fields globally 3. Ignore null fields (Class Level) 4. Ignore null fields (Field Level) 5. Download Source Code 6. References P.S Tested with Jackson 2.17.0 1. Jackson default include […]
This article uses the Jackson framework to parse JSON Array in Java. Table of contents: 1. Download Jackson 2. Parse JSON Array with Jackson 3. Convert List of Objects to JSON Array 4. Write JSON Array with a list label 5. Parse JSON Array contains a list label 6. Download Source Code 7. References P.S […]
This article shows how to parse JSON using Gson. Table of contents: 1. Download Google Gson 2. A Java Object 3. Parse JSON String using Gson 4. Parse JSON Array using Gson 5. Convert Java Object to JSON using Gson 6. Unstructured JSON 7. Download Source Code 8. References P.S Tested with Gson 2.10.1 1. […]
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(); // […]
Find the major version of the Java class file, and we will know which JDK version compiled the Java class. Below are some popular Java LTS released and their major version numbers. Java SE Major Version Hex 17 61 0x3D 11 55 0x37 8 52 0x34 If the major version of the Java class file […]
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> […]
To send email in Java, we need JavaMail pom.xml <dependency> <groupId>com.sun.mail</groupId> <artifactId>javax.mail</artifactId> <version>1.6.2</version> </dependency> 1. Send Email Send a normal email in text format. SendEmailSMTP.java package com.mkyong; import com.sun.mail.smtp.SMTPTransport; import javax.mail.Message; import javax.mail.MessagingException; import javax.mail.Session; import javax.mail.internet.InternetAddress; import javax.mail.internet.MimeMessage; import java.util.Date; import java.util.Properties; public class SendEmailSMTP { // for example, smtp.mailgun.org private static final String […]
This article focus on a few of the commonly used methods to read a file in Java. Files.lines, return a Stream (Java 8) Files.readString, returns a String (Java 11), max file size 2G. Files.readAllBytes, returns a byte[] (Java 7), max file size 2G. Files.readAllLines, returns a List<String> (Java 8) BufferedReader, a classic old friend (Java […]
In Java, we can use Files.write to create and write to a file. String content = "…"; Path path = Paths.get("/home/mkyong/test.txt"); // string -> bytes Files.write(path, content.getBytes(StandardCharsets.UTF_8)); The Files.write also accepts an Iterable interface; it means this API can write a List to a file. List<String> list = Arrays.asList("a", "b", "c"); Files.write(path, list); Short History […]
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 […]
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 […]
In Java, we use equals() to compare string value. String str1 = "apple"; // compare strings case-sensitive if (str1.equals("apple")) { System.out.println("I have an apple."); } String str2 = "apple"; // compare strings case-insensitive if (str2.equalsIgnoreCase("APPLE")) { System.out.println("I have an APPLE."); } Output Terminal I have an apple. I have an APPLE. In this article, we […]
In Java, we can use JSONAssert to unit test JSON data easily. 1. Maven pom.xml <dependency> <groupId>org.skyscreamer</groupId> <artifactId>jsonassert</artifactId> <version>1.5.0</version> </dependency> 2. JSON Object To test the JSON Object, Strict or not, fields order does not matter. If the extended fields are matter, enable the strict mode. 2.1 When the strictMode is off. JSONObject actual = […]
The Java project is using log4j2, but look like some components are used SLF4J logging and caused the following error message: SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder". SLF4J: Defaulting to no-operation (NOP) logger implementation SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details. Solution To fix it, we need a SLF4J Bridge. Puts log4j-slf4j-impl in the classpath. pom.xml […]
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 […]
The Files.walk API is available since Java 8; it helps to walk a file tree at a given starting path. Topics Files.walk() method signature List all files List all folders or directories Find files by file extension Find files by filename Find files by file size Update all files last modified date 1. Files.walk() method […]
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 […]
