Long live the Scanner class, a few examples for self-reference. 1. Read Input 1.1 Read input from the console. JavaScanner1.java package com.mkyong.io; import java.util.Scanner; public class JavaScanner1 { public static void main(String[] args) { try (Scanner scanner = new Scanner(System.in)) { System.out.print("Please enter your name: "); String input = scanner.nextLine(); System.out.println("name : " + input); […]
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 […]
A simple Java source code to download a web page from the internet. JavaDownloadWebPage.java package com.mkyong; import java.io.BufferedReader; import java.io.IOException; import java.io.InputStream; import java.io.InputStreamReader; import java.net.URL; import java.net.URLConnection; public class JavaDownloadWebPage { public static void main(String[] args) throws IOException { String result = downloadWebPage("https://mkyong.com"); System.out.println(result); } private static String downloadWebPage(String url) throws IOException { StringBuilder […]
In this article, we will show you how to attach the JDK source code into the IntelliJ IDEA? By default, IntelliJ IDEA doesn’t come with JDK source code, and the auto decompile feature is useful; however, we still need to jump in the source code to read the source code comments or docs to understand […]
In Java, we can use Apache commons-text, StringEscapeUtils.escapeHtml4(str) to escape HTML characters. pom.xml <dependency> <groupId>org.apache.commons</groupId> <artifactId>commons-text</artifactId> <version>1.8</version> </dependency> JavaEscapeHtmlExample.java package com.mkyong.html; // make sure import the correct commons-text package import org.apache.commons.text.StringEscapeUtils; // @deprecated as of 3.6, use commons-text StringEscapeUtils instead //import org.apache.commons.lang3.StringEscapeUtils; public class JavaEscapeHtmlExample { public static void main(String[] args) { String html = […]
A jsoup HTML parser example to show you how to parse and get all HTML hyperlinks from a web page: pom.xml <dependency> <groupId>org.jsoup</groupId> <artifactId>jsoup</artifactId> <version>1.12.1</version> </dependency> JsoupFindLinkSample.java package com.mkyong; import org.jsoup.Jsoup; import org.jsoup.nodes.Document; import org.jsoup.nodes.Element; import org.jsoup.select.Elements; import java.io.IOException; import java.util.HashSet; import java.util.Set; public class JsoupFindLinkSample { public static void main(String[] args) throws IOException { […]
In Java, we can use jsoup, a Java HTML parser, to parse a HTML code and pretty print it. pom.xml <dependency> <groupId>org.jsoup</groupId> <artifactId>jsoup</artifactId> <version>1.12.1</version> </dependency> JavaPrettyPrintHTML.java package com.mkyong; import org.jsoup.Jsoup; import org.jsoup.nodes.Document; public class JavaPrettyPrintHTML { public static void main(String[] args) { String html = "<html><body><h1>hello world</h1></body></html>"; System.out.println(html); // original Document doc = Jsoup.parse(html); // […]
This example shows you how to read input from the console using the standard java.util.Scanner class. JavaScanner.java package com.mkyong.io; import java.util.Scanner; public class JavaScanner { public static void main(String[] args) { try (Scanner scanner = new Scanner(System.in)) { System.out.print("Please enter your name: "); String input = scanner.nextLine(); System.out.println("name : " + input); System.out.print("Please enter your […]
In Java, we can use System.console() to read input from the console. JavaSample.java package com.mkyong.io; import java.io.Console; public class JavaSample { public static void main(String[] args) { // jdk 1.6 Console console = System.console(); System.out.print("Enter your name: "); String name = console.readLine(); System.out.println("Name is: " + name); System.out.print("Enter your password: "); // Reads a password […]
A simple Java program tries to download something from the internet: try (InputStream is = new URL(url).openStream(); BufferedReader br = new BufferedReader(new InputStreamReader(is))) { while ((line = br.readLine()) != null) { result.append(line); } } But, some web servers return the following HTTP 403 errors? Exception in thread "main" java.io.IOException: Server returned HTTP response code: 403 […]
Here’s a Java example to print a name 1000 times without looping or recursion, instead, use String concatenation and simple math. Just for fun. JavaNoLoop.java package com.mkyong.samples; public class JavaNoLoop { public static void main(String[] args) { String s1 = "Mkyong\n"; String s3 = s1 + s1 + s1; String s10 = s3 + s3 […]
This article shows you how to use –enable-preview to enable the preview language features in Java 12, 13 and above. Note Read this JEP 12: Preview Language and VM Features P.S All preview features are disabled by default. On JDK 12: # compile javac Example.java // Do not enable any preview features javac –release 12 […]
This example shows you how to write Java ternary operator. Here’s the syntax condition ? get_this_if_true : get_this_if_false Java ternary operator syntax (n > 18) ? true : false; (n == true) ? 1 : 0; (n == null) ? n.getValue() : 0; 1. Java ternary operator 1.1 Java example without ternary operator. JavaExample1.java package […]
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 […]
This article shows you how to use the new Java 11 HttpClient APIs to send HTTP GET/POST requests, and some frequent used examples. HttpClient httpClient = HttpClient.newBuilder() .version(HttpClient.Version.HTTP_2) .followRedirects(HttpClient.Redirect.NORMAL) .connectTimeout(Duration.ofSeconds(20)) .proxy(ProxySelector.of(new InetSocketAddress("proxy.yourcompany.com", 80))) .authenticator(Authenticator.getDefault()) .build(); 1. Synchronous Example This example sends a GET request to https://httpbin.org/get, and print out the response header, status code, and […]
This article shows you how to use the OkHttp library to send an HTTP GET/POST requests and some frequent used examples. P.S Tested with OkHttp 4.2.2 pom.xml <dependency> <groupId>com.squareup.okhttp3</groupId> <artifactId>okhttp</artifactId> <version>4.2.2</version> </dependency> 1. Synchronous Get Request OkHttpExample1.java package com.mkyong.http; import okhttp3.Headers; import okhttp3.OkHttpClient; import okhttp3.Request; import okhttp3.Response; import java.io.IOException; public class OkHttpExample1 { // only […]
This article shows you how to use Apache HttpClient to perform an HTTP basic authentication. P.S Tested with HttpClient 4.5.10 pom.xml <dependency> <groupId>org.apache.httpcomponents</groupId> <artifactId>httpclient</artifactId> <version>4.5.10</version> </dependency> Start a simple Spring Security WebApp providing HTTP basic authentication, and test it with the HttpClient 1. Basic Authentication The key is to configure CredentialsProvider and pass it to […]
In Java 13, the JEP 354: Switch Expressions extends the previous Java 12 Switch Expressions by adding a new yield keyword to return a value from the switch expression. P.S Switch expressions are a preview feature and are disabled by default. Note This is a standard feature in Java 14. 1. No more value breaks! […]
Java 13 reached General Availability on 17 September 2019, download Java 13 here or this openJDK archived. Java 13 features. 1. JEP 350 Dynamic CDS Archives 2. JEP 351 ZGC: Uncommit Unused Memory 3. JEP-353 Reimplement the Legacy Socket API 4. JEP-354 Switch Expressions (Preview) 5. JEP-355 Text Blocks (Preview) Java 13 developer features. Switch […]
Java 12, JEP 325: Switch Expressions enhanced the traditional switch statement to support the following new features: Multiple case labels Switch expression returning value via break (replaced with yield in Java 13 switch expressions) Switch expression returning value via label rules (arrow) P.S Switch expressions are a preview feature and are disabled by default. A […]
In Java, index starts at 0, we can get the last index of a list via this formula: list.size() – 1 JavaExample1.java package com.mkyong.test; import java.util.Arrays; import java.util.List; public class JavaExample1 { public static void main(String[] args) { List<Integer> list = Arrays.asList(1, 2, 3, 4, 5); System.out.println(list.get(list.size() – 1)); System.out.println(list.get(list.size() – 2)); System.out.println(list.get(list.size() – 3)); […]
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) -> { […]
Below example, the jdbcTemplate.queryForList returns an object of Integer and we try to convert it into a Long directly: public List<Customer> findAll() { String sql = "SELECT * FROM CUSTOMER"; List<Customer> customers = new ArrayList<>(); List<Map<String, Object>> rows = jdbcTemplate.queryForList(sql); for (Map row : rows) { Customer obj = new Customer(); obj.setID(((Long) row.get("ID"))); // the […]
In Java, we can combine RandomAccessFile and FileChannel to lock a file before writing. LockFileAndWrite.java package com.mkyong; import java.io.File; import java.io.IOException; import java.io.RandomAccessFile; import java.nio.channels.FileLock; import java.util.concurrent.TimeUnit; public class LockFileAndWrite { public static void main(String[] args) { writeFileWithLock(new File("D:\\server.log"), "mkyong"); } public static void writeFileWithLock(File file, String content) { // auto close and release the […]
In Java, we can use the Apache Commons IO ReversedLinesFileReader to read the last few lines of a File. pom.xml <dependency> <groupId>commons-io</groupId> <artifactId>commons-io</artifactId> <version>2.6</version> </dependency> 1. Test Data A server log file sample d:\\server.log a b c d 1 2 3 4 5 2. Read Last Line 2.1 Read the last 3 lines of a […]
Java 8 stream and regular expression examples. Note Learn the basic regular expression at Wikipedia 1. String.matches(regex) 1.1 This example, check if the string is a number. JavaRegEx1.java package com.mkyong.regex; import java.util.Arrays; import java.util.List; public class JavaRegEx1 { public static void main(String[] args) { List<String> numbers = Arrays.asList("1", "20", "A1", "333", "A2A211"); for (String number […]
As title, we can use flatMap to convert it. Java9Example1.java package com.mkyong.test; import java.util.Arrays; import java.util.List; import java.util.Scanner; import java.util.stream.Collectors; public class Java9Example1 { public static void main(String[] args) { List<String> numbers = Arrays.asList("1", "2", "A", "B", "C1D2E3"); List<List<String>> collect = numbers.stream() .map(x -> new Scanner(x).findAll("\\D+") .map(m -> m.group()) .collect(Collectors.toList()) ) .collect(Collectors.toList()); collect.forEach(x -> System.out.println(x)); […]
Call the .values() method of the enum class to return an array, and loop it with the for loop: for (EnumClass obj : EnumClass.values()) { System.out.println(obj); } For Java 8, convert an enum into a stream and loop it: Stream.of(EnumClass.values()).forEach(System.out::println); 1. For Loop Enum 1.1 An enum to contain a list of the popular JVM […]
In Java, we can use Long.valueOf() to convert an Integer to a Long TestInteger.java package com.mkyong.example; public class TestInteger { public static void main(String[] args) { Integer num = 1; System.out.println(num); // 1 Long numInLong = Long.valueOf(num); System.out.println(numInLong); // 1 } } References Long.valueOf JavaDocs)
The following Java examples will print a list of all the prime numbers up to 1,000: 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 101 103 107 109 113 127 131 137 139 149 151 157 163 167 […]
If Java 8, DateTimeFormatter, else SimpleDateFormat to change the date format in a String. 1. DateTimeFormatter (Java 8) Convert the String to LocalDateTime and change the date format with DateTimeFormatter DateFormatExample1.java package com.mkyong; import java.time.LocalDateTime; import java.time.format.DateTimeFormatter; public class DateFormatExample1 { // date format 1 private static final DateTimeFormatter dateFormatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss.S"); // date […]
Different operating system has a different new line or line separator string: UNIX, Linux or Mac OSX = \n Windows = \r\n NewLineExample.java package com.mkyong; public class NewLineExample { public static void main(String[] args) { String original = "Hello World Java"; System.out.println(original); // add new line String originalNewLine = "Hello\nWorld\nJava"; System.out.println(originalNewLine); } } Output Hello […]
Only Streams whose source are an IO channel like Files.lines(Path, Charset) need to be closed. Read this Stream JavaDocs Streams have a BaseStream.close() method and implement AutoCloseable, but nearly all stream instances do not actually need to be closed after use. Generally, only streams whose source is an IO channel (such as those returned by […]
In Java, -Xms set initial Java heap size, while -Xmx set the maximum Java heap size. 1. Some java -Xms -Xmx examples : # Start with 128MB of memory, and allow the Java process to use up to 1024MB of memory. java -Xms128m -Xmx1024m # Start with 256MB of memory, and allow the Java process […]
In Java, we have many ways to convert a File to a String. A text file for testing later. c:\\projects\\app.log A B C D E 1. Java 11 – Files.readString A new method Files.readString is added in java.nio.file.Files, it makes reading a string from File much easier. FileToString1.java package com.mkyong; import java.io.IOException; import java.nio.file.Files; import […]
In Java, there are many ways to write a String to a File. 1. Java 11 – Files.writeString Finally, a new method added in java.nio to save a String into a File easily. StringToFileJava11.java package com.mkyong; import java.io.IOException; import java.nio.charset.StandardCharsets; import java.nio.file.Files; import java.nio.file.Paths; import java.nio.file.StandardOpenOption; public class StringToFileJava11 { public static void main(String[] args) […]
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); […]
