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‎java-basic/pom.xml‎

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<?xml version="1.0" encoding="UTF-8"?>
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<project xmlns="http://maven.apache.org/POM/4.0.0"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
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http://maven.apache.org/xsd/maven-4.0.0.xsd">
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<modelVersion>4.0.0</modelVersion>
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<groupId>com.mkyong</groupId>
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<artifactId>io</artifactId>
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<version>1.0</version>
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<name>java-basic</name>
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<url>https://mkyong.com</url>
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<properties>
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<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
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<maven.compiler.source>11</maven.compiler.source>
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<maven.compiler.target>11</maven.compiler.target>
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<java.version>11</java.version>
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<junit.version>5.4.0</junit.version>
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</properties>
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<dependencies>
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<dependency>
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<groupId>org.junit.jupiter</groupId>
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<artifactId>junit-jupiter-params</artifactId>
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<version>${junit.version}</version>
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<scope>test</scope>
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</dependency>
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</dependencies>
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<build>
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<finalName>java-basic</finalName>
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<plugins>
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-compiler-plugin</artifactId>
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<version>3.8.1</version>
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<configuration>
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<source>${java.version}</source>
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<target>${java.version}</target>
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</configuration>
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</plugin>
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</plugins>
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</build>
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</project>

‎java-basic/src/main/java/com/mkyong/collections/map/InitMap.java‎

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package com.mkyong.collections.map;
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import java.util.Collections;
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import java.util.HashMap;
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import java.util.Map;
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import static java.util.stream.Collectors.toMap;
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public class InitMap {
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public static Map<String, String> map5;
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static {
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// mutable Map
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map5 = new HashMap<>();
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map5.put("key1", "value1");
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map5.put("key2", "value2");
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map5.put("key3", "value3");
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}
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public static void main(String[] args) {
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// normal way, mutable Map
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Map<String, String> map = new HashMap<>();
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map.put("key1", "value1");
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map.put("key2", "value2");
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map.put("key3", "value3");
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System.out.println(map.getClass()); // java.util.HashMap
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// immutable Map
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Map<String, String> immutableMap = Collections.unmodifiableMap(map);
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System.out.println(immutableMap.getClass()); // java.util.Collections$UnmodifiableMap
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//immutableMap.put("key4", "value4"); // UnsupportedOperationException
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// @since 1.3
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// single entry immutable Map
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Map<String, String> map1 = Collections.singletonMap("key1", "value1");
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System.out.println(map1.getClass()); // java.util.Collections$SingletonMap
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//map1.put("key2", "value2"); // throws UnsupportedOperationException
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// Java 9, max is 10 entries or elements, immutable map
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Map<String, String> map2 = Map.of(
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"key1", "value1",
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"key2", "value2",
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"key3", "value3"
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);
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System.out.println(map2.getClass()); // java.util.ImmutableCollections$MapN
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//map2.put("key3", "value4"); // throws UnsupportedOperationException
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// Java 9, no limit of Map entry, immutable Map.
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Map<String, String> map3 = Map.ofEntries(
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Map.entry("key1", "value1"),
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Map.entry("key2", "value2"),
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Map.entry("key3", "value3")
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);
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System.out.println(map3.getClass()); // java.util.ImmutableCollections$MapN
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//map3.put("key3", "value4"); // throws UnsupportedOperationException
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// It works for all Java version, mutable Map.
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Map<String, String> map4 = createMap();
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System.out.println(map4.getClass()); // java.util.HashMap
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map4.put("key4", "value4"); // no problem.
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// static init
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System.out.println(map5.getClass()); // java.util.HashMap
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map5.put("key4", "value4"); // no problem.*/
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// Java 8, mutable Map, do not use this, just for reference
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/*Map<String, String> map6 = Stream.of(
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new AbstractMap.SimpleEntry<>("key1", "value1"),
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new AbstractMap.SimpleEntry<>("key2", "value2"),
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new AbstractMap.SimpleEntry<>("key3", "value3"))
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.collect(
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toMap(AbstractMap.SimpleEntry::getKey,
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AbstractMap.SimpleEntry::getValue)
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);
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System.out.println(map6.getClass()); // java.util.HashMap
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map6.put("key4", "value4"); // no problem.*/
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// classic way, loop a Map
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for (Map.Entry<String, String> entry : map.entrySet()) {
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System.out.println("Key : " + entry.getKey() + " Value : " + entry.getValue());
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}
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}
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private static Map<String, String> createMap() {
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Map<String, String> map = new HashMap<>();
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map.put("key1", "value1");
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map.put("key2", "value2");
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map.put("key3", "value3");
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return map;
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}
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}

‎java-basic/src/test/resources/database.properties‎

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datasource.url=jdbc:mysql://localhost/test?useSSL=false
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datasource.username=test
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datasource.password=password
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datasource.driver-class-name=com.mysql.jdbc.Driver

‎java-basic/src/test/resources/json/file1.json‎

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{
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"name": "unit test",
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"age": 38
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}

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