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Add Stage 2 Lesson 3
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-4
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16 files changed

+507
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‎「一入 Java 深似海 」/代码/segmentfault/deep-in-java/stage-1/lesson-4/pom.xml

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<version>1.0-SNAPSHOT</version>
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</parent>
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<modelVersion>4.0.0</modelVersion>
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<name>「一入 Java 深似海 」系列 :: 第一期 :: 第四节</name>
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<artifactId>stage-1-lesson-4</artifactId>
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‎「一入 Java 深似海 」/代码/segmentfault/deep-in-java/stage-2/lesson-1/pom.xml

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<relativePath>../pom.xml</relativePath>
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</parent>
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<modelVersion>4.0.0</modelVersion>
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<name>「一入 Java 深似海 」系列 :: 第二期 :: 第一节</name>
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<artifactId>stage-2-lesson-1</artifactId>
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<dependencies>

‎「一入 Java 深似海 」/代码/segmentfault/deep-in-java/stage-2/lesson-1/src/main/java/com/segmentfault/deep/in/java/modular/CollectionRemoveDemoInJava8.java

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package com.segmentfault.deep.in.java.modular;
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import java.util.ArrayList;
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import java.util.Iterator;
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import java.util.List;
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public class CollectionRemoveDemoInJava8 {
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public static void main(String[] args) {
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List<Integer> values = new ArrayList(List.of(1, 2, 3));
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for (int i = 0; i < values.size(); i++) {
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Object value = values.get(i);
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values.remove(value);
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}
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}
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}

‎「一入 Java 深似海 」/代码/segmentfault/deep-in-java/stage-2/lesson-2/pom.xml

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<artifactId>stage-2</artifactId>
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<groupId>com.segmentfault</groupId>
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<version>1.0-SNAPSHOT</version>
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<relativePath>../pom.xml</relativePath>
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</parent>
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<modelVersion>4.0.0</modelVersion>
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<name>「一入 Java 深似海 」系列 :: 第二期 :: 第二节</name>
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<artifactId>stage-2-lesson-2</artifactId>
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<build>

‎「一入 Java 深似海 」/代码/segmentfault/deep-in-java/stage-2/lesson-3/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 http://maven.apache.org/xsd/maven-4.0.0.xsd">
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<parent>
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<artifactId>stage-2</artifactId>
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<groupId>com.segmentfault</groupId>
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<version>1.0-SNAPSHOT</version>
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<relativePath>../pom.xml</relativePath>
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</parent>
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<modelVersion>4.0.0</modelVersion>
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<name>「一入 Java 深似海 」系列 :: 第二期 :: 第二节</name>
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<artifactId>stage-2-lesson-3</artifactId>
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<build>
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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>3.8.0</version>
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<configuration>
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<source>11</source>
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<target>11</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>

‎「一入 Java 深似海 」/代码/segmentfault/deep-in-java/stage-2/lesson-3/src/main/java/com/segmentfault/deep/in/java/collection/advanced/CheckedTypeCollectionDemo.java

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package com.segmentfault.deep.in.java.collection.advanced;
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import java.util.ArrayList;
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import java.util.Arrays;
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import java.util.Collections;
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import java.util.List;
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public class CheckedTypeCollectionDemo {
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public static void main(String[] args) {
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// List 元素类型是 java.lang.Integer
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List<Integer> values = new ArrayList<>(Arrays.asList(1, 2, 3));
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// values.add("1"); // 编译错误
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// 泛型是编译时检查，运行时擦写
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// 引用 List<Integer> 类型的对象 values
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List referencedValues = values;
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System.out.println(referencedValues == values);
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referencedValues.add("A"); // 添加 "A" 进入 List<Integer> values
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// 运行时的数据 List<Integer> == List<Object> == List
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// values.add("1") // 运行时允许，因为成员类型是 Object
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for (Object value : values) {
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System.out.println(value);
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}
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// values
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// [0] = 1, [1] = 2, [2] = 3, [3] = "A"
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// 创建时尚未检查内部的数据是否类型相同，操作时做检查，Wrapper 模式（装饰器模式）的运用
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// Collections.checked* 接口是弥补泛型集合在运行时中的擦写中的不足
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// 强约束：编译时利用 Java 泛型、运行时利用 Collections.checked* 接口
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List<Integer> checkedTypeValues = Collections.checkedList(values, Integer.class);
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// checkedTypeValues.add("1"); // 编译错误
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// 运行时检查
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// 又将 checkedTypeValues 引用给 referencedValues
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referencedValues = checkedTypeValues;
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System.out.println(referencedValues == values);
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System.out.println(referencedValues == checkedTypeValues);
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// 添加 "B" 进入 referencedValues
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referencedValues.add("B");
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}
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}

‎「一入 Java 深似海 」/代码/segmentfault/deep-in-java/stage-2/lesson-3/src/main/java/com/segmentfault/deep/in/java/collection/advanced/CollectionRemoveDemo.java

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package com.segmentfault.deep.in.java.collection.advanced;
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import java.util.ArrayList;
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import java.util.Iterator;
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import java.util.List;
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public class CollectionRemoveDemo {
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public static void main(String[] args) {
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List<Integer> values = new ArrayList(List.of(1, 2, 3));
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// 常规做法
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Iterator<Integer> iterator = values.iterator();
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while (iterator.hasNext()) {
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iterator.next(); // 需要执行，调整游标
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iterator.remove();
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}
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values = new ArrayList(List.of(1, 2, 3));
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//　成功删除
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int size = values.size();
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for (int i = 0; i < size; i++) {
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Object value = values.get(0);
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values.remove(value);
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System.out.println(values.size());
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}
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values = new ArrayList(List.of(1, 2, 3));
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size = values.size();
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for (int i = 0; i < size; i++) {
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values.remove(0);
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}
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// 失败删除: for-each 语法
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values = new ArrayList(List.of(1, 2, 3));
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for (Integer value : values) {
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values.remove(value);
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}
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}
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}

‎「一入 Java 深似海 」/代码/segmentfault/deep-in-java/stage-2/lesson-3/src/main/java/com/segmentfault/deep/in/java/collection/advanced/EmptyCollectionDemo.java

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package com.segmentfault.deep.in.java.collection.advanced;
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import java.util.Collections;
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import java.util.List;
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public class EmptyCollectionDemo {
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public static void main(String[] args) {
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// 对自己（严格）：所有的返回接口类型的方法禁止返回 null
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// 对别人（宽容）：要做 null 判断（尤其在 RPC 场景）
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// 集合方法入参：
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// 1. 如果能用 Iterable 尽量用
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// 2. 其次是 Collection
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// 3. 再者是 List 或 Set
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// 禁止使用具体类型，比如：ArrayList，LinkedHashSet
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}
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public static List<String> getIdsList(String name) {
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if(name ==null || name.length() < 1){
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return Collections.emptyList();
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}
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// 只读 empty List
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// 实现 Java 序列化
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return Collections.emptyList();
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}
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}

‎「一入 Java 深似海 」/代码/segmentfault/deep-in-java/stage-2/lesson-3/src/main/java/com/segmentfault/deep/in/java/collection/advanced/FailFastVsFailSafeDemo.java

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package com.segmentfault.deep.in.java.collection.advanced;
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import java.util.ArrayList;
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import java.util.Collection;
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import java.util.ConcurrentModificationException;
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import java.util.List;
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import java.util.concurrent.CopyOnWriteArrayList;
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public class FailFastVsFailSafeDemo {
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public static void main(String[] args) {
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demoFailFast();
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demoFailSafe();
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}
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private static void demoFailSafe() {
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removeForEach(new CopyOnWriteArrayList<>(List.of(1, 2, 3)));
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}
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private static void demoFailFast() {
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removeForEach(new ArrayList(List.of(1, 2, 3)));
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}
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private static void removeForEach(Collection<?> values) {
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try {
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// 如果是 Fail-Fast 设计的话，那么会抛出 java.util.ConcurrentModificationException，如：ArrayList
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// 如果是 Fail-Safe 设计的话，那么就没有关系，如 java.util.concurrent.CopyOnWriteArrayList
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for (Object value : values) {
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values.remove(value);
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}
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System.out.println("集合删除成功，目前空间大小：" + values.size());
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} catch (ConcurrentModificationException e) {
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System.out.println("Fail-Fast 异常");
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}
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}
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}

‎「一入 Java 深似海 」/代码/segmentfault/deep-in-java/stage-2/lesson-3/src/main/java/com/segmentfault/deep/in/java/collection/advanced/IdentityHashMapDemo.java

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package com.segmentfault.deep.in.java.collection.advanced;
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import java.util.HashMap;
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import java.util.IdentityHashMap;
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import java.util.Map;
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import java.util.Objects;
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public class IdentityHashMapDemo {
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public static void main(String[] args) {
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// 如果类覆盖了 Object 的 equals(Object) 方法，那么 hashCode() 方法需不需要覆盖？
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// 说明：不强制覆盖，建议实现，注意不要将 hashCode() 作为 equals 方法的实现，可参考
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// Objects.hash(Object...) 以及 Arrays.hashCode(Object[])，hashCode() 是一个计算较重的实现
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// equals 通常是做对象属性的比较
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// 如果类覆盖了 Object 的 hashCode() ，那么 equals(Object) 方法 方法需不需要覆盖？
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// 说明：必须实现，hashCode() 是用于 Hash 计算，比如普通的 HashMap，由于不同对象的 hashCode() 方法可能返回相同的数据
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// 原因一：int 数据范围 2^31-1，原因二：hashCode() 方法计算问题
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// 当不同对象 hashCode() 相同是，再做对象 equals(Object) 方法比较
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demoHashMap();
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// 场景，需要对对象本身做鉴别
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demoIdentityHashMap();
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// System.identityHashCode() 与 覆盖 hashCode() 方法的差异
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// Object a = new Object();
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// demoIdentityHashCodeAndHashCode(a, a);
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//
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// Object b = new Object();
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// demoIdentityHashCodeAndHashCode(a, b);
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String string1 = "1";
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String string2 = "1";
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// demoIdentityHashCodeAndHashCode(string1, string2);
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// 不同对象
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string2 = new String("1");
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demoIdentityHashCodeAndHashCode(string1, string2);
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}
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private static void demoIdentityHashCodeAndHashCode(Object a, Object b) {
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System.out.printf("System.identityHashCode(%s) == %d \n", a, System.identityHashCode(a));
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System.out.printf("%s.hashCode() == %d \n", a, a.hashCode());
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System.out.printf("System.identityHashCode(%s) == System.identityHashCode(%s) == %s \n", a, b,
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System.identityHashCode(a) == System.identityHashCode(b));
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System.out.printf("%s.hashCode() == %s.hashCode() == %s \n", a, b, a.hashCode() == b.hashCode());
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}
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private static void demoMap(Map<String, Integer> map) {
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System.out.println("A" == new String("A")); // false
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System.out.println("A".equals(new String("A"))); // true
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System.out.println("A".hashCode() == new String("A").hashCode()); // true
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map.put("A", 1);
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map.put(new String("A"), 1);
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System.out.println(map.size());
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}
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private static void demoIdentityHashMap() {
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demoMap(new IdentityHashMap<>());
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}
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private static void demoHashMap() {
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demoMap(new HashMap<>());
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}
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private String name;
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private int age;
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@Override
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public boolean equals(Object o) {
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if (this == o) return true;
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if (o == null || getClass() != o.getClass()) return false;
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IdentityHashMapDemo that = (IdentityHashMapDemo) o;
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return age == that.age &&
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Objects.equals(name, that.name);
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}
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@Override
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public int hashCode() {
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return Objects.hash(name, age);
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}
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}

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