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Commitc2786e5

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Dev-AliGhasemiohbusiluwatar
authored
add monitor design pattern (iluwatar#1640)
* add monitor design pattern .* add extra line and change compiler version to 11 in pom.xml.* encapsulate getBalance method .* update puml file .* export uml as png .* duplicate codes eliminated .* update tag* change the format of pom.xml* using logger to print* change AtomicRefrence to type inference var* explanations added !* Update monitor/README.mdCo-authored-by: Ilkka Seppälä <iluwatar@users.noreply.github.com>* Update monitor/README.mdCo-authored-by: Ilkka Seppälä <iluwatar@users.noreply.github.com>* Update monitor/src/main/java/com/iluwatar/monitor/Main.javaCo-authored-by: Ilkka Seppälä <iluwatar@users.noreply.github.com>* Update monitor/src/main/java/com/iluwatar/monitor/Main.javaCo-authored-by: Ilkka Seppälä <iluwatar@users.noreply.github.com>* Update monitor/src/main/java/com/iluwatar/monitor/Main.javaCo-authored-by: Ilkka Seppälä <iluwatar@users.noreply.github.com>* Update monitor/src/main/java/com/iluwatar/monitor/Main.javaCo-authored-by: Ilkka Seppälä <iluwatar@users.noreply.github.com>* e.printStackTrace have changed to logger to prints standard output (STD OUT) .* add programmatic example .* Delete mvnw* mvnw.cmd deleted .* added mvnw from master* AddUnitTest* Add language to readme.mdCo-authored-by: Subhrodip Mohanta <subhrodipmohanta@gmail.com>Co-authored-by: Ilkka Seppälä <iluwatar@users.noreply.github.com>Co-authored-by: Subhrodip Mohanta <subhromo@cisco.com>Co-authored-by: Subhrodip Mohanta <contact@subho.xyz>
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‎monitor/README.md

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---
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layout:pattern
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title:Monitor
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folder:monitor
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permalink:/patterns/monitor/
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categories:Concurrency
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language:en
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tags:
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-Performance
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---
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##Intent
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Monitor pattern is used to create thread-safe objects and prevent conflicts between threads in concurrent applications.
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##Explanation
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In plain words
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>Monitor pattern is used to enforce single-threaded access to data. Only one thread at a time is allowed to execute code within the monitor object.
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Wikipedia says
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>In concurrent programming (also known as parallel programming), a monitor is a synchronization construct that allows threads to have both mutual exclusion and the ability to wait (block) for a certain condition to become false. Monitors also have a mechanism for signaling other threads that their condition has been met.
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**Programmatic Examples**
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Consider there is a bank that transfers money from an account to another account with transfer method . it is`synchronized` mean just one thread can access to this method because if many threads access to it and transfer money from an account to another account in same time balance changed !
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```
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class Bank {
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private int[] accounts;
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Logger logger;
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public Bank(int accountNum, int baseAmount, Logger logger) {
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this.logger = logger;
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accounts = new int[accountNum];
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Arrays.fill(accounts, baseAmount);
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}
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public synchronized void transfer(int accountA, int accountB, int amount) {
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if (accounts[accountA] >= amount) {
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accounts[accountB] += amount;
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accounts[accountA] -= amount;
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logger.info("Transferred from account :" + accountA + " to account :" + accountB + " , amount :" + amount + " . balance :" + getBalance());
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}
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}
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```
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getBalance always return total amount and the total amount should be same after each transfers
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```
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private synchronized int getBalance() {
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int balance = 0;
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for (int account : accounts) {
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balance += account;
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}
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return balance;
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}
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}
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```
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##Class diagram
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![alt text](./etc/monitor.urm.png"Monitor class diagram")
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##Applicability
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Use the Monitor pattern when
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* we have a shared resource and there is critical section .
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* you want to create thread-safe objects .
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* you want to achieve mutual exclusion in high level programming language .

‎monitor/etc/monitor.urm.png

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‎monitor/etc/monitor.urm.puml

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@startuml
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Main-Bank : use
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classMain{
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+main(args : String[]) : void
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}
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classBank{
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-accounts : int[]
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+Bank (accountNum : int , baseAccount : int)
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+transfer(accountA : int , accountB : int , amount : int) : void {synchronized}
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-getBalance() : void {synchronized}
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}
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@enduml

‎monitor/pom.xml

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<?xml version="1.0" encoding="UTF-8"?>
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<!--
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The MIT License
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Copyright © 2014-2021 Ilkka Seppälä
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Permission is hereby granted, free of charge, to any person obtaining a copy
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of this software and associated documentation files (the "Software"), to deal
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in the Software without restriction, including without limitation the rights
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to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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copies of the Software, and to permit persons to whom the Software is
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furnished to do so, subject to the following conditions:
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The above copyright notice and this permission notice shall be included in
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all copies or substantial portions of the Software.
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THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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THE SOFTWARE.
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-->
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<projectxmlns="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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<modelVersion>4.0.0</modelVersion>
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<parent>
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<artifactId>java-design-patterns</artifactId>
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<groupId>com.iluwatar</groupId>
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<version>1.24.0-SNAPSHOT</version>
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</parent>
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<artifactId>monitor</artifactId>
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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-engine</artifactId>
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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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<plugins>
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<!-- Maven assembly plugin is invoked with default setting which we have
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in parent pom and specifying the class having main method-->
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-assembly-plugin</artifactId>
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<executions>
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<execution>
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<configuration>
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<archive>
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<manifest>
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<mainClass>com.iluwatar.abstractdocument.Main</mainClass>
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</manifest>
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</archive>
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</configuration>
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</execution>
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</executions>
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</plugin>
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</plugins>
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</build>
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</project>
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packagecom.iluwatar.monitor;
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importjava.util.Arrays;
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importjava.util.logging.Logger;
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// Bank class implements the Monitor pattern
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publicclassBank {
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privateint[]accounts;
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Loggerlogger;
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publicBank(intaccountNum,intbaseAmount,Loggerlogger) {
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this.logger =logger;
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accounts =newint[accountNum];
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Arrays.fill(accounts,baseAmount);
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}
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publicsynchronizedvoidtransfer(intaccountA,intaccountB,intamount) {
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if (accounts[accountA] >=amount) {
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accounts[accountB] +=amount;
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accounts[accountA] -=amount;
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logger.info("Transferred from account :" +accountA +" to account :" +accountB +" , amount :" +amount +" . balance :" +getBalance());
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}
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}
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publicsynchronizedintgetBalance() {
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intbalance =0;
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for (intaccount :accounts) {
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balance +=account;
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}
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returnbalance;
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}
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publicint[]getAccounts() {
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returnaccounts;
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}
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}
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/*
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* The MIT License
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* Copyright © 2014-2021 Ilkka Seppälä
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be included in
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* all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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packagecom.iluwatar.monitor;
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importjava.util.*;
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importjava.util.concurrent.ExecutorService;
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importjava.util.concurrent.Executors;
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importjava.util.logging.Logger;
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/**
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* <p>The Monitor pattern is used in concurrent algorithms to achieve mutual exclusion.</p>
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*
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* <p>Bank is a simple class that transfers money from an account to another account using
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* {@link Bank#transfer}. It can also return the balance of the bank account stored in the bank.</p>
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*
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* <p>Main class uses ThreadPool to run threads that do transactions on the bank accounts.</p>
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*/
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publicclassMain {
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publicstaticvoidmain(String[]args) {
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Loggerlogger =Logger.getLogger("monitor");
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varbank =newBank(4,1000,logger);
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Runnablerunnable = () -> {
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try {
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Thread.sleep((long) (Math.random() *1000));
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Randomrandom =newRandom();
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for (inti =0;i <1000000;i++)
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bank.transfer(random.nextInt(4),random.nextInt(4), (int) (Math.random() *1000));
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}catch (InterruptedExceptione) {
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logger.info(e.getMessage());
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}
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};
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ExecutorServiceexecutorService =Executors.newFixedThreadPool(5);
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for (inti =0;i <5;i++) {
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executorService.execute(runnable);
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}
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}
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}
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packagecom.iluwater.java;
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importcom.iluwatar.monitor.Bank;
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importorg.junit.jupiter.api.AfterAll;
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importorg.junit.jupiter.api.BeforeAll;
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importorg.junit.jupiter.api.Test;
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importstaticorg.junit.jupiter.api.Assertions.*;
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importstaticorg.junit.jupiter.api.Assumptions.*;
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importjava.util.logging.Logger;
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publicclassBankTest {
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privatestaticBankbank;
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privatestaticfinalintACCOUNT_NUM =4;
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privatestaticfinalintBASE_AMOUNT =1000;
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privatestaticfinalLoggerLOGGER =Logger.getLogger("monitor");
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@BeforeAll
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publicstaticvoidSetup() {
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bank =newBank(ACCOUNT_NUM,BASE_AMOUNT,LOGGER);
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}
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@Test
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publicvoidGetAccountHaveNotBeNull() {
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assertNotNull(bank.getAccounts());
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}
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@Test
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publicvoidLengthOfAccountsHaveToEqualsToAccountNumConstant() {
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assumeTrue(bank.getAccounts() !=null);
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assertEquals(ACCOUNT_NUM,bank.getAccounts().length);
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}
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@Test
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publicvoidTransferMethodHaveToTransferAmountFromAnAccountToOtherAccount() {
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bank.transfer(0,1,1000);
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intaccounts[] =bank.getAccounts();
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assertEquals(0,accounts[0]);
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assertEquals(2000,2000);
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}
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@Test
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publicvoidBalanceHaveToBeOK() {
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assertEquals(4000,bank.getBalance());
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}
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@AfterAll
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publicstaticvoidTearDown() {
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bank =null;
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}
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}

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