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Add memento pattern with bounded undo/redo for catalog queries (#16)
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‎section-04-behavioral-design-patterns/13-memento/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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<modelVersion>4.0.0</modelVersion>
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<groupId>com.luv2code</groupId>
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<artifactId>java-design-patterns</artifactId>
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<version>1.0</version>
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<properties>
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<maven.compiler.source>24</maven.compiler.source>
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<maven.compiler.target>24</maven.compiler.target>
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<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
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</properties>
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</project>

‎section-04-behavioral-design-patterns/13-memento/src/main/java/com/luv2code/designpatterns/behavioral/memento/CatalogQueryHistory.java‎

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package com.luv2code.designpatterns.behavioral.memento;
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import java.util.ArrayDeque;
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import java.util.Deque;
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/**
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* Role: Caretaker
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*
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* Maintains bounded undo and redo history.
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* Stores QuerySnapshot objects without
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* inspecting or modifying their contents.
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*/
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public class CatalogQueryHistory {
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private static final int MAX_HISTORY = 20;
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private Deque<CatalogQueryState.QuerySnapshot> undoCollection = new ArrayDeque<>();
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private Deque<CatalogQueryState.QuerySnapshot> redoCollection = new ArrayDeque<>();
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private void pushBounded(Deque<CatalogQueryState.QuerySnapshot> theCollection,
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CatalogQueryState.QuerySnapshot snapshot) {
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if (theCollection.size() == MAX_HISTORY) {
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theCollection.removeLast();
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}
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theCollection.push(snapshot);
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}
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public void saveBeforeChange(CatalogQueryState originator) {
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redoCollection.clear();
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pushBounded(undoCollection, originator.createSnapshot());
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}
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public boolean undo(CatalogQueryState originator) {
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if (undoCollection.isEmpty()) {
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return false;
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}
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pushBounded(redoCollection, originator.createSnapshot());
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originator.restore(undoCollection.pop());
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return true;
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}
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public boolean redo(CatalogQueryState originator) {
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if (redoCollection.isEmpty()) {
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return false;
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}
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pushBounded(undoCollection, originator.createSnapshot());
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originator.restore(redoCollection.pop());
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return true;
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}
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}
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‎section-04-behavioral-design-patterns/13-memento/src/main/java/com/luv2code/designpatterns/behavioral/memento/CatalogQueryState.java‎

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package com.luv2code.designpatterns.behavioral.memento;
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import java.util.LinkedHashMap;
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import java.util.LinkedHashSet;
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import java.util.Map;
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import java.util.Set;
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/**
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* Role: Originator
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*
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* Maintains the internal state of the course catalog query.
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* Responsible for creating and restoring snapshots.
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*
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* Encapsulation is preserved because only the Originator
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* can access the contents of the Memento.
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*/
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public class CatalogQueryState {
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private String searchText = "";
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private Map<String, Set<String>> filters = new LinkedHashMap<>();
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private String sortField = "name";
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private SortDirection sortDirection = SortDirection.ASCENDING;
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public void setSearchText(String theSearchText) {
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searchText = (theSearchText == null) ? "" : theSearchText;
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}
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public void addFilter(String key, String value) {
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Set<String> values = filters.get(key);
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if (values == null) {
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values = new LinkedHashSet<>();
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filters.put(key, values);
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}
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values.add(value);
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}
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public void removeFilter(String key, String value) {
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Set<String> values = filters.get(key);
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if (values != null) {
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values.remove(value);
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if (values.isEmpty()) {
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filters.remove(key);
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}
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}
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}
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public void clearFilters() {
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filters.clear();
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}
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public void setSort(String theSortField, SortDirection theSortDirection) {
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sortField = theSortField;
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sortDirection = theSortDirection;
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}
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@Override
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public String toString() {
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return "CatalogQueryState{" +
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"searchText='" + searchText + '\'' +
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", filters=" + filters +
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", sortField='" + sortField + '\'' +
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", sortDirection=" + sortDirection +
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'}';
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}
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// Add some snapshot operations
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private Map<String, Set<String>> deepCopy(Map<String, Set<String>> original) {
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Map<String, Set<String>> copy = new LinkedHashMap<>();
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for (Map.Entry<String, Set<String>> entry : original.entrySet()) {
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Set<String> valuesCopy = new LinkedHashSet<>(entry.getValue());
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copy.put(entry.getKey(), valuesCopy);
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}
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return copy;
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}
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public QuerySnapshot createSnapshot() {
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return new QuerySnapshot(
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searchText,
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deepCopy(this.filters),
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sortField,
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sortDirection
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);
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}
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public void restore(QuerySnapshot snapshot) {
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searchText = snapshot.searchText;
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filters = deepCopy(snapshot.filters);
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sortField = snapshot.sortField;
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sortDirection = snapshot.sortDirection;
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}
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/**
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* Role: Memento
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*
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* Encapsulated snapshot of CatalogQueryState.
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* The state is accessible only to the Originator.
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*/
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static class QuerySnapshot {
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private final String searchText;
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private final Map<String, Set<String>> filters;
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private final String sortField;
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private final SortDirection sortDirection;
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private QuerySnapshot(
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String searchText,
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Map<String, Set<String>> filters,
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String sortField,
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SortDirection sortDirection) {
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this.searchText = searchText;
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this.filters = filters;
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this.sortField = sortField;
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this.sortDirection = sortDirection;
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}
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}
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}
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‎section-04-behavioral-design-patterns/13-memento/src/main/java/com/luv2code/designpatterns/behavioral/memento/MainApp.java‎

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package com.luv2code.designpatterns.behavioral.memento;
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/**
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* Role: Client
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*
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* Demonstrates usage of the Memento pattern
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* for Course Catalog Query History with
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* bounded undo and redo support
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*/
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public class MainApp {
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public static void main(String[] args) {
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//------------------------------------
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// Setup
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//------------------------------------
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CatalogQueryState queryState = new CatalogQueryState();
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CatalogQueryHistory queryHistory = new CatalogQueryHistory();
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System.out.println("Initial state:");
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System.out.println(queryState);
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//------------------------------------
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// Build Query
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//------------------------------------
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queryHistory.saveBeforeChange(queryState);
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queryState.setSearchText("spring boot");
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queryHistory.saveBeforeChange(queryState);
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queryState.addFilter("level", "beginner");
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queryHistory.saveBeforeChange(queryState);
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queryState.addFilter("topic", "security");
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queryHistory.saveBeforeChange(queryState);
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queryState.setSort("rating", SortDirection.DESCENDING);
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System.out.println("\nAfter building query:");
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System.out.println(queryState);
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//------------------------------------
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// Remove a filter
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//------------------------------------
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queryHistory.saveBeforeChange(queryState);
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queryState.removeFilter("topic", "security");
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System.out.println("\nAfter removing topic=security");
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System.out.println(queryState);
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//------------------------------------
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// Clear all filters
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//------------------------------------
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queryHistory.saveBeforeChange(queryState);
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queryState.clearFilters();
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System.out.println("\nAfter clearing all filters");
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System.out.println(queryState);
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//------------------------------------
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// UNDO clear
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//------------------------------------
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boolean undoWorked = queryHistory.undo(queryState);
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System.out.println("\nAfter UNDO (restore previous state)");
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System.out.println(queryState);
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System.out.println("Undo worked: " + undoWorked);
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//------------------------------------
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// UNDO remove
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//------------------------------------
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queryHistory.undo(queryState);
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System.out.println("\nAfter UNDO (restore removed filter)");
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System.out.println(queryState);
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//------------------------------------
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// Change Sort
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//------------------------------------
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queryHistory.saveBeforeChange(queryState);
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queryState.setSort("name", SortDirection.ASCENDING);
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System.out.println("\nAfter changing sort to name ASCENDING");
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System.out.println(queryState);
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//------------------------------------
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// UNDO Sort change
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//------------------------------------
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queryHistory.undo(queryState);
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System.out.println("\nAfter UNDO (restore previous sort)");
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System.out.println(queryState);
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//------------------------------------
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// REDO
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//------------------------------------
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queryHistory.redo(queryState);
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System.out.println("\nAfter REDO");
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System.out.println(queryState);
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//------------------------------------
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// REDO clearing rule
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//------------------------------------
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queryHistory.saveBeforeChange(queryState);
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queryState.addFilter("level", "advanced");
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System.out.println("\nAfter new change (redo history cleared)");
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System.out.println(queryState);
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boolean redoWorked = queryHistory.redo(queryState);
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System.out.println("\nTry REDO after new change (should be false): " + redoWorked);
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}
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}
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‎section-04-behavioral-design-patterns/13-memento/src/main/java/com/luv2code/designpatterns/behavioral/memento/SortDirection.java‎

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package com.luv2code.designpatterns.behavioral.memento;
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/**
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* Represents the sort direction for displaying courses.
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*/
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public enum SortDirection {
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ASCENDING,
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DESCENDING
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}

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