refactor: separate collection state from UI navigation

app/src/main/java/ru/project/DataCollection.java

@@ -0,0 +1,47 @@
+package ru.project;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+public class DataCollection {
+
+  private final List<String> elements = new ArrayList<>();
+
+  public void add(String element) {
+    elements.add(element);
+  }
+
+  public void sort() {
+    Collections.sort(elements);
+  }
+
+  public boolean contains(String element) {
+    return elements.contains(element);
+  }
+
+  public int count(String element) {
+    int count = 0;
+
+    for (String current : elements) {
+      if (current.equals(element)) {
+        count++;
+      }
+    }
+
+    return count;
+  }
+
+  public List<String> getElements() {
+    return Collections.unmodifiableList(elements);
+  }
+
+  @Override
+  public String toString() {
+    if (elements.isEmpty()) {
+      return "Коллекция пуста";
+    }
+
+    return String.join(", ", elements);
+  }
+}