fix merge issue; refactoring

java-collections-conversions/src/main/java/com/baeldung/convertToMap/ConvertToMap.java

@@ -0,0 +1,33 @@
+package com.baeldung.convertToMap;
+
+import java.util.*;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+
+public class ConvertToMap {
+    public Map<String, String> listToMap(List<Book> books) {
+        return books.stream().collect(Collectors.toMap(Book::getIsbn, Book::getName));
+    }
+
+    public Map<Integer, Book> listToMapWithDupKeyError(List<Book> books) {
+        return books.stream().collect(Collectors.toMap(Book::getReleaseYear, Function.identity()));
+    }
+
+    public Map<Integer, Book> listToMapWithDupKey(List<Book> books) {
+        return books.stream().collect(Collectors.toMap(Book::getReleaseYear, Function.identity(), (existing, replacement) -> existing));
+    }
+
+    public Map<Integer, Book> listToConcurrentMap(List<Book> books) {
+        return books.stream().collect(Collectors.toMap(Book::getReleaseYear, Function.identity(), (o1, o2) -> o1, ConcurrentHashMap::new));
+    }
+
+    public TreeMap<String, Book> listToSortedMap(List<Book> books) {
+        return books.stream()
+                .sorted(Comparator.comparing(Book::getName))
+                .collect(Collectors.toMap(Book::getName, Function.identity(), (o1, o2) -> o1, TreeMap::new));
+    }
+
+
+}
+