BAEL-2936

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

@@ -0,0 +1,48 @@
+package com.baeldung.convertToMap;
+
+public class Book {
+    private String name;
+    private int releaseYear;
+    private String isbn;
+
+
+    @Override
+    public String toString() {
+        return "Book{" +
+                "name='" + name + '\'' +
+                ", releaseYear=" + releaseYear +
+                ", isbn='" + isbn + '\'' +
+                '}';
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public int getReleaseYear() {
+        return releaseYear;
+    }
+
+    public void setReleaseYear(int releaseYear) {
+        this.releaseYear = releaseYear;
+    }
+
+    public String getIsbn() {
+        return isbn;
+    }
+
+    public void setIsbn(String isbn) {
+        this.isbn = isbn;
+    }
+
+    public Book(String name, int releaseYear, String isbn) {
+        this.name = name;
+        this.releaseYear = releaseYear;
+        this.isbn = isbn;
+    }
+}
+

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

@@ -0,0 +1,34 @@
+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(),
+                (o1, o2) -> o1));
+    }
+
+    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));
+    }
+
+
+}
+

java-collections-conversions/src/test/java/com/baeldung/convertToMap/ConvertToMapUnitTest.java

@@ -0,0 +1,50 @@
+package com.baeldung.convertToMap;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.ConcurrentHashMap;
+
+import static org.junit.Assert.*;
+
+import org.junit.Before;
+import org.junit.Test;
+
+
+public class ConvertToMapUnitTest {
+
+    private List<Book> bookList;
+    private ConvertToMap convertToMap = new ConvertToMap();
+
+    @Before
+    public void init() {
+        bookList = new ArrayList<>();
+        bookList.add(new Book("The Fellowship of the Ring", 1954, "0395489318"));
+        bookList.add(new Book("The Two Towers", 1954, "0345339711"));
+        bookList.add(new Book("The Return of the King", 1955, "0618129111"));
+    }
+
+    @Test
+    public void whenConvertFromListToMap() {
+        assertTrue(convertToMap.listToMap(bookList).size() == 3);
+    }
+
+    @Test(expected = IllegalStateException.class)
+    public void whenMapHasDuplicateKey_without_merge_function_then_runtime_exception() {
+        convertToMap.listToMapWithDupKeyError(bookList);
+    }
+
+    @Test
+    public void whenMapHasDuplicateKey_with_merge_function() {
+        assertTrue(convertToMap.listToMapWithDupKey(bookList).size() == 2);
+    }
+
+    @Test
+    public void whenCreateConcurrentHashMap() {
+        assertTrue(convertToMap.listToConcurrentMap(bookList) instanceof ConcurrentHashMap);
+    }
+
+    @Test
+    public void whenMapisSorted() {
+        assertTrue(convertToMap.listToSortedMap(bookList).firstKey().equals("The Fellowship of the Ring"));
+    }
+}