BAEL 3131 guide to java hash map (#7758)

* BAEL-3131 Guide to Java HashMap

* Update java-collections-maps-2/src/test/java/com/baeldung/map/ProductUnitTest.java

Co-Authored-By: KevinGilmore <kpg102@gmail.com>

java-collections-maps-2/README.md

@@ -1,6 +1,7 @@
 ## Relevant Articles:
 - [Map of Primitives in Java](https://www.baeldung.com/java-map-primitives)
 - [Copying a HashMap in Java](https://www.baeldung.com/java-copy-hashmap)
+- [Guide to Java HashMap]()
 - [Guide to WeakHashMap in Java](https://www.baeldung.com/java-weakhashmap)
 - [Map to String Conversion in Java](https://www.baeldung.com/java-map-to-string-conversion)
 - [Iterate over a Map in Java](https://www.baeldung.com/java-iterate-map)

java-collections-maps-2/src/main/java/com/baeldung/map/Product.java

@@ -0,0 +1,133 @@
+package com.baeldung.map;
+
+import java.util.*;
+
+public class Product {
+
+    private String name;
+    private String description;
+    private List<String> tags;
+
+    public Product(String name, String description) {
+        this.name = name;
+        this.description = description;
+        this.tags = new ArrayList<>();
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public String getDescription() {
+        return description;
+    }
+
+    public List<String> getTags() {
+        return tags;
+    }
+
+    public Product addTagsOfOtherProdcut(Product product) {
+        this.tags.addAll(product.getTags());
+        return this;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+
+        if (this == o) {
+            return true;
+        }
+        if (o == null || getClass() != o.getClass()) {
+            return false;
+        }
+
+        Product product = (Product) o;
+        return Objects.equals(name, product.name) &&
+                Objects.equals(description, product.description);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(name, description);
+    }
+
+    public static void forEach() {
+
+        HashMap<String, Product> productsByName = new HashMap<>();
+        productsByName.forEach( (key, product)
+                        -> System.out.println("Key: " + key + " Product:" + product.getDescription())
+                //do something with the key and value
+        );
+
+        //Prior to Java 8:
+        for(Map.Entry<String, Product> entry : productsByName.entrySet()) {
+            Product product =  entry.getValue();
+            String key = entry.getKey();
+            //do something with the key and value
+        }
+    }
+
+    public static void getOrDefault() {
+
+        HashMap<String, Product> productsByName = new HashMap<>();
+        Product chocolate = new Product("chocolate", "something sweet");
+        Product defaultProduct = productsByName.getOrDefault("horse carriage", chocolate);
+        Product bike = productsByName.getOrDefault("E-Bike", chocolate);
+
+        //Prior to Java 8:
+        Product bike2 = productsByName.containsKey("E-Bike")
+                ? productsByName.get("E-Bike")
+                : chocolate;
+        Product defaultProduct2 = productsByName.containsKey("horse carriage")
+                ? productsByName.get("horse carriage")
+                : chocolate;
+    }
+
+    public static void putIfAbsent() {
+
+        HashMap<String, Product> productsByName = new HashMap<>();
+        Product chocolate = new Product("chocolate", "something sweet");
+        productsByName.putIfAbsent("E-Bike", chocolate);
+
+        //Prior to Java 8:
+        if(productsByName.containsKey("E-Bike")) {
+            productsByName.put("E-Bike", chocolate);
+        }
+    }
+
+    public static void merge() {
+
+        HashMap<String, Product> productsByName = new HashMap<>();
+        Product eBike2 = new Product("E-Bike", "A bike with a battery");
+        eBike2.getTags().add("sport");
+        productsByName.merge("E-Bike", eBike2, Product::addTagsOfOtherProdcut);
+
+        //Prior to Java 8:
+        if(productsByName.containsKey("E-Bike")) {
+            productsByName.get("E-Bike").addTagsOfOtherProdcut(eBike2);
+        } else {
+            productsByName.put("E-Bike", eBike2);
+        }
+    }
+
+    public static void compute() {
+
+        HashMap<String, Product> productsByName = new HashMap<>();
+        Product eBike2 = new Product("E-Bike", "A bike with a battery");
+
+        productsByName.compute("E-Bike", (k,v) -> {
+            if(v != null) {
+                return v.addTagsOfOtherProdcut(eBike2);
+            } else {
+                return eBike2;
+            }
+        });
+
+        //Prior to Java 8:
+        if(productsByName.containsKey("E-Bike")) {
+            productsByName.get("E-Bike").addTagsOfOtherProdcut(eBike2);
+        } else {
+            productsByName.put("E-Bike", eBike2);
+        }
+    }
+}

java-collections-maps-2/src/test/java/com/baeldung/map/ProductUnitTest.java

@@ -0,0 +1,124 @@
+package com.baeldung.map;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.HashMap;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class ProductUnitTest {
+
+
+    @Test
+    public void getExistingValue() {
+        HashMap<String, Product> productsByName = new HashMap<>();
+
+        Product eBike = new Product("E-Bike", "A bike with a battery");
+        Product roadBike = new Product("Road bike", "A bike for competition");
+
+        productsByName.put(eBike.getName(), eBike);
+        productsByName.put(roadBike.getName(), roadBike);
+
+        Product nextPurchase = productsByName.get("E-Bike");
+
+        assertEquals("A bike with a battery", nextPurchase.getDescription());
+    }
+
+    @Test
+    public void getNonExistingValue() {
+        HashMap<String, Product> productsByName = new HashMap<>();
+
+        Product eBike = new Product("E-Bike", "A bike with a battery");
+        Product roadBike = new Product("Road bike", "A bike for competition");
+
+        productsByName.put(eBike.getName(), eBike);
+        productsByName.put(roadBike.getName(), roadBike);
+
+        Product nextPurchase = productsByName.get("Car");
+
+        assertNull(nextPurchase);
+    }
+
+    @Test
+    public void getExistingValueAfterSameKeyInsertedTwice() {
+        HashMap<String, Product> productsByName = new HashMap<>();
+
+        Product eBike = new Product("E-Bike", "A bike with a battery");
+        Product roadBike = new Product("Road bike", "A bike for competition");
+        Product newEBike = new Product("E-Bike", "A bike with a better battery");
+
+        productsByName.put(eBike.getName(), eBike);
+        productsByName.put(roadBike.getName(), roadBike);
+        productsByName.put(newEBike.getName(), newEBike);
+
+        Product nextPurchase = productsByName.get("E-Bike");
+
+        assertEquals("A bike with a better battery", nextPurchase.getDescription());
+    }
+
+    @Test
+    public void getExistingValueWithNullKey() {
+        HashMap<String, Product> productsByName = new HashMap<>();
+
+        Product defaultProduct = new Product("Chocolate", "At least buy chocolate");
+
+        productsByName.put(null, defaultProduct);
+        productsByName.put(defaultProduct.getName(), defaultProduct);
+
+        Product nextPurchase = productsByName.get(null);
+        assertEquals("At least buy chocolate", nextPurchase.getDescription());
+
+        nextPurchase = productsByName.get("Chocolate");
+        assertEquals("At least buy chocolate", nextPurchase.getDescription());
+    }
+
+    @Test
+    public void insertSameObjectWithDifferentKey() {
+        HashMap<String, Product> productsByName = new HashMap<>();
+
+        Product defaultProduct = new Product("Chocolate", "At least buy chocolate");
+
+        productsByName.put(null, defaultProduct);
+        productsByName.put(defaultProduct.getName(), defaultProduct);
+
+        assertSame(productsByName.get(null), productsByName.get("Chocolate"));
+    }
+
+    @Test
+    public void checkIfKeyExists() {
+        HashMap<String, Product> productsByName = new HashMap<>();
+
+        Product eBike = new Product("E-Bike", "A bike with a battery");
+
+        productsByName.put(eBike.getName(), eBike);
+
+        assertTrue(productsByName.containsKey("E-Bike"));
+    }
+
+    @Test
+    public void checkIfValueExists() {
+        HashMap<String, Product> productsByName = new HashMap<>();
+
+        Product eBike = new Product("E-Bike", "A bike with a battery");
+
+        productsByName.put(eBike.getName(), eBike);
+
+        assertTrue(productsByName.containsValue(eBike));
+    }
+
+    @Test
+    public void removeExistingKey() {
+        HashMap<String, Product> productsByName = new HashMap<>();
+
+        Product eBike = new Product("E-Bike", "A bike with a battery");
+        Product roadBike = new Product("Road bike", "A bike for competition");
+
+        productsByName.put(eBike.getName(), eBike);
+        productsByName.put(roadBike.getName(), roadBike);
+
+        productsByName.remove("E-Bike");
+
+        assertNull(productsByName.get("E-Bike"));
+    }
+
+}