Collections - Beginning Java 8 Language Features - page 568

The comparison operations methods deal with comparing two Map s for equality. Examples of methods in this
category are as follows:

•

boolean equals (Object o)

int hashCode()
The HashMap , LinkedHashMap , and WeakHashMap are three of the available implementation classes
for the Map interface.
The HashMap allows one null value as a key and multiple null values as the values. The following snippet of code
demonstrates how to create and use a Map . A HashMap does not guarantee any specific iteration order of entries in the Map .

•

// Create a map using HashMap as the implementation class
Map<String, String> map = new HashMap<>();

// Put an entry to the map - "John" as the key and "[phone]" as the value
map.put("John", "[phone]");

The LinkedHashMap is another implementation class for the Map interface. It stores entries in the Map using a
doubly linked list. It defines the iteration ordering as the insertion order of the entries. If you want to iterate over
entries in a Map in its insertion order, you need to use LinkedHashMap instead of HashMap as the implementation class.
Listing 12-29 demonstrates how to use a Map . Note that the methods remove() and get() return the value of a
key. If the key does not exist in the Map , they return a null . You must use the containsKey() method to check if a key
exists in a Map or use the getOrDefault() method that lets you specify the default value in case the key does not exist
in the map. The toString() method returns a well-formatted string for all entries in the Map . It places all entries inside
braces ( {} ). Each entry is formatted in key=value format. A comma separates two entries. The toString() method of
the Map returns a string like {key1=value1, key2=value2, key3=value3 ...} .

Listing 12-29. Using a Map

// MapTest.java
package com.jdojo.collections;

import java.util.HashMap;
import java.util.Map;

public class MapTest {
public static void main(String[] args) {
// Create a map and add some key-value pairs
Map<String,String> map = new HashMap<>();
map.put("John", "[phone]");
map.put("Richard", "[phone]");
map.put("Donna", "[phone]");
map.put("Ken", "[phone]");

// Print the details
printDetails(map);

// Remove all entries from the map
map.clear();
System.out.printf("%nRemoved all entries from the map.%n%n");