In the tutorial, JavaSampleApproach will introduce you how to work with Java HashMap
I. Concepts
Java HashMap is a construction of Map interface, so it provides all methods of Map interface.
Related post: How to work with Java Map Interface
– It permits a null key and null values
HashMap provides 3 Constructors:
– HashMap(): default initial capacity (16) and the default load factor (0.75).
– HashMap(int initialCapacity, float loadFactor)
– HashMap(Map m): constructs a new HashMap with the same mappings as the specified Map.
Note: In common case, we use empty constructor HashMap() & HashMap(Map m) constructors.
Example:
//a void (no arguments) constructor Map<Integer, String> map = new HashMap<Integer, String>(); //a constructor with a single argument of type Map Map<Integer, String> newMap = new HashMap<Integer, String>(map); |
For insert and retrieve a key-value mapping, we uses: put() & get() methods.
– put(K key, V value)
– putAll(Map m)
– get(Object key)
Example:
// PUT operators map.put(1, "one"); map.put(2, "two"); map.put(3, "three"); map.put(4, "four"); map.put(null, null); map.put(5, null); // GET operators String value1 = map.get(1); String value2 = map.get(2); |
Important! HashMap is not synchronized, so a best practice: should use it with single thread programming.
In case of multi-thread programming, we can use Collections.synchronizedMap to protect data for consistency but a performance problem will exist.
Sample:
Map m = Collections.synchronizedMap(new HashMap(...)); |
HashMap is fail-fast, it means if a map is structurally modified at any time after the iterator is created, a ConcurrentModificationException will be throwed.
Example:
for (Map.Entry<Integer, String> entry : map.entrySet()) { map.remove(entry.getKey()); } |
Exception:
Exception in thread "main" java.util.ConcurrentModificationException at java.util.HashMap$HashIterator.nextNode(Unknown Source) at java.util.HashMap$EntryIterator.next(Unknown Source) at java.util.HashMap$EntryIterator.next(Unknown Source) at com.javasampleapproach.HashMapTutorial.main(HashMapTutorial.java:35) |
II. Practice
Full Example:
package com.javasampleapproach; import java.util.HashMap; import java.util.Map; public class HashMapTutorial { public static void main(String[] args) { Map<Integer, String> map = new HashMap<Integer, String>(); // PUT operators System.out.println("===PUT Operator==="); map.put(1, "one"); map.put(2, "two"); map.put(3, "three"); map.put(4, "four"); map.put(null, null); map.put(5, null); System.out.println("map size after put: " + map.size()); // GET operators System.out.println("===GET Operator==="); String value1 = map.get(1); String value2 = map.get(2); System.out.println(value1); System.out.println(value2); System.out.println("###Loop a Map with entrySet()"); // LOOP a Map for (Map.Entry<Integer, String> entry : map.entrySet()) { System.out.println("Key : " + entry.getKey() + " Value : " + entry.getValue()); } // fail-fast case // because the Map is modified while iteration // Throw Exception System.out.println("Test FAIL-FAST!"); for (Map.Entry<Integer, String> entry : map.entrySet()) { map.remove(entry.getKey()); } } } |
Result:
===PUT Operator=== map size after put: 6 ===GET Operator=== one two ###Loop a Map with entrySet() Key : null Value : null Key : 1 Value : one Key : 2 Value : two Key : 3 Value : three Key : 4 Value : four Key : 5 Value : null Test FAIL-FAST! Exception in thread "main" java.util.ConcurrentModificationException at java.util.HashMap$HashIterator.nextNode(Unknown Source) at java.util.HashMap$EntryIterator.next(Unknown Source) at java.util.HashMap$EntryIterator.next(Unknown Source) at com.javasampleapproach.HashMapTutorial.main(HashMapTutorial.java:37) |
III. Sourcecode
Last updated on June 4, 2017.