Java HashMap key value storage and retrieval
101,723
Solution 1
I use these three ways to iterate a map. All methods (keySet
, values
, entrySet
) return a collection.
// Given the following map
Map<KeyClass, ValueClass> myMap;
// Iterate all keys
for (KeyClass key : myMap.keySet())
System.out.println(key);
// Iterate all values
for (ValueClass value : myMap.values())
System.out.println(value);
// Iterate all key/value pairs
for (Entry<KeyClass, ValueClass> entry : myMap.entrySet())
System.out.println(entry.getKey() + " - " + entry.getValue());
Since Java 8 i often use Stream
s with lambda expressions.
// Iterate all keys
myMap.keySet().stream().forEach(key -> System.out.println(key));
// Iterate all values
myMap.values().parallelStream().forEach(value -> System.out.println(value));
// Iterate all key/value pairs
myMap.entrySet().stream().forEach(entry -> System.out.println(entry.getKey() + " - " + entry.getValue()));
Solution 2
map.keySet()
would give you all the keys
Solution 3
//import statements
import java.util.HashMap;
import java.util.Iterator;
import java.util.TreeMap;
// hashmap test class
public class HashMapTest {
public static void main(String args[]) {
HashMap<Integer,String> hashMap = new HashMap<Integer,String>();
hashMap.put(91, "India");
hashMap.put(34, "Spain");
hashMap.put(63, "Philippines");
hashMap.put(41, "Switzerland");
// sorting elements
System.out.println("Unsorted HashMap: " + hashMap);
TreeMap<Integer,String> sortedHashMap = new TreeMap<Integer,String>(hashMap);
System.out.println("Sorted HashMap: " + sortedHashMap);
// hashmap empty check
boolean isHashMapEmpty = hashMap.isEmpty();
System.out.println("HashMap Empty: " + isHashMapEmpty);
// hashmap size
System.out.println("HashMap Size: " + hashMap.size());
// hashmap iteration and printing
Iterator<Integer> keyIterator = hashMap.keySet().iterator();
while(keyIterator.hasNext()) {
Integer key = keyIterator.next();
System.out.println("Code=" + key + " Country=" + hashMap.get(key));
}
// searching element by key and value
System.out.println("Does HashMap contains 91 as key: " + hashMap.containsKey(91));
System.out.println("Does HashMap contains India as value: " + hashMap.containsValue("India"));
// deleting element by key
Integer key = 91;
Object value = hashMap.remove(key);
System.out.println("Following item is removed from HashMap: " + value);
}
}
Solution 4
You can use keySet()
to retrieve the keys.
You should also consider adding typing in your Map, e.g :
Map<Integer, String> hm = new HashMap<Integer, String>();
hm.put(1,"godric gryfindor");
hm.put(2,"helga hufflepuff");
hm.put(3,"rowena ravenclaw");
hm.put(4,"salazaar slytherin");
Set<Integer> keys = hm.keySet();
Solution 5
public static void main(String[] args) {
HashMap<String, String> hashmap = new HashMap<String, String>();
hashmap.put("one", "1");
hashmap.put("two", "2");
hashmap.put("three", "3");
hashmap.put("four", "4");
hashmap.put("five", "5");
hashmap.put("six", "6");
Iterator<String> keyIterator = hashmap.keySet().iterator();
Iterator<String> valueIterator = hashmap.values().iterator();
while (keyIterator.hasNext()) {
System.out.println("key: " + keyIterator.next());
}
while (valueIterator.hasNext()) {
System.out.println("value: " + valueIterator.next());
}
}
![Arjun K P](https://i.stack.imgur.com/mcKCF.jpg?s=256&g=1)
Comments
-
Arjun K P almost 2 years
I want to store values and retrieve them from a Java HashMap.
This is what I have so far:
public void processHashMap() { HashMap hm = new HashMap(); hm.put(1,"godric gryfindor"); hm.put(2,"helga hufflepuff"); hm.put(3,"rowena ravenclaw"); hm.put(4,"salazaar slytherin"); }
I want to retrieve all Keys and Values from the HashMap as a Java Collection or utility set (for example
LinkedList
).I know I can get the value if I know the key, like this:
hm.get(1);
Is there a way to retrieve key values as a list?
-
Arjun K P about 12 yearsthanks for ur reply bro.....so is it possible for me to typecast the return value into a LinkedList........??
-
user3339147 over 10 yearsA complete example on basic operations of HashMap
-
Willie Cheng about 8 yearsplease follow this URL it will be useful to lift your content quality up
-
jmj over 6 yearsreturned value is one of the type of
Set
, so not directly but you can construct from it