Initialization of a Java HashMap means creating an instance of the HashMap class and adding elements to it. The standard way to initialize a HashMap is by using the new keyword. However, we will use the lambda expression in this article. Lambda expressions are methods without any names. It was introduced with the release of Java8. And, HashMap is a class of Java Collection Framework that implements Map Interface. It stores its element in key-value pairs. The Key is an object that is used to retrieve value associated with it. Example 1 The following Java program explains how to initialize ... Read More

Read-only Hashtable would mean users won’t be able to add or remove elements from it. Let us first create a Hashtable with key-value pair −Hashtablehash = new Hashtable(); hash.put("1", "A"); hash.put("2", "B"); hash.put("3", "C"); hash.put("4", "D"); hash.put("5", "E"); hash.put("6", "F"); hash.put("7", "G");Now, use unmodifiableMap() to form a Hashtable read-only −Mapm = Collections.unmodifiableMap(hash);Example Live Demoimport java.util.Collections; import java.util.Hashtable; import java.util.Map; public class Demo {    public static void main(String[] s) {       Hashtablehash = new Hashtable();       hash.put("1", "A");       hash.put("2", "B");       hash.put("3", "C");       hash.put("4", "D");       hash.put("5", ... Read More

Let’s say the following is our Integer array −Integer arr[] = { 50, 100, 150, 200, 250, 300 };Set the above integer array in HashSet −Setset = new HashSet(Arrays.asList(arr));Now, let us check whether the HashSet contains certain value or not −set.contains(150)TRUE is returned if the value is in the List, else FALSE is the return value.Example Live Demoimport java.util.Arrays; import java.util.HashSet; import java.util.Set; public class Demo {    public static void main(String[] a) {       Integer arr[] = { 50, 100, 150, 200, 250, 300 };       Setset = new HashSet(Arrays.asList(arr));       System.out.println(set.contains(200));     ... Read More

Let’s say we have the following SortedSet −SortedSet set = new TreeSet(); set.add("T"); set.add("R"); set.add("S"); set.add("Q"); set.add("V"); set.add("U"); set.add("W");Now, after displaying the above elements with Iterator, you can insert values like this to a SortedSetset.add("Z"); set.add("Y");Example Live Demoimport java.util.Iterator; import java.util.SortedSet; import java.util.TreeSet; public class Demo {    public static void main(String[] argv) throws Exception {       SortedSet set = new TreeSet();       set.add("T");       set.add("R");       set.add("S");       set.add("Q");       set.add("V");       set.add("U");       set.add("W");       Iterator i = set.iterator();     ... Read More

To get Sub Map from TreeMap, let us first create a TreeMap and set key-value pair −TreeMaptMap = new TreeMap(); tMap.put("1", "A"); tMap.put("2", "B"); tMap.put("3", "C"); tMap.put("4", "D"); tMap.put("5", "E"); tMap.put("6", "F"); tMap.put("7", "G");Now, let’s say you need to get submap from 3 to 7. For that, use the method submap() as −=SortedMapmap = tMap.subMap("3", "7");Example Live Demoimport java.util.SortedMap; import java.util.TreeMap; public class Demo {    public static void main(String[] args) {       TreeMaptMap = new TreeMap();       tMap.put("1", "A");       tMap.put("2", "B");       tMap.put("3", "C");       tMap.put("4", "D");     ... Read More

Let’s say the following is our Map with unsorted keys −HashMapmap = new HashMap(); map.put("1", "A"); map.put("6", "B"); map.put("3", "C"); map.put("7", "D"); map.put("5", "E"); map.put("2", "F"); map.put("4", "G"); map.put("8", "H");Now, sort the above Map. The result of tMap below would be a sorted map by key −MaptMap = new TreeMap(map);Example Live Demoimport java.util.HashMap; import java.util.Map; import java.util.TreeMap; public class Demo {    public static void main(String[] args) {       HashMapmap = new HashMap();       map.put("1", "A");       map.put("6", "B");       map.put("3", "C");       map.put("7", "D");       map.put("5", "E");   ... Read More

In this article, we will convert a HashMap to a TreeMap in Java. A HashMap is used for storing data as key-value pairs, but it doesn’t keep the keys in any particular order. A TreeMap sorts the keys in ascending order. By converting a HashMap to a TreeMap, we can ensure that the keys are stored in a sorted way. Problem StatementGiven a HashMap filled with key-value pairs, we need to convert it into a TreeMap so that the entries are sorted by the keys.Input ("1", "A"), ("2", "B"), ("3", "C"), ("4", "D"), ("5", "E"), ("6", "F"), ("7", ... Read More

A read-only map in Java is a type of Map that cannot be modified after it has been created. This means that once the map has been set to be read-only, we cannot add, remove, or update any of the key-value pairs.Converting a Map to a read only map We can convert a mutable map to an immutable map, a read-only map using the Collections.unmodifiableMap() method. This method provides a way to make sure that the map cannot be modified once it has been wrapped in this method. Following are the steps to convert a Map to a read-only map ... Read More

Yes, you can set null as key in Java HashMap. For this, let’s first create a HashMap with key and value pair −Mapmap = new HashMap(); map.put("Football", "A"); map.put("Squash", "B"); map.put("Cricket", "C"); map.put("Hockey", "D"); map.put("Rugby", "E");Now, let’s add null value as key −map.put(null, "H");You can try to get the value for key as “null” −map.get(null);Example Live Demoimport java.util.HashMap; import java.util.Map; public class Demo {    public static final void main(String[] args) {       Mapmap = new HashMap();       map.put("Football", "A");       map.put("Squash", "B");       map.put("Cricket", "C");       map.put("Hockey", "D");     ... Read More

First, create a HashMap, which is to be iterated −Mapmap = new LinkedHashMap(); map.put("Jack", "0"); map.put("Tim", "1"); map.put("David", "2"); map.put("Tom", "3"); map.put("Kevin", "4");Now, use Iterator to map through keyset −Iterator iterator = map.keySet().iterator();Iterate through all the pairs −while (iterator.hasNext()) {    String resKey = (String) iterator.next();    System.out.println("Rank of " + resKey + " is " + map.get(resKey)); }Example Live Demoimport java.util.Iterator; import java.util.LinkedHashMap; import java.util.Map; public class Demo {    public static void main(String[] args) {       Mapmap = new LinkedHashMap();       map.put("Jack", "0");       map.put("Tim", "1");       map.put("David", "2");     ... Read More