Java Hashmap Methods: Examples with Explanations

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To all Java lovers out there, yet again we meet! Now that you know what is Java 8, let’s dive deeper into the fundamentals of this bad guy. Today, we are going to discuss on one of the most monumental classes in Java; and that is Hashmap. So before we get started, what does HashMap mean exactly? Well, let us break it down for you. It can be defined in 2 terms. First of all, it is a class that is used in the process of maintaining key and value pairs. And secondly, in more simpler terms, a hashmap is a simple and very efficient way to retrieve data in Java.

What Are Java Hashmap Values

Generally, it can be denoted in the following ways:

HashMap<Key, Value> or HashMap<K, V>.

Hashmap is more or less identical to Hashtable; only differentiating from the latter with two unique exceptions.

  • First of all, the methods used in Hashmap are not synchronized therefore allowing for null values and null key which is not the case in Hashtable.
  • Furthermore, Hashmap is not comprised of an ordered collection. This means that keys and Java hashmap values inserted into the Hashmap will not be returned in a similar order to which they were initially inserted. This can only be done by importing the java.util.HashMap so that you can have access to the Hashmap classes.

Here is a prime example of the use of Hashmap in Java:

package javaassignmenthelp.net;
import java.util.HashMap;
import java.util.Map;
import java.util.Iterator;
import java.util.Set;
public class Details {
public static void main(String args[]) {
/* This is how to declare HashMap */
HashMap<Integer, String> hmap = new HashMap<Integer, String>();
/*Adding elements to HashMap*/
hmap.put(12, "John");
hmap.put(2, "Hellen");
hmap.put(7, "Rita");
hmap.put(49, "Eric");
hmap.put(3, "Stephen")
/* Display content using Iterator*/
Set set = hmap.entrySet();
Iterator iterator = set.iterator();
while(iterator.hasNext()) {
Map.Entry mentry = (Map.Entry)iterator.next();
System.out.print("key is: "+ mentry.getKey() + " & Value is: ");
System.out.println(mentry.getValue());
}
/* Get values based on key*/
String var= hmap.get(2);
System.out.println("Value at index 2 is: "+var);
/* Remove values based on key*/
hmap.remove(3);
System.out.println("Map key and values after removal:");
Set set2 = hmap.entrySet();
Iterator iterator2 = set2.iterator();
while(iterator2.hasNext()) {
Map.Entry mentry2 = (Map.Entry)iterator2.next();
System.out.print("Key is: "+mentry2.getKey() + " & Value is: ");
System.out.println(mentry2.getValue());
}
}
}

The above code will give out the output shown below:

key is: 49 & Value is: Eric
key is: 2 & Value is: Hellen
key is: 3 & Value is: Stephen
key is: 7 & Value is: Rita
key is: 12 & Value is: John
Value at index 2 is: Hellen
Map key and values after removal:
Key is: 49 & Value is: Eric
Key is: 2 & Value is: Hellen
Key is: 7 & Value is: Rita
Key is: 12 & Value is: John

Nearly all the vital Java Hashmap methods have been covered extensively in this example. However, let’s discuss a few more that are just as helpful and important.

java hashmap values

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More Java Equals Methods

  • Void Clear(). It is utilized in the removal of every key and value pair that is present in a specified Map. As shown below:
import java.util.HashMap;
public class EmptyHashMapExample {
public static void main(String[] args) {
//create HashMap object
HashMap hMap = new HashMap();
//add key value pairs to HashMap
hMap.put("1","One");
hMap.put("2","Two");
hMap.put("3","Three");
hMap.clear();
System.out.println("Total key value pairs in HashMap are : " + hMap.size());
}
}
  • Object Clone(). Object cloning is used to return the copies of a map in order to utilize them in another map. For example:
class Eric15 implements Cloneable{
int rollno;
String name;
Eric15(int rollno,String name){
this.rollno=rollno;
this.name=name;
}
public Object clone()throws CloneNotSupportedException{
return super.clone();
}
public static void main(String args[]){
try{
Eric15 s1=new Student18(101,"gene");
Eric15 s2=(Student18)s1.clone();
System.out.println(s1.rollno+" "+s1.name);
System.out.println(s2.rollno+" "+s2.name);
}catch(CloneNotSupportedException c){}
}
}
  • Boolean ContainsKey(Object Key). This is a unique boolean function whose task is to return a true or false depending on whether the key specified is actually present in the map.
  • Boolean ContainsValue(Object Value). This is a function is more or less the same as the contains key() function. Instead, it searches for the specified value in the present map instead of the key.
  • Value Get(Object Key). This function is used to return the value of the key that was specified.
  • Boolean IsEmpty(). The first thing this method does is to check if the present map is empty or not. If there are no keys or value that are present, then the function will return a true statement. If key-values are present, then the map returns a false statement (else false)
  • Set KeySet(). This function returns a section of the keys that were initially fetched from the map.
  • Value Put(Key K, Value v). As initially used in the first example, this function automatically places a key value onto the map. It is also known as the Java hashmap put method
  • Int Size(). This function automatically returns the number of key-value mappings
  • Collection values(). This method returns a set of values of the map.
  • Java Hashmap Equals Method. Hashmaps are normally compared with the use of the equals() only in the case where both the key and value objects can be compared using the same exact method.
  • Value Remove(Object Key). This function eliminates the key value pair for a given key. If you have a look at the first example, you can clearly see its use.
  • Void PutAll(Map M). This function copies one element of a map and proceeds to copy them to another specified map.

Need Help?

These are just some of the honorable mentions. As you can see, Hashmap is an important feature in Java that carries along with it a set of vital methods and functions that every starting or experienced programmer can implement in their daily coding. To find out more about Hashmap features, how to get good at utilizing them, and how they can improve your code; just get in touch with us here at JavaAssignmentHelp.net. We will be more than happy to get you started and tutor you on the basics as you excel in your development as a solid programmer.

Give us a call and let’s conquer Java hashmap methods together!