Java HashMap Example

One of my favorite of all the Collections classes of Java is the HashMap. Its easy to use and easy to understand. HashMap is actually a hash table implementation of the Map interface. The HashMap class is equivalent of HashTable only that the HashMap allows null values and keys. Moreover hashmap is unsynchronized compared to HashTable. Since this is a lot faster because it is not synchronized however it would not be thread safe.

In dealing with this class, always remember that the order of elements is changing over time. In other words the placement of element on this pool of objects is dispersed.

Now that we have described the HashMap let’s take a look on a simple example.

Basic Example of HashMap

package com.javatutorialhq.java.examples;

import java.util.HashMap;

/*
 * Basic HashMap example 
 * 
 */


public class HashMapBasicExample {

	public static void main(String[] args) {
		
		// initialization of our map
		HashMap<String, String> student = new HashMap<>();
		
		// adding elements to our hashmap
		student.put("age", "32");
		student.put("gender","male");
		student.put("name","Ryan");
		student.put("birthday","May 12, 1982");
		
		// getting the contents of our hashmap
		System.out.println("Name:"+student.get("name"));
		System.out.println("Name:"+student.get("age"));
		System.out.println("Name:"+student.get("gender"));
		System.out.println("Name:"+student.get("birthday"));
		
	}

}

The above example will yield on the following result

Name:Ryan
Name:32
Name:male
Name:May 12, 1982

The above example is very simple though the following lessons has been learned:

• How to instantiate a hashmap. Specifying the type of key value pair
• Its also been shown how to add elements on the hashmap using the put method.
• The get() method were used to access the corresponding value of the specified key argument.

Now that we have know the basics of dealing with HashMap, lets try a more concrete example

HashMap of Objects

This example deals with the use of hashmap in storing of objects. There are a lot of things that happen to this example but to give you an overview here’s what it have.

• Create a Student class which takes a string on its constructor.
• The constructor must able to parse the string into tokens and assign it using the setters and getters of the Student class.
• On the main Class create a init() method that instantiate objects of Students by calling its constructors
• The init method should be able to return a HashMap with id as the key in getting Student values.
• Print all the id number on the hashmap and then the student age with respect to the id number.

With the above requirements, please try to create the implementation and compare below for my solution

Here is the Student Class

package com.javatutorialhq.java.examples.utilities;

import java.util.StringTokenizer;

public class Student {

	public Student(String s) {
		/*
		 * Pattern is id|name|bday|age|course
		 */

		// initializer our tokenizer
		StringTokenizer st = new StringTokenizer(s, "|");
		// set the values
		this.setStudentid(st.nextToken());
		this.setName(st.nextToken());
		this.setBirthday(st.nextToken());
		this.setAge(st.nextToken());
		this.setCourse(st.nextToken());

	}

	public Student() {
		super();
	}

	String name;

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public String getAge() {
		return age;
	}

	public void setAge(String age) {
		this.age = age;
	}

	public String getBirthday() {
		return birthday;
	}

	public void setBirthday(String birthday) {
		this.birthday = birthday;
	}

	public String getCourse() {
		return course;
	}

	public void setCourse(String course) {
		this.course = course;
	}

	public String getGender() {
		return gender;
	}

	public void setGender(String gender) {
		this.gender = gender;
	}

	public String getStudentid() {
		return studentid;
	}

	public void setStudentid(String studentid) {
		this.studentid = studentid;
	}

	String age;
	String birthday;
	String course;
	String gender;
	String studentid;

}

And below is our Main Class

package com.javatutorialhq.java.examples;

import java.util.HashMap;

import com.javatutorialhq.java.examples.utilities.Student;

/*
 * Example of Advance use of hashmap
 * 
 */

public class HashMapObjectExample {

	public static void main(String[] args) {

		HashMap<String, Student> map = new HashMap<String, Student>();
		map = init();
		// iterate to all the keys stored on our hashmap
		for (String s : map.keySet()) {
			// print the student id and age
			System.out.println("Student with id number:" + s + " is "
					+ map.get(s).getAge() + " years old");
		}
	}

	private static HashMap<String, Student> init() {
		// initialize our student objects
		Student s1 = new Student(
				"100212554|Gary Valenciano|January 10, 1993|21|BS Math");
		Student s2 = new Student(
				"101292574|Vic Castro|May 10, 1995|18|Mechaninal Engineering");
		Student s3 = new Student(
				"201212444|Joel Uy|February 17, 1994|20|BS Education");
		Student s4 = new Student(
				"172324448|Louise Valenciano|December 10, 1985|29|Medicine");

		// initialize of hashmap
		HashMap<String, Student> map = new HashMap<String, Student>();

		// assign each student id as key and the student objects as values on
		// our hashmap
		map.put(s1.getStudentid(), s1);
		map.put(s2.getStudentid(), s2);
		map.put(s3.getStudentid(), s3);
		map.put(s4.getStudentid(), s4);

		return map;
	}

}

Sample Output:

Running the above java example will give the following console output:

HashMap of Objects

There are a lot of things that can be learned from the above example. It looks very complicated, but it is very easy because we only applied the basics of Java. Let’s take a look below on the knowledge that we have used:

• The example above has the following code snippet

  HashMap<String, Student> map = new HashMap<String, Student>();

Basically we just instantiate a hashmap that takes a String object as its keys and a Student as its values. This is basic of Generics. We have used Generics to avoid mistake in passing arguments to our HashMap. To make it short this HashMap wont accept Integer or any other Object type as its keys. Moreover the value will not accept any other object other than Student object or its subclass.

• The use of the constructor to initialize the values of the student class. This is very simple idea, pass a string to the constructor and then use StringTokenizer to broken down the string argument into tokes.
• Use of the setters and getters to set and access the attributes of Student.
• We have also used one helpful method of the HashMap class which is the keySet(). This method returns a Set which corresponds to the keys on our HashMap. We then use these keys to get the values on our HashMap and eventually we now have access to all the attributes of Student object.

The following are the detailed list of HasMap methods and descriptions. We have also provided links to examples of each method on the list.