Java program to print only unique characters in a string

In this article we will learn to implement a Java program to print only unique characters in a string. A Java program is provided below which reads a string from the user, and prints out only the unique characters in the string in insertion order.

First we find out the frequency of each character in the string and then print out only the characters that have frequency as 1. To maintain the insertion order, we use LinkedHashMap class from collections framework.

The program is as follows:

import java.util.*;

public class Driver
{
	public static void main(String[] args)
	{
		System.out.println("Enter input: ");
		Scanner input = new Scanner(System.in);
		String s = input.next();
		input.close();
		LinkedHashMap<Character, Integer> list = new LinkedHashMap<Character, Integer>();
		for(Character c : s.toCharArray())
		{
			if(list.containsKey(c))
			{
				list.put(c, list.get(c)+1);
			}
			else
			{
				list.put(c, 1);
			}
		}
		for(Map.Entry e: list.entrySet())
		{
			if((int)e.getValue() == 1)
				System.out.println(e.getKey());
		}
	}
}

Input and output for the above program is as follows:

Input:
Enter input: 
yaadccdekp

Output:
y
e
k
p

For printing in ascending order, use TreeMap class instead of LinkedHashMap class.

Suryateja Pericherla

Suryateja Pericherla, at present is a Research Scholar (full-time Ph.D.) in the Dept. of Computer Science & Systems Engineering at Andhra University, Visakhapatnam. Previously worked as an Associate Professor in the Dept. of CSE at Vishnu Institute of Technology, India.

He has 11+ years of teaching experience and is an individual researcher whose research interests are Cloud Computing, Internet of Things, Computer Security, Network Security and Blockchain.

He is a member of professional societies like IEEE, ACM, CSI and ISCA. He published several research papers which are indexed by SCIE, WoS, Scopus, Springer and others.