Java Program to Print all Unique Words of a String

Last Updated : 17 Oct, 2022

Java program to print all unique words present in the string. The task is to print all words occurring only once in the string.

Illustration:

Input  : Welcome to Geeks for Geeks.
Output : Welcome 
         to
         for
Input  : Java is great.Python is also great.
Output : Java
         Python
         also

Methods:

This can be done in the following ways:

Using nested loops
Using Map
Using frequency() method of Collections

Naive approach: Using nested loops

The idea to count the occurrence of the string in the string and print if count equals one

Extract words from string using split() method and store them in an array.
Iterate over the word array using for loop.
Use another loop to find the occurrence of the current word the array.
If the second occurrence is found increment count and set the word to ""
If the count of the current word is equal to one print it

Example:

Java

// Java Program to Print all unique words
// Using nested loops

// Main class
public class GFG {

    // Method 1
    // To print the unique words
    static void printUniqueWords(String str)
    {
        // Maintaining a count variable
        int count;

        // Extract words from string
        // using split() method
        String[] words = str.split("\\W");

        // Iterating over the words array
        for (int i = 0; i < words.length; i++) {

            // Setting count of current word to one
            count = 1;

            for (int j = i + 1; j < words.length; j++) {
                if (words[i].equalsIgnoreCase(words[j])) {

                    // If word found later in array
                    // increment the count variable
                    count++;

                    words[j] = "";
                }
            }

            // If count of current word is one print it
            if (count == 1 && words[i] != "")

                // Print statement
                System.out.println(words[i]);
        }
    }

    // Method 2
    // Main driver method
    public static void main(String[] args)
    {
        // Custom input string
        String str = "Welcome to geeks for geeks";

        // Calling the method 1 to print all unique words
        // in above string passed as argument
        printUniqueWords(str);
    }
}

Output

Welcome
to
for

Note: Time complexity is of order n^2 where space complexity is of order n

Method 2: Using Map

Approach: The idea is to use Map to keep track of words already occurred. But first, we have to extract all words from a String, as a string may contain many sentences with punctuation marks.

Create an empty Map.
Extract words from string using split() method and store them in an array.
Iterate over the word array.
Check if the word is already present in the Map or not.
If a word is present in the map, increment its value by one.
Else store the word as the key inside the map with value one.
Iterate over the map and print words whose value is equal to one.

Example:

Java

// Java Program to Print all Unique Words
// Using Map

// Importing utility classes from java.util package
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.Map;

// Main class
public class GFG {

    // Method 1
    // To print all unique words in the string
    static void printUniqueWords(String str)
    {
        // Create a new Map by creating object of HashMap
        // class
        HashMap<String, Integer> map
            = new LinkedHashMap<String, Integer>();

        // Extract words from string
        // using split() method
        String[] words = str.split("\\W");

        // Iterating over the words array
        // using for each loop
        for (String word : words) {

            // If the word is present in array then
            //
            if (map.containsKey(word)) {

                //  Increment its value by one
                // using map.get() method
                map.put(word, map.get(word) + 1);
            }

            // Else store the word inside map
            // with value one
            else
                map.put(word, 1);
        }

        // Iterate over the map using for each loop
        for (Map.Entry<String, Integer> entry :
             map.entrySet()) {

            // If value of words equals unity
            if (entry.getValue() == 1)

                // Print all those words
                System.out.println(entry.getKey());
        }
    }

    // Method 2
    // Main driver method
    public static void main(String[] args)
    {
        // Custom input string
        String str = "Welcome to geeks for geeks";

        // Calling the Method1 to
        // print all unique words in above string
        printUniqueWords(str);
    }
}

Output

Welcome
to
for

Note: Time complexity is of the order of n where space complexity is of order n. Hence, it is the optimal approach.

Method 3 : Using Collections.frequency() . If the occurrence of word in string is 1 , then the word is unique.

Java

// Java Program to Print all unique words

// Main class
import java.util.*;
public class Main{
    public static void main(String[] args)
    {
        String str = "Welcome to geeks for geeks";
        String[] words = str.split("\\W");
        List<String> al = new ArrayList<>(Arrays.asList(words));
        for(String x:al)  {
            if(Collections.frequency(al,x)==1){
                System.out.println(x);
            }
        }  
         
    }
}