Find Duplicate Strings in a List Using Java | Java Interview Question

In this article, we will see how to find duplicate strings and their counts from an array or a list using Java. This is one of the common programming questions asked in technical interviews.

In this program, we will use both Map and List, so it is also a good example for understanding Java Collections. You can find a few more Java Collection interview questions.

Today, we will see how to find duplicate strings in a list and count how many times each string is repeated.

Logic

The logic is quite simple:

• First, we create a Map to store the key-value pair. The key will be the array element, and the value will be the number of times that element occurs.

• Then, we iterate through the array or list and add each element to the Map. If the element is already present in the Map, we increase its count by 1.

• After completing the iteration, the Map will contain each string along with the number of times it occurs.

• Finally, we iterate through the Map and check for entries where the count is greater than 1. These are the duplicate or repeated strings.

CountDuplicate.java

package com.techbyteslearn.lab.basic;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;

public class CountDuplicate {
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<String>(
Arrays.asList("JDG", "AA", "AA", "JAVA", "JavaScript", "Java", "Stream", "hibernate", "Hibernate"));
System.out.println("Input List = " + list);
Map<String, Integer> map = new HashMap<String, Integer>();
for (int i = 0; i < list.size(); i++) {
if (map.isEmpty()) {
map.put(list.get(i).toUpperCase(), 1);
} else if (map.containsKey(list.get(i).toUpperCase())) {
map.put(list.get(i).toUpperCase(), map.get(list.get(i).toUpperCase()) + 1);
} else {
map.put(list.get(i).toUpperCase(), 1);
}
}
int counter = 0;
for (Entry<String, Integer> entry : map.entrySet()) {
if (entry.getValue() > 1) {
counter++;
System.out.println("String Found " + entry.getKey() + " with count " + entry.getValue());
}
}
System.out.println("Total Duplicate String - " + counter);
}
}

Output

Input List = [JDG, AA, AA, JAVA, JavaScript, Java, Stream, hibernate, Hibernate]

String Found: AA with count 2
String Found: JAVA with count 2
String Found: HIBERNATE with count 2

Total Duplicate Strings: 3

 

Happy Learning!

Reverse a String in Java Using Java API

Reverse a string using java existing API is really very easy. Below example will show you how to reverse the string using StringBuilder class.

package com.techbyteslearn.www;

public class ReverseString {

    /**
     * @param args
     */
    public static void main(String[] args) {

        String inputString = "javadevelopersguide";

        System.out.println("Before Reverse:: " + inputString);

        // You can do this using StringBuffer also, based on your requirement.
        // If you are concerned about thread safety, use StringBuffer.
        StringBuilder builder = new StringBuilder(inputString).reverse();

        // After reverse using StringBuilder
        inputString = builder.toString();

        System.out.println("After Reverse:: " + inputString);
    }
}
​

Output :-

Before Reverse::javadevelopersguide

After Reverse:: ediugsrepolevedavaj