A palindrome is a word, number, or sequence that can be read the same forwards and backwards, like level, madam, and 121. Writing a palindrome program in Java is an excellent way to introduce beginners to the concepts of loops, recursion, string manipulation, and conditional logic. It also develops logical thinking and problem-solving skills using a variety of approaches, such as StringBuilder, stack, or two-pointer techniques. In this blog, you will discover how to check whether a string or a number is a palindrome using different approaches in Java.
Table of Contents:
What is a Palindrome?
A palindrome is a word, number, or sequence that reads the same when you read it from forward or backward. For example, level and 121 are considered palindromes because their sequence remains unchanged when reversed.
In Java, writing a palindrome program helps in understanding the manipulation of strings and numbers. It also helps in understanding the basic logic, conditions, and the working of the looping structure. Depending on whether you are working on a string or a number, you can write a string palindrome program in Java and a palindrome number program in Java.
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Methods to Check String Palindrome in Java
There are many methods used to determine whether a given string is a palindrome or not in Java. Each method uses a different logic but produces the same result. Let us explore the common methods that are used to write a palindrome program in Java.
1. Using Naive Approach in Java
The naive approach is the simplest way to check whether the string is a palindrome or not. In this method, we manually reverse the string and then compare it with the original string. If both the strings are equal, then it is considered a palindrome string.
Steps:
- Take an input string.
- Reverse the string with the help of a loop.
- Compare the reversed string with the original string.
- If both the strings match, print “Palindrome”.
Example:
Output:
Explanation: Here, the String is reversed with the help of a for loop, which is then checked with the original string.
2. Using Two Pointer Method in Java
The two-pointer technique is more efficient than reversing the entire string, as we have two unique pointers: one pointer is at the front of the string, and the other pointer is at the back of the string. Both pointers will move towards the center of the string, comparing the characters in each step. If the characters don’t match up, then the string is not a palindrome. The two-pointer method is faster since we don’t create a new string, therefore using less memory.
Example:
Output:
Explanation: Here, i and j are used as two pointers to check whether the string is a palindrome or not.
3. Using Recursive Method in Java
The recursive method is an optimal method for checking if a string is a palindrome. It works by comparing the first and the last characters and then calling the same function for the remaining part of the string.
Example:
Output:
Explanation: Here, a recursive function is used to check whether the given string is a palindrome or not, which is done by calling the function itself.
4. Using Stack in Java
A stack works on the LIFO principle, which stands for Last In, First Out. When we push all the characters of the strings and pop them one by one, we get the reversed version of the string. This makes the stack one of the best ways to check whether the string is a palindrome or not.
Example:
Output:
Explanation: Here, the stack is used to reverse the strings.
5. Using StringBuilder in Java
The StringBuilder is used to make string manipulation easy in Java. It contains a built-in reverse() function, which is used to reverse the order of the string.
Example:
Output:
Explanation: Here, the reverse() function is used to change the order of the string, which helps in finding out whether the string is a palindrome or not.
Methods to Check a Number Palindrome in Java
Numbers can also be checked for palindromes, which is very similar to strings. A palindrome number is a number that remains the same when reversed. For example, 121, 1331, and 1221.
1. Using Iterative Method in Java
The iterative approach is commonly used to check palindrome numbers. It uses loops to reverse the digits of a number and then compare the reverse value with the original number.
Example:
Output:
Explanation: In this code, a while loop is used for checking the conditions and reversing the digits.
2. Using Recursive Method in Java
The recursive method is used to check whether the number is a palindrome or not. In this method, the function calls itself to reverse the digits step by step.
Example:
Output:
Explanation: Here, a recursive function is used to check whether the given number is a palindrome or not, which is done by calling the function itself.
3. Using BigInteger Palindrome Detection in Java
When you are working with large numbers that cannot be fit in the range of regular integer types. In this case, you can use the BigInteger class in Java. It allows managing the numbers with unlimited length, and this method is very useful for checking the big values for palindromes.
Example:
Output:
Explanation: Here, the BigInteger is converted to a string, and with the help of a StringBuilder, it is reversed.
4. Using Scanner to Find Palindrome in Java
In Java, the Scanner class is utilized for obtaining input from users. This way, a user can input any number, and then the program will check whether the input number is a palindrome or not. This approach makes the palindrome program in Java using Scanner more flexible and interactive.
Output:
Explanation: Here, the Scanner class receives the user input and assigns it to a variable. The loop will reverse the digits of this variable and then check to see if it is equal to the original number to determine if it is a palindrome or not.
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Time and Space Complexity Analysis
| Method |
Description |
Time Complexity |
Space Complexity |
| Naive / Iterative Method |
This method is used to reverse a string or a number and then compare it with the original one. |
Time Complexity: O(n). |
Space Complexity: O(n) for string, O(1) for number. |
| Two Pointer Method |
This method uses two pointers from both ends, which are used to compare the characters with each other directly. |
Time Complexity is: O(n). |
Space Complexity is: O(1). |
| Recursive Method |
It uses function calls to compare the first and last characters repeatedly. |
Time Complexity is: O(n). |
Space Complexity is: O(n). |
| Stack Method |
It pushes all characters into a stack, then pops them to form a reverse string. |
Time Complexity is: O(n). |
Space Complexity is: O(n). |
| StringBuilder Method |
It uses StringBuilder.reverse() to reverse and compare the string. |
Time Complexity is: O(n). |
Space Complexity is: O(n). |
| BigInteger Palindrome |
It is used to convert a large number to a string and reverse it using StringBuilder. |
Time Complexity is: O(n). |
Space Complexity is: O(n). |
Best Practices for Palindrome Program in Java
- Utilize Meaningful Variable Names: It is always a good practice to name variables in a clear and meaningful way. For instance, instead of the standard single-letter variables such as i, j, use more descriptive variable names such as original, reversed, start, and end.
- Trim and Normalize Input: Normalize the input by removing extra spaces, and then check the input against the palindrome condition while converting the input to all lowercase characters.
- Utilize Built-in Methods: You can reverse the strings with less manual looping and shorter code using the built-in methods of Java.
- Check for Input Validation: Always validate the input and determine whether the user entered a valid input or not.
Common Mistakes to Avoid in Palindrome Program in Java
- Ignoring Case Sensitivity: One of such mistakes is comparing characters without changing them to the same case. Always make the input in lower or upper case, then check a palindrome.
- Failure to Eliminate Spaces or Special Characters: Make sure to remove the extra spaces, commas, and symbols when you are working with strings, as ignoring these may affect your output.
- Incorrect Loop Conditions: Beginners who are new to programming often make mistakes in applying the loop conditions in the wrong way. Always ensure that the loop runs properly from the start index to the end index.
- Using the Wrong Data Type for Large Numbers: When checking the palindrome for a number, using int for a large number can cause overflow. Always use long or BigInteger.
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Conclusion
Getting started with Palindrome programs in Java is the best way to improve your programming skills and logical thinking because you get to learn how strings, numbers, loops, recursion, and built-in Java functions work. Practicing different methods thoroughly helps you to write a clean and efficient palindrome program in Java.
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Palindrome Program in Java- FAQs
Q1. What data types can be used to check for palindrome numbers in Java?
You can use int, long, or BigInteger depending on the size of the number. BigInteger is ideal for handling very large values that exceed the range of primitive types.
Q2. Can a single character or a blank string be considered a palindrome?
Yes, both a single character and an empty string are considered palindromes because they read the same forward and backward.
Q3. How can we handle case sensitivity when checking string palindromes?
You can convert the input string to lowercase or uppercase before checking, using str.toLowerCase() to ensure accurate comparison.
Q4. Is it possible to check for palindrome words within a sentence in Java?
Yes, by splitting the sentence using spaces or punctuation and then checking each word separately to see if it is a palindrome.
Q5. What is the most efficient approach for checking a palindrome string in Java?
The two-pointer method is the most efficient as it uses less memory and compares characters directly without creating a reversed string.