Java Program to check Palindrome

 

Strings is such a favorite in Java Interviews, that you can expect at least one question on this topic. One of the important concept is Reversing a String, which we have covered earlier. 

Logic can be applied using existing knowledge to solve a variety of questions. We'll see the same today. 

A Palindrome is a String which if read in reverse order, is still the same. Eg. Radar, kayak etc. 

See where we are coming at?

So if you're asked to check if a String is a Palidrome, your knowledge of reversing a String can come in handy!

So, we'll simply reverse a String, and compare if it matches our current String. That's it!

For revision, our Reversal function was:

/**
 * Reverse a String using for loop
 * @author computengine.com
 * @param input
 * @return
 */
public String reverseString(String input) {
	
	System.out.println("Input String: "+input);
	
	//Create a String with no data
	String output = "";
	
	//Loop through the Input String from the last character and come to the first character. 
	for(int i=input.length()-1;i>=0;i--) {
		//store each character into the output string.
		output = output + input.charAt(i);
	}
	
	System.out.println("Reversed String: "+output);
	
	return output;
}

We'll use the same function to solve the next problem.

If you notice carefully, the revereString function is taking an Input of a String, and Returning Output as a String. That was cleverly written so that it can be reused!

Re-usability is an important concept which you'll find at the workplace. It allows us to make use of a function once written for a variety of purposes. We'll re-use this code now for our next problem. 

We had created a separate class for Reversing a String. We don't have to rewrite that code again. We'll simply create an object of the class and call the function for our use. 

ReverseString rs = new ReverseString();

Now when we call the revereString function, it will return a String to us. We need to compare that. 

Strings have a built in function for comparison. 

input.equalsIgnoreCase(rs.reverseString(input)

We're using ignore case, because we don't want our comparison to be case sensitive. If your requirement is to have a case sensitive comparison, you can use input.equals(xyz)

Now, we would want to return a return of the comparison. Here's how we can do that: 

// if a string is equal to its reverse, its a palindrome
if (input.equalsIgnoreCase(rs.reverseString(input)))
    return true;
else
    return false;

That's quite compact right?

Here is the complete code for your reference: 

package coding;

public class CheckPalindrome {

	public static void main(String[] args) {

		CheckPalindrome cp = new CheckPalindrome();

		System.out.println("Input String is Palindrome: " + cp.checkPalindrome("radar"));

	}

	/**
	 * Check if String is palindrome
	 * 
	 * @author computengine.com
	 * @param input
	 * @return
	 */
	public boolean checkPalindrome(String input) {

		ReverseString rs = new ReverseString();

		// if a string is equal to its reverse, its a palindrome
		if (input.equalsIgnoreCase(rs.reverseString(input)))
			return true;
		else
			return false;

	}

}

Running a Sample output for a Palindrome String: 

Input String: radar
Reversed String: radar
Input String is Palindrome: true

And we can try for a Non Palindrome String:

Input String: Hello
Reversed String: olleH
Input String is Palindrome: false

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