Thread: Need help with replace function

My code(pasted below) basically replaces A, B, C with C, P, Q.
My new requirement is, I am going to create a file and then it has to replace the A, B, C of file content with C, P, Q. How do I do it?
for example: let's say the name of the file is the filename.txt.So what changes do I have to make to existing code, please help

public class replacefunction1 {
 
 
 
	public static void main(String[] args) {
		// TODO Auto-generated method stub
 
		 File file = new File("filename.txt"); 
 
 
 
	    HashMap<Character, Character> replacements = new HashMap<>();
	    replacements.put('a', 'c');
	    replacements.put('b', 'p');
	    replacements.put('c', 'q');
 
	    String input =  "abcd";
	    StringBuilder output = new StringBuilder();
	    for (Character c : input.toCharArray()) {
	        output.append(replacements.getOrDefault(c, c));
	    }
	    System.out.println(output.toString());
 
 
 
	}