Hello!

I'm new to this site and fairly new to Java, I've tried creating a program that takes a file as input and edits specific substrings in said file, compiles without problems but the file remains unchanged.

import java.io.*;

public class Uppg10_OH  {
    static class MinHistoria  {
        private String namnInfil;
        private String gammaltNamn;
        private String nyttNamn;
        
        public MinHistoria(String inFilNamn, String gNamn, String nNamn) {
        
            namnInfil       = inFilNamn;
            gammaltNamn     = gNamn;
            nyttNamn        = nNamn;
        }

        public void transformera() throws IOException {
    
            try {
             
                FileInputStream stream = new FileInputStream("C:\\test.txt");
                DataInputStream in = new DataInputStream(stream);
                BufferedReader br = new BufferedReader( new InputStreamReader(in));
            
                String stringRad;
                while ((stringRad = br.readLine()) != null) {
                    if (stringRad.contains(gammaltNamn)) {
                        stringRad.replaceAll(gammaltNamn, nyttNamn);
                    }
                
                }
                in.close();
                br.close();
                
            }   
            catch (Exception e) {
                System.err.println("Error: " + e.getMessage());
            }
        }
        
        public static void main (String args[]) throws IOException {
                  MinHistoria mh = new MinHistoria("C:\\test.txt", "cat", "dog");
                  mh.transformera();
        }
    }
    
}

I'm not quite sure where the problem lies, any help would be greatly appreciated!

I would like to save it in a different file, haven't yet gotten quite that far.
Should have been clear on this, sorry!

You read in the data, change it in memory, then exit. Until you write the code to write the changed data to a file you will not see any changes.
You can check your code so far by printing stringRad at line 29

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