string - Java replace method problems -

i trying project computer science class called "encryption/decryption"

the code goes follows

import java.io.*;  import java.util.*;  import java.io.*;  import java.util.*;   public class tester {  public static void main(string args[]) {      scanner kbreader = new scanner(system.in);      system.out.print("enter sentence encrypted: ");      string sntnc = kbreader.nextline();      system.out.println("original sentence = " + sntnc);       crypto mycryptobj = new crypto();      string encryptdsntnc = mycryptobj.encrypt(sntnc);      system.out.println("encrypted sentence = " + encryptdsntnc);       string decryptdsntnc = mycryptobj.decrypt(encryptdsntnc);      system.out.println("decrypted sentence = " + decryptdsntnc);    }  }   class crypto {   public string encrypt(string sntnc) {      sntnc = sntnc.replace("m", "ssad");      sntnc = sntnc.replace("b", "dug>?/");     sntnc = sntnc.replace("g", "jeb..w");      sntnc = sntnc.replace("v", "ag',r");      return sntnc;  }   public string decrypt(string sntnc) {      sntnc = sntnc.replace("ag',r", "v");      sntnc = sntnc.replace("ssad", "m");      sntnc = sntnc.replace("jeb..w", "g");       sntnc = sntnc.replace("dug>?/", "b");      return sntnc;    }  } 

the problem isn't in tester class, it's in crypto class.

the input is: this big morning. , code should output:

enter sentence encrypted: big morning. original sentence: big morning. encrypted sentence: ag',rery dug>?/ijeb..w ssadorninjeb..w. decrypted sentence: big morning. 

but instead encrypted sentence line printing:

this ag',rery dujeb..w>?/ijeb..w ssadorninjeb..w. 

the sntnc.replace method replacing replaced letters. how fix won't replace things twice?

the problem comes logic of replacing strings:

sntnc = sntnc.replaceall("b", "dug>?/");  // <-- replaces "g" here sntnc = sntnc.replaceall("g", "jeb..w");  // <-- "g" getting replaced here 

a simple solution drop using replaceall. making code works re-ordering replaceall calls fragile: don't know in future change getting replaced , break everything.

the solution simpler when building string iterating on characters. because have distinct temporary container when building strings, there no conflicts when replacements happening.

public string encrypt(string sntnc) {       stringbuilder sb = new stringbuilder();      (char ch : sntnc.tochararray()) {          if (ch == 'm') sb.append("ssad");          else if (ch == 'b') sb.append("dug>?/");          else if (ch == 'g') sb.append("jeb..w");          else if (ch == 'v') sb.append("ag',r");          else sb.append(ch);      }      return sb.tostring(); }