Here is a java code that accept the string from the user using BufferedReader and replace the character sequence "cat" with dog".

import java.io.*;

class ReplaceString{
    public static void main(String[] args){
        try{
            BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
            System.out.print("Enter string: ");
            String str=br.readLine();
            String replaceString=str.replace("cat","dog");
            System.out.println("New String: "+replaceString);
        }
         catch(Exception e){}
    }
}