FilterWriter.java

import java.io.IOException;
import java.io.Writer;
import java.util.function.BiPredicate;

public class FilterWriter extends Writer{  
   private Writer writer;
   private BiPredicate<Character, Character> pred;
   private char previous;
  
   public FilterWriter(Writer writer, BiPredicate<Character, Character> pred, char prev) {
       this.writer = writer;
       this.pred = pred;
       this.previous = prev;

   }

   @Override
   public void write(char[] data, int off, int len) throws IOException {

       for (int i = off; i < off + len; i++){
           char c = data[i];
           if (pred.test(previous, c)){
               writer.write(c);
               previous = c;
           }  
          
       }      
   }

   @Override
   public void flush() throws IOException {
       writer.flush();      
   }

   @Override
   public void close() throws IOException {
       writer.close();
   }

}