Question

P9.17 (For Java, and can you please explain as well.)

Declare an interface Filter as follows:

public interface Filter
{
boolean accept(Object x);
}

Write a method: public static ArrayList collectAll(ArrayList objects, Filter f) that returns all objects in the objects list that are accepted by the given filter.

Provide a class ShortWordFilter whose filter method accepts all strings of length < 5.

Then write a program that asks the user for input and output textfile names, reads all words from an input file, puts them into an ArrayList, calls collectAll, and prints a list of the short words to the output file.

Answer 1

import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Scanner;


interface Filter
{
boolean accept(Object x);
}

class ShortWordFilter implements Filter {
        @Override
        public boolean accept(Object x) {
                return x.toString().length() < 5;
        }
}

class FilterDemo {
        
        public static ArrayList collectAll(ArrayList objects, Filter f)  {
                ArrayList result = new ArrayList();
                for(Object obj: objects) {
                        if(f.accept(obj)) {
                                result.add(obj);
                        }
                }
                return result;
        }
        
        public static void main(String args[]) throws FileNotFoundException {
                String inputFile = "input.txt";
                String outputFile = "output.txt";
                
                Filter f = new ShortWordFilter();
                
                ArrayList words = new ArrayList();
                Scanner reader = new Scanner(new File(inputFile));
                while(reader.hasNext()) {
                        words.add(reader.next());
                }
                
                ArrayList shortWords = collectAll(words, f);
                PrintWriter writer = new PrintWriter(new File(outputFile));
                for(Object s: shortWords) {
                        writer.println(s);
                }
        }
}