Functional Java

Func.java

public interface Func<TReturn, TArg0> {
  public TReturn call(TArg0 arg0);
}

Func2.java

public interface Func2<TReturn, TArg0, TArg1> {
  public TReturn call(TArg0 arg0, TArg1 arg1);
}

Iter.java

import java.util.*;

public class Iter {
  public static <T> Iterable<T> filter(Collection<T> list, Predicate<T> predicate) {
    Collection<T> filtered = new ArrayList<T>();
    for (T item : list) {
      if (predicate.call(item)) {
        filtered.add(item);
      }
    }
    return filtered;
  }

  public static <TReturn, TItem> Iterable<TReturn> map(Collection<TItem> list, Func<TReturn, TItem> func) {
    Collection<TReturn> mapped = new ArrayList<TReturn>();
    for (TItem item : list) {
      mapped.add(func.call(item));
    }
    return mapped;
  }

  public static <TReturn, TItem> TReturn reduce(Collection<TItem> list, TReturn initial, Func2<TReturn, TReturn, TItem> func) {
    TReturn acc = initial;
    for (TItem item : list) {
      acc = func.call(acc, item);
    }
    return acc;
  }
}

Predicate.java

public interface Predicate<T> {
  public boolean call(T object);
}

X.java

import java.util.*;

public class X {
  public static void main(String[] args) {
    List<Double> grades = Arrays.asList(1.0, 1.0, 3.5, 4.5, 5.5, 6.5, 7.5, 8.5, 9.5, 10.0);

    System.out.println("All grades:");

    for (double grade : grades) {
      System.out.println(grade);
    }


    System.out.println();
    System.out.println("Only >= 6.0:");
    Iterable<Double> gradesPassed = Iter.filter(grades, new Predicate<Double>() {
      public boolean call(Double grade) {
        return grade > 6.0;
      }
    });
    for (double grade : gradesPassed) {
      System.out.println(grade);
    }


    System.out.println();
    System.out.println("Passed:");
    Iterable<String> gradesPassed2 = Iter.map(grades, new Func<String, Double>() {
      public String call(Double grade) {
        if (grade > 6.0) {
          return String.format("Passed (%.1f)", grade);
        } else {
          return String.format("Failed (%.1f)", grade);
        }
      }
    });
    for (String grade : gradesPassed2) {
      System.out.println(grade);
    }


    System.out.println();
    System.out.println("Average:");
    Double sum = Iter.reduce(grades, 0d, new Func2<Double, Double, Double>() {
      public Double call(Double sum, Double grade) {
        return sum +  grade;
      }
    });
    System.out.format("Arithmeric average.: %.1f", sum / grades.size());
  }
}