OnJava8-Examples/generics/Functional.java

//: generics/Functional.java
// ©2015 MindView LLC: see Copyright.txt
import java.math.*;
import java.util.concurrent.atomic.*;
import java.util.*;
import static net.mindview.util.Print.*;

// Different types of function objects:
interface Combiner<T> { T combine(T x, T y); }
interface UnaryFunction<R,T> { R function(T x); }
interface Collector<T> extends UnaryFunction<T,T> {
  T result(); // Extract result of collecting parameter
}
interface UnaryPredicate<T> { boolean test(T x); }

public class Functional {
  // Calls the Combiner object on each element to combine
  // it with a running result, which is finally returned:
  public static <T> T
  reduce(Iterable<T> seq, Combiner<T> combiner) {
    Iterator<T> it = seq.iterator();
    if(it.hasNext()) {
      T result = it.next();
      while(it.hasNext())
        result = combiner.combine(result, it.next());
      return result;
    }
    // If seq is the empty list:
    return null; // Or throw exception
  }
  // Take a function object and call it on each object in
  // the list, ignoring the return value. The function
  // object can act as a collecting parameter, so it is
  // returned at the end.
  public static <T> Collector<T>
  forEach(Iterable<T> seq, Collector<T> func) {
    for(T t : seq)
      func.function(t);
    return func;
  }
  // Creates a list of results by calling a
  // function object for each object in the list:
  public static <R,T> List<R>
  transform(Iterable<T> seq, UnaryFunction<R,T> func) {
    List<R> result = new ArrayList<>();
    for(T t : seq)
      result.add(func.function(t));
    return result;
  }
  // Applies a unary predicate to each item in a sequence,
  // and returns a list of items that produced "true":
  public static <T> List<T>
  filter(Iterable<T> seq, UnaryPredicate<T> pred) {
    List<T> result = new ArrayList<>();
    for(T t : seq)
      if(pred.test(t))
        result.add(t);
    return result;
  }
  // To use the above generic methods, we create
  // function objects to adapt to our particular needs:
  static class IntegerAdder implements Combiner<Integer> {
    @Override
    public Integer combine(Integer x, Integer y) {
      return x + y;
    }
  }
  static class
  IntegerSubtracter implements Combiner<Integer> {
    @Override
    public Integer combine(Integer x, Integer y) {
      return x - y;
    }
  }
  static class
  BigDecimalAdder implements Combiner<BigDecimal> {
    @Override
    public BigDecimal combine(BigDecimal x, BigDecimal y) {
      return x.add(y);
    }
  }
  static class
  BigIntegerAdder implements Combiner<BigInteger> {
    @Override
    public BigInteger combine(BigInteger x, BigInteger y) {
      return x.add(y);
    }
  }
  static class
  AtomicLongAdder implements Combiner<AtomicLong> {
    @Override
    public AtomicLong combine(AtomicLong x, AtomicLong y) {
      // Not clear whether this is meaningful:
      return new AtomicLong(x.addAndGet(y.get()));
    }
  }
  // We can even make a UnaryFunction with an "ulp"
  // (Units in the last place):
  static class BigDecimalUlp
  implements UnaryFunction<BigDecimal,BigDecimal> {
    @Override
    public BigDecimal function(BigDecimal x) {
      return x.ulp();
    }
  }
  static class GreaterThan<T extends Comparable<T>>
  implements UnaryPredicate<T> {
    private T bound;
    public GreaterThan(T bound) { this.bound = bound; }
    @Override
    public boolean test(T x) {
      return x.compareTo(bound) > 0;
    }
  }
  static class MultiplyingIntegerCollector
  implements Collector<Integer> {
    private Integer val = 1;
    @Override
    public Integer function(Integer x) {
      val *= x;
      return val;
    }
    @Override
    public Integer result() { return val; }
  }
  public static void main(String[] args) {
    // Generics, varargs & boxing working together:
    List<Integer> li = Arrays.asList(1, 2, 3, 4, 5, 6, 7);
    Integer result = reduce(li, new IntegerAdder());
    print(result);

    result = reduce(li, new IntegerSubtracter());
    print(result);

    print(filter(li, new GreaterThan<>(4)));

    print(forEach(li,
      new MultiplyingIntegerCollector()).result());

    print(forEach(filter(li, new GreaterThan<>(4)),
      new MultiplyingIntegerCollector()).result());

    MathContext mc = new MathContext(7);
    List<BigDecimal> lbd = Arrays.asList(
      new BigDecimal(1.1, mc), new BigDecimal(2.2, mc),
      new BigDecimal(3.3, mc), new BigDecimal(4.4, mc));
    BigDecimal rbd = reduce(lbd, new BigDecimalAdder());
    print(rbd);

    print(filter(lbd,
      new GreaterThan<>(new BigDecimal(3))));

    // Use the prime-generation facility of BigInteger:
    List<BigInteger> lbi = new ArrayList<>();
    BigInteger bi = BigInteger.valueOf(11);
    for(int i = 0; i < 11; i++) {
      lbi.add(bi);
      bi = bi.nextProbablePrime();
    }
    print(lbi);

    BigInteger rbi = reduce(lbi, new BigIntegerAdder());
    print(rbi);
    // The sum of this list of primes is also prime:
    print(rbi.isProbablePrime(5));

    List<AtomicLong> lal = Arrays.asList(
      new AtomicLong(11), new AtomicLong(47),
      new AtomicLong(74), new AtomicLong(133));
    AtomicLong ral = reduce(lal, new AtomicLongAdder());
    print(ral);

    print(transform(lbd,new BigDecimalUlp()));
  }
} /* Output:
28
-26
[5, 6, 7]
5040
210
11.000000
[3.300000, 4.400000]
[11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47]
311
true
265
[0.000001, 0.000001, 0.000001, 0.000001]
*///:~