Initial commit for 4.2

src/main/java/_4/_2/Filter.java

@@ -0,0 +1,9 @@
+package _4._2;
+
+public abstract class Filter implements Sequence {
+	protected Sequence sequence;
+
+	public Filter(Sequence sequence) {
+		this.sequence = sequence;
+	}
+}

src/main/java/_4/_2/Naturals.java

@@ -0,0 +1,10 @@
+package _4._2;
+
+public class Naturals implements Sequence {
+	private int last;
+
+	@Override
+	public int getNext() {
+		return ++this.last;
+	}
+}

src/main/java/_4/_2/Primes.java

@@ -0,0 +1,15 @@
+package _4._2;
+
+public class Primes extends Filter {
+	public Primes() {
+		super(new Naturals());
+		this.sequence.getNext(); // remove 1 form the sequence
+	}
+
+	@Override
+	public int getNext() {
+		int ret = this.sequence.getNext(); // also removes ret form the sequence
+		this.sequence = new ZapMultiples(ret, this.sequence);
+		return ret;
+	}
+}

src/main/java/_4/_2/Sequence.java

@@ -0,0 +1,5 @@
+package _4._2;
+
+public interface Sequence {
+	int getNext();
+}

src/main/java/_4/_2/ZapMultiples.java

@@ -0,0 +1,19 @@
+package _4._2;
+
+public class ZapMultiples extends Filter {
+	private final int divisor;
+
+	public ZapMultiples(int divisor, Sequence sequence) {
+		super(sequence);
+		if(divisor < 2) {
+			throw new IllegalArgumentException("divisor must be >= 2");
+		}
+		this.divisor = divisor;
+	}
+
+	@Override
+	public int getNext() {
+		int next = this.sequence.getNext();
+		return next % divisor == 0 ? this.getNext() : next;
+	}
+}