diff --git a/src/main/java/oving5/BinaryComputingIterator.java b/src/main/java/oving5/BinaryComputingIterator.java
index 18fbe86..072b0b6 100644
--- a/src/main/java/oving5/BinaryComputingIterator.java
+++ b/src/main/java/oving5/BinaryComputingIterator.java
@@ -4,18 +4,63 @@ import java.util.Iterator;
 import java.util.function.BinaryOperator;
 
 public class BinaryComputingIterator {
+	private Iterator<Double> iterator1;
+	private Iterator<Double> iterator2;
+
+	private Double default1;
+	private Double default2;
+
+	private BinaryOperator<Double> operator;
+
 	BinaryComputingIterator(Iterator<Double> iterator1, Iterator<Double> iterator2, BinaryOperator<Double> operator) {
+		if (iterator1 == null || iterator2 == null || operator == null) {
+			throw new IllegalArgumentException("argument cannot be null");
+		}
+		this.iterator1 = iterator1;
+		this.iterator2 = iterator2;
+		this.operator = operator;
 	}
 
 	BinaryComputingIterator(Iterator<Double> iterator1, Iterator<Double> iterator2, Double default1, Double default2,
 			BinaryOperator<Double> operator) {
+		if (iterator1 == null || iterator2 == null || operator == null) {
+			throw new IllegalArgumentException("argument cannot be null");
+		}
+		this.iterator1 = iterator1;
+		this.iterator2 = iterator2;
+		this.operator = operator;
+		this.default1 = default1;
+		this.default2 = default2;
 	}
 
 	boolean hasNext() {
+		if (!iterator1.hasNext() && !iterator2.hasNext()) {
+			return false;
+		}
+		if ((iterator1.hasNext() || default1 != null) && (iterator2.hasNext() || default2 != null)) {
+			return true;
+		}
 		return false;
 	}
 
 	Double next() {
-		return 0.0;
+		if (hasNext()) {
+			Double a = 0.0;
+			if (iterator1.hasNext()) {
+				a = iterator1.next();
+			} else {
+				a = default1;
+			}
+
+			Double b = 0.0;
+			if (iterator2.hasNext()) {
+				b = iterator2.next();
+			} else {
+				b = default2;
+			}
+
+			return operator.apply(a, b);
+		}
+		throw new IllegalArgumentException("empty things");
 	}
 }
diff --git a/src/main/java/oving5/debugging/StringMergingIterator.java b/src/main/java/oving5/debugging/StringMergingIterator.java
index edc53c5..d1016bf 100644
--- a/src/main/java/oving5/debugging/StringMergingIterator.java
+++ b/src/main/java/oving5/debugging/StringMergingIterator.java
@@ -27,18 +27,17 @@ public class StringMergingIterator implements Iterator<String> {
 
 		String result = null;
 
-		if (!this.first.hasNext()) {
+		if (this.turnSwitch && this.first.hasNext()) {
 			result = this.first.next();
-		} else if (!this.second.hasNext()) {
+		} else if (!this.turnSwitch && this.second.hasNext()) {
 			result = this.second.next();
 		} else {
-			if (this.turnSwitch) {
+			if (this.first.hasNext()) {
 				result = this.first.next();
-				this.turnSwitch = false;
-			}
-			if (!this.turnSwitch) {
+			} else if (this.second.hasNext()) {
 				result = this.second.next();
-				this.turnSwitch = true;
+			} else {
+				throw new NoSuchElementException();
 			}
 		}
 
diff --git a/src/test/java/oving5/BinaryComputingIteratorTest.java.unimplemented b/src/test/java/oving5/BinaryComputingIteratorTest.java
similarity index 100%
rename from src/test/java/oving5/BinaryComputingIteratorTest.java.unimplemented
rename to src/test/java/oving5/BinaryComputingIteratorTest.java