Move the source files into a new default directory.

src/main/java/org/nwapw/abacus/tree/NumberNode.java

@@ -0,0 +1,48 @@
+package org.nwapw.abacus.tree;
+
+import org.nwapw.abacus.number.NaiveNumber;
+import org.nwapw.abacus.number.NumberInterface;
+
+/**
+ * A node implementation that represents a single number.
+ */
+public class NumberNode extends TreeNode {
+
+    /**
+     * The number that is represented by this number node.
+     */
+    private NumberInterface number;
+
+    /**
+     * Creates a number node with no number.
+     */
+    public NumberNode(){
+        number = null;
+    }
+
+    /**
+     * Creates a new number node with the given double value.
+     * @param newNumber the number for which to create a number node.
+     */
+    public NumberNode(NumberInterface newNumber){
+        this.number = newNumber;
+    }
+
+    /**
+     * Gets the number value of this node.
+     * @return the number value of this node.
+     */
+    public NumberInterface getNumber() {
+        return number;
+    }
+
+    @Override
+    public <T> T reduce(Reducer<T> reducer) {
+        return reducer.reduceNode(this);
+    }
+
+    @Override
+    public String toString() {
+        return number != null ? number.toString() : "null";
+    }
+}