Rename nodes to more general names.

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

@@ -0,0 +1,108 @@
+package org.nwapw.abacus.tree;
+
+/**
+ * A tree node that represents an operation being applied to two operands.
+ */
+public class BinaryNode extends TreeNode {
+
+    /**
+     * The operation being applied.
+     */
+    private String operation;
+    /**
+     * The left node of the operation.
+     */
+    private TreeNode left;
+    /**
+     * The right node of the operation.
+     */
+    private TreeNode right;
+
+    private BinaryNode() {
+    }
+
+    /**
+     * Creates a new operation node with the given operation
+     * and no child nodes.
+     *
+     * @param operation the operation.
+     */
+    public BinaryNode(String operation) {
+        this(operation, null, null);
+    }
+
+    /**
+     * Creates a new operation node with the given operation
+     * and child nodes.
+     *
+     * @param operation the operation.
+     * @param left      the left node of the expression.
+     * @param right     the right node of the expression.
+     */
+    public BinaryNode(String operation, TreeNode left, TreeNode right) {
+        this.operation = operation;
+        this.left = left;
+        this.right = right;
+    }
+
+    /**
+     * Gets the operation in this node.
+     *
+     * @return the operation in this node.
+     */
+    public String getOperation() {
+        return operation;
+    }
+
+    /**
+     * Gets the left sub-expression of this node.
+     *
+     * @return the left node.
+     */
+    public TreeNode getLeft() {
+        return left;
+    }
+
+    /**
+     * Sets the left sub-expression of this node.
+     *
+     * @param left the sub-expression to apply.
+     */
+    public void setLeft(TreeNode left) {
+        this.left = left;
+    }
+
+    /**
+     * Gets the right sub-expression of this node.
+     *
+     * @return the right node.
+     */
+    public TreeNode getRight() {
+        return right;
+    }
+
+    /**
+     * Sets the right sub-expression of this node.
+     *
+     * @param right the sub-expression to apply.
+     */
+    public void setRight(TreeNode right) {
+        this.right = right;
+    }
+
+    @Override
+    public <T> T reduce(Reducer<T> reducer) {
+        T leftReduce = left.reduce(reducer);
+        T rightReduce = right.reduce(reducer);
+        if (leftReduce == null || rightReduce == null) return null;
+        return reducer.reduceNode(this, leftReduce, rightReduce);
+    }
+
+    @Override
+    public String toString() {
+        String leftString = left != null ? left.toString() : "null";
+        String rightString = right != null ? right.toString() : "null";
+
+        return "(" + leftString + operation + rightString + ")";
+    }
+}