/** class for nodes used in a binary tree */package dataStructures;public class BinaryTreeNode {    // package visible data members    Object element;    BinaryTreeNode leftChild; // left subtree    BinaryTreeNode rightChild; // right subtree    // constructors    public BinaryTreeNode() {    }    public BinaryTreeNode(Object theElement) {        element = theElement;    }    public BinaryTreeNode(Object theElement, BinaryTreeNode theleftChild,            BinaryTreeNode therightChild) {        element = theElement;        leftChild = theleftChild;        rightChild = therightChild;    }    // accessor methods    public BinaryTreeNode getLeftChild() {        return leftChild;    }    public BinaryTreeNode getRightChild() {        return rightChild;    }    public Object getElement() {        return element;    }    // mutator methods 设值方法    public void setLeftChild(BinaryTreeNode theLeftChild) {        leftChild = theLeftChild;    }    public void setRightChild(BinaryTreeNode theRightChild) {        rightChild = theRightChild;    }    public void setElement(Object theElement) {        element = theElement;    }    // output method    public String toString() {        return element.toString();    }}