Java Program to Implement Expression Tree

This is a Java Program to implement Expression Tree. Expression Tree is used in evaluating expressions.

Here is the source code of the Java program to implement Expression Tree. The Java program is successfully compiled and run on a Windows system. The program output is also shown below.

/**
 * Java Program to Implement Expression Tree Algorithm
 */
 
import java.util.Scanner;
 
/** Class ExpressionTree **/
class ExpressionTree
{
    /** class TreeNode **/
    class TreeNode
    {    
        char data;
        TreeNode left, right;
 
        /** constructor **/
        public TreeNode(char data)
        {
            this.data = data;
            this.left = null;
            this.right = null;
        }
    } 
 
    /** class StackNode **/
    class StackNode
    {
        TreeNode treeNode;
        StackNode next;
 
        /** constructor **/
        public StackNode(TreeNode treeNode)
        {
            this.treeNode = treeNode;
            next = null;
        }
    }
 
    private static StackNode top;
 
    /** constructor **/
    public ExpressionTree()
    {
        top = null;
    }
 
    /** function to clear tree **/
    public void clear()
    {
        top = null;
    }
 
    /** function to push a node **/
    private void push(TreeNode ptr)
    {
        if (top == null)
            top = new StackNode(ptr);
        else
        {
            StackNode nptr = new StackNode(ptr);
            nptr.next = top;
            top = nptr;
        }
    }
 
    /** function to pop a node **/
    private TreeNode pop()
    {
        if (top == null)
            throw new RuntimeException("Underflow");
        else
        {
            TreeNode ptr = top.treeNode;
            top = top.next;
            return ptr;
        }
    }
 
    /** function to get top node **/
    private TreeNode peek()
    {
        return top.treeNode;
    }
 
    /** function to insert character **/
    private void insert(char val)
    {
        try
        {
            if (isDigit(val))
            {
                TreeNode nptr = new TreeNode(val);
                push(nptr);
            }
            else if (isOperator(val))
            {
                TreeNode nptr = new TreeNode(val);
                nptr.left = pop();
                nptr.right = pop();
                push(nptr);
            }
        }
        catch (Exception e)
        {
            System.out.println("Invalid Expression");
        }
    }
 
    /** function to check if digit **/
    private boolean isDigit(char ch)
    {
        return ch >= '0' && ch <= '9';
    }
 
    /** function to check if operator **/
    private boolean isOperator(char ch)
    {
        return ch == '+' || ch == '-' || ch == '*' || ch == '/';
    }
 
    /** function to convert character to digit **/
    private int toDigit(char ch)
    {
        return ch - '0';
    }
 
    /** function to build tree from input */
    public void buildTree(String eqn)
    {
        for (int i = eqn.length() - 1; i >= 0; i--)
            insert(eqn.charAt(i));
    }
 
    /** function to evaluate tree */
    public double evaluate()
    {
        return evaluate(peek());
    }
 
    /** function to evaluate tree */
    public double evaluate(TreeNode ptr)
    {
        if (ptr.left == null && ptr.right == null)
            return toDigit(ptr.data);
        else
        {
            double result = 0.0;
            double left = evaluate(ptr.left);
            double right = evaluate(ptr.right);
            char operator = ptr.data;
 
            switch (operator)
            {
            case '+' : result = left + right; break;
            case '-' : result = left - right; break;
            case '*' : result = left * right; break;
            case '/' : result = left / right; break;
            default  : result = left + right; break;
            }
            return result;
        }
    }
 
    /** function to get postfix expression */
    public void postfix()
    {
        postOrder(peek());
    }
 
    /** post order traversal */
    private void postOrder(TreeNode ptr)
    {
        if (ptr != null)
        {
            postOrder(ptr.left);            
            postOrder(ptr.right);
            System.out.print(ptr.data);            
        }    
    }
 
    /** function to get infix expression */
    public void infix()
    {
        inOrder(peek());
    }
 
    /** in order traversal */
    private void inOrder(TreeNode ptr)
    {
        if (ptr != null)
        {
            inOrder(ptr.left);
            System.out.print(ptr.data);
            inOrder(ptr.right);            
        }    
    }
 
    /** function to get prefix expression */
    public void prefix()
    {
        preOrder(peek());
    }
 
    /** pre order traversal */
    private void preOrder(TreeNode ptr)
    {
        if (ptr != null)
        {
            System.out.print(ptr.data);
            preOrder(ptr.left);
            preOrder(ptr.right);            
        }    
    }
}
 
/** class ExpressionTreeTest **/
public class ExpressionTreeTest
{
    public static void main(String[] args)
    {
        Scanner scan = new Scanner(System.in);
        System.out.println("Expression Tree Test");
 
        /** make object of ExpressionTree **/
        ExpressionTree et = new ExpressionTree();
 
        System.out.println("\nEnter equation in prefix form");
        et.buildTree(scan.next());
 
        System.out.print("\nPrefix  : ");
        et.prefix();
        System.out.print("\n\nInfix   : ");
        et.infix();
        System.out.print("\n\nPostfix : ");
        et.postfix();
        System.out.println("\n\nEvaluated Result : "+ et.evaluate());
    }
}

Expression Tree Test
 
Enter equation in prefix form
+-+7*/935/82*/625
 
Prefix  : +-+7*/935/82*/625
 
Infix   : 7+9/3*5-8/2+6/2*5
 
Postfix : 793/5*+82/-62/5*+
 
Evaluated Result : 33.0