***Can you help me to write a java program to evaluate a mathematical expression written in a a Prefix Order ??
***

@hala.qaddoumi
Have you tried something on your own?
If yes pelase post the code and tell us the errors

Ah, building an expression (infix, prefix, postfix) tree! That takes me back! :-) Fortunately, I still have my copy of Knuth handy, including the last volume recently published! :-)

*i have postfix code.
on postfix we store the operator on the stack and the operand as output , but the prefix i don't get the idea of it .. :(
*

package Hallosh;
import java.util.Stack;
import java.util.StringTokenizer;
public class Driver {
  public static void main(String[] args) {
    // Taken from Daniel Liang – Intro to Java Prog.
    // the input is a correct postfix expression
String expression = "1 2 + 3 *";

try {
      System.out.println( evaluateExpression(expression) );
    }
catch (Exception ex) {
      System.out.println("Wrong expression");
    }
  }
/** Evaluate an expression **********************************************/
public static int evaluateExpression(String expression) {
    // Create operandStack to store operands
    Stack<Integer> operandStack = new Stack<Integer>();
    // Extract operands and operators
StringTokenizer tokens = new StringTokenizer(expression, " +-/*%", true);
// Phase 1: Scan tokens
while (tokens.hasMoreTokens()) {
      String token = tokens.nextToken().trim(); // Extract a token
      if (token.length() == 0) { // Blank space
        continue; // Back to the while loop to extract the next token
      }
else if (token.charAt(0) == '+' || token.charAt(0) == '-' ||
               token.charAt(0) == '*' || token.charAt(0) == '/') {
              processAnOperator(token.charAt(0), operandStack);
      }
else {  // An operand scanned
              // Push an operand to the stack
operandStack.push(new Integer(token));
      }
    }
// Return the result
return ((Integer)(operandStack.pop())).intValue();
}
/** Process one operator: Take an operator from operatorStack and
   *  apply it on the operands in the operandStack */
  public static void processAnOperator(char op, Stack operandStack) {

if (op == '+') {
      int op1 = ((Integer)(operandStack.pop())).intValue();
      int op2 = ((Integer)(operandStack.pop())).intValue();
      operandStack.push(new Integer(op2 + op1));
    }
else if (op == '-') {
      int op1 = ((Integer)(operandStack.pop())).intValue();
      int op2 = ((Integer)(operandStack.pop())).intValue();
      operandStack.push(new Integer(op2 - op1));
    }
else if ((op == '*')) {
      int op1 = ((Integer)(operandStack.pop())).intValue();
      int op2 = ((Integer)(operandStack.pop())).intValue();
      operandStack.push(new Integer(op2 * op1));
    }
else if (op == '/') {
      int op1 = ((Integer)(operandStack.pop())).intValue();
      int op2 = ((Integer)(operandStack.pop())).intValue();
      operandStack.push(new Integer(op2 / op1));
    }
  }
}
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