Logical expression evaluator

Question

hi,
Can anybody provide code snippet/example or any reference to evaluate
a String ?
thanx in advance.

Answer 1

You can use Jacc (find it on Google) to parse and evaluate the expression.
But, you can also create a tree representing the expression and evaluate it each time you want :
abstract class Node {
   public abstract Object evaluate();
abstract class BinaryOperator extends Node {
   protected Node leftOperand;
   protected Node rightOperand;
   protected BinaryOperator (Node lo, Node ro) {
      leftOperand =lo;
      rightOperand = ro;
abstract class Leaf extends Node{
   protected Object value;
class Value extends Leaf {
   public Value(Object val) {
       value = val;
   public Object evaluate() {
       return value;
class Variable extends Leaf {
   String name;
   public Variable (String name) {
       this.name= name;
   public void setValue(Object val) {
      value = val;
   public Object evaluate() {
       return value;
class Equal extends BinaryOperator {
   public Equal(Node lo, Node ro) {
      super(lo,ro);
   public Object evaluate() {
      return new Boolean(leftOperand.evaluate().equals(rightOperand.evaluate()));
class NotEqual extends BinaryOperator {
   public NotEqual(Node lo, Node ro) {
      super(lo,ro);
   public Object evaluate() {
      return new Boolean(!leftOperand.evaluate().equals(rightOperand.evaluate()));
class Inferior extends BinaryOperator {
   public Inferior (Node lo, Node ro) {
      super(lo,ro);
   public Object evaluate() {
      Object o1 = leftOperand.evaluate();     
      Object o2 = rightOperand.evaluate();
      if (o1 instanceof Number && o2 instanceof Number) {
      return new Boolean(((Number)o1).doubleValue()<((Number)o2).doubleValue());
      return new Boolean(false);
class Or extends BinaryOperator {
   public Or(Node lo, Node ro) {
      super(lo,ro);
   public Object evaluate() {
      Object o1 = leftOperand.evaluate();     
      Object o2 = rightOperand.evaluate();
      if (o1 instanceof Boolean && o2 instanceof Boolean ) {
      return new Boolean(((Boolean)o1).booleanValue()||((Boolean)o2).booleanValue());
      return new Boolean(false); // if correct it never happens
...After that you build the tree :
Variable var1 = new Variable("STN");
Value v1 = new Value("E");
BinaryOperator bo1 = new Equal(var1,v1);
Variable var2 = new Variable("STN");
Value v2 = new Value("R");
BinaryOperator bo2 = new Equal(var2,v2);
BinaryrOperator bo3 = new Or(bo1,bo2);
// We instanciate the value of the variables for each row :
for (int i = 0; i<rowNumber;i++) {
    var1.setValue(stn);
var2.setValue(stn[i]);
res = bo3.evaluate();
//You must make this automatically.
I hope this helps,
Denis