Expression Engine

interface Expression<T> {
    /**
     * Evaluates the expression and returns the result
     */
    T evaluate() throws ExpressionEvaluationException;
    
    /**
     * Returns the set of variable names referenced in this expression
     */
    Set<String> getVariableNames();
    
    /**
     * Sets the value of a variable used in the expression
     */
    void setVariable(String name, Object value);
}

interface ExpressionFactory {
    /**
     * Creates an expression from a string representation
     */
    <T> Expression<T> createExpression(String expressionString);
    
    /**
     * Creates a boolean expression for conditional logic
     */
    Expression<Boolean> createBooleanExpression(String expressionString);
}

class ExpressionEvaluationException extends Exception {
    ExpressionEvaluationException(String message);
    ExpressionEvaluationException(String message, Throwable cause);
}

class ExpressionModel {
    String getExpressionString();
    void setExpressionString(String expressionString);
    Map<String, Object> getVariables();
    void setVariable(String name, Object value);
}

class ExpressionContext {
    Locale getLocale();
    Map<String, Object> getVariables();
    long getCompanyId();
    long getGroupId();
    long getUserId();
}

@Component
public class ExpressionExample {
    
    @Reference
    private ExpressionFactory expressionFactory;
    
    public void evaluateFormRules() throws ExpressionEvaluationException {
        // Create a boolean expression for field visibility
        Expression<Boolean> visibilityExpression = expressionFactory.createBooleanExpression(
            "getValue('country') == 'US' && getValue('age') >= 18"
        );
        
        // Set field values
        visibilityExpression.setVariable("country", "US");
        visibilityExpression.setVariable("age", 25);
        
        // Evaluate the expression
        boolean isVisible = visibilityExpression.evaluate();
        System.out.println("Field should be visible: " + isVisible);
        
        // Create a calculation expression
        Expression<Number> calculationExpression = expressionFactory.createExpression(
            "getValue('quantity') * getValue('price') * (1 + getValue('tax_rate'))"
        );
        
        calculationExpression.setVariable("quantity", 5);
        calculationExpression.setVariable("price", 19.99);
        calculationExpression.setVariable("tax_rate", 0.08);
        
        Number totalCost = calculationExpression.evaluate();
        System.out.println("Total cost: $" + totalCost);
    }
}

class ExpressionConstants {
    // Built-in functions available in expressions
    String FUNCTION_BETWEEN = "between";
    String FUNCTION_CONTAINS = "contains";
    String FUNCTION_GET_VALUE = "getValue";
    String FUNCTION_IS_EMAIL_ADDRESS = "isEmailAddress";
    String FUNCTION_IS_URL = "isURL";
    String FUNCTION_MATCH = "match";
    String FUNCTION_SUM = "sum";
    
    // Expression operators
    String OPERATOR_AND = "&&";
    String OPERATOR_OR = "||";
    String OPERATOR_NOT = "!";
    String OPERATOR_EQUALS = "==";
    String OPERATOR_NOT_EQUALS = "!=";
    String OPERATOR_GREATER_THAN = ">";
    String OPERATOR_LESS_THAN = "<";
}