Full Code Examples - Applied Java Patterns - page 238

Interpreter

The Expression hierarchy is at the heart of the Interpreter pattern. It defines the grammar that can be used to
create and evaluate expressions. The Expression interface is the foundation for all expressions, and defines the
interpret method that performs an evaluation.

Table A-1 lists the interface and corresponding information.

Table A-1. Purpose of the Expression interface and its implementers

Expression Common interface for all expressions

ConstantExpression Represents a constant value

VariableExpression Represents a variable value, obtained by calling a method on some class

CompoundExpression A pair of comparison expressions that evaluate to a boolean result

AndExpression The logical “and” of two expressions

OrExpression The logical “or” of two expressions

ComparisonExpression A pair of expressions that evaluate to a boolean result

EqualsExpression P erforms an equals method comparison between the two expressions

ContainsExpression

Checks to see if the first String expression contains the second one

Example A.51 Expression.java
1. public interface Expression{
2. void interpret(Context c);
3. }

Example A.52 ConstantExpression.java
1. import java.lang.reflect.Method;
2. import java.lang.reflect.InvocationTargetException;
3. public class ConstantExpression implements Expression{
4. private Object value;
5.
6. public ConstantExpression(Object newValue){
7. value = newValue;
8. }
9.
10. public void interpret(Context c){
11. c.addVariable(this, value);
12. }
13. }

Example A.53 VariableExpression.java
1. import java.lang.reflect.Method;
2. import java.lang.reflect.InvocationTargetException;
3. public class VariableExpression implements Expression{
4. private Object lookup;
5. private String methodName;
6.
7. public VariableExpression(Object newLookup, String newMethodName){
8. lookup = newLookup;
9. methodName = newMethodName;
10. }
11.
12. public void interpret(Context c){
13. try{
14. Object source = c.get(lookup);
15. if (source != null){
16. Method method = source.getClass().getMethod(methodName, null);
17. Object result = method.invoke(source, null);
18. c.addVariable(this, result);
19. }
20. }
21. catch (NoSuchMethodException exc){ }
22. catch (IllegalAccessException exc){ }
23. catch (InvocationTargetException exc){ }
24. }
25. }