org.nfunk.jep
Class EvaluatorVisitor

java.lang.Object
  org.nfunk.jep.EvaluatorVisitor

All Implemented Interfaces:

EvaluatorI, ParserVisitor

Direct Known Subclasses:

VectorEvaluator, XEvaluatorVisitor

public class EvaluatorVisitor
extends java.lang.Object
implements ParserVisitor, EvaluatorI

This class is used for the evaluation of an expression. It uses the Visitor design pattern to traverse the function tree and evaluate the expression using a stack.

Function nodes are evaluated by first evaluating all the children nodes, then applying the function class associated with the node. Variable and constant nodes are evaluated by pushing their value onto the stack.

Some changes implemented by rjm. Nov 03. Added hook to SpecialEvaluationI. Clears stack before evaluation. Simplifies error handling by making visit methods throw ParseException. Changed visit(ASTVarNode node) so messages not calculated every time.

Field Summary

protected static boolean	debug Debug flag
protected java.util.Stack	stack Stack used for evaluating the expression
protected SymbolTable	symTab The symbol table for variable lookup
protected boolean	trapNullValues TrapNull

Constructor Summary

EvaluatorVisitor()
Constructor.

Method Summary

java.lang.Object	eval(Node node) Evaluates a given node, in the current context.
java.lang.Object	eval(PostfixMathCommandI pfmc, Node[] children) Evaluates a PostfixMathCommandI with given arguments.
java.lang.Object	getValue(Node topNode, SymbolTable symTab_in) Returns the value of the expression as an object.
boolean	isTrapNullValues() Tests whether null variable values are trapped by evaluator.
void	setTrapNullValues(boolean b) Sets the behaviour when a variable's value is null.
java.lang.Object	visit(ASTConstant node, java.lang.Object data) Visit a constant node.
java.lang.Object	visit(ASTFunNode node, java.lang.Object data) Visit a function node.
java.lang.Object	visit(ASTStart node, java.lang.Object data) This method should never be called when evaluating a normal expression.
java.lang.Object	visit(ASTVarNode node, java.lang.Object data) Visit a variable node.
java.lang.Object	visit(SimpleNode node, java.lang.Object data) This method should never be called when evaluation a normal expression.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

stack

protected java.util.Stack stack

Stack used for evaluating the expression

symTab

protected SymbolTable symTab

The symbol table for variable lookup

debug

protected static final boolean debug

Debug flag

See Also:

Constant Field Values

trapNullValues

protected boolean trapNullValues

TrapNull

Constructor Detail

EvaluatorVisitor

public EvaluatorVisitor()

Constructor. Initialise the stack member

Method Detail

getValue

public java.lang.Object getValue(Node topNode,
                                 SymbolTable symTab_in)
                          throws ParseException

Returns the value of the expression as an object. The expression tree is specified with its top node. The algorithm uses a stack for evaluation.

The symTab parameter can be null, if no variables are expected in the expression. If a variable is found, an error is added to the error list.

An exception is thrown, if an error occurs during evaluation.

Returns:

The value of the expression as an object.

Throws:

ParseException - if there is a problem with the evaluation.

eval

public java.lang.Object eval(Node node)
                      throws ParseException

Evaluates a given node, in the current context.

Specified by:

eval in interface EvaluatorI

Parameters:

node - The node to evaluate

Returns:

result of the evaluation

Throws:

ParseException - if errors occur during evaluation;

eval

public java.lang.Object eval(PostfixMathCommandI pfmc,
                             Node[] children)
                      throws ParseException

Evaluates a PostfixMathCommandI with given arguments. Not used in normal use.

Parameters:

pfmc - the command to evaluate.

children - the parameters to the function.

Returns:

the value of the function

Throws:

ParseException

visit

public java.lang.Object visit(SimpleNode node,
                              java.lang.Object data)
                       throws ParseException

This method should never be called when evaluation a normal expression.

Specified by:

visit in interface ParserVisitor

Throws:

ParseException

visit

public java.lang.Object visit(ASTStart node,
                              java.lang.Object data)
                       throws ParseException

This method should never be called when evaluating a normal expression.

Specified by:

visit in interface ParserVisitor

Throws:

ParseException

visit

public java.lang.Object visit(ASTFunNode node,
                              java.lang.Object data)
                       throws ParseException

Visit a function node. The values of the child nodes are first pushed onto the stack. Then the function class associated with the node is used to evaluate the function.

If a function implements SpecialEvaluationI then the evaluate method of PFMC is called.

Specified by:

visit in interface ParserVisitor

Throws:

ParseException

visit

public java.lang.Object visit(ASTVarNode node,
                              java.lang.Object data)
                       throws ParseException

Visit a variable node. The value of the variable is obtained from the symbol table (symTab) and pushed onto the stack.

Specified by:

visit in interface ParserVisitor

Throws:

ParseException

visit

public java.lang.Object visit(ASTConstant node,
                              java.lang.Object data)

Visit a constant node. The value of the constant is pushed onto the stack.

Specified by:

visit in interface ParserVisitor

isTrapNullValues

public boolean isTrapNullValues()

Tests whether null variable values are trapped by evaluator.

Returns:

true is nulls are trapped

setTrapNullValues

public void setTrapNullValues(boolean b)

Sets the behaviour when a variable's value is null. If true an exception will be thrown is a variable value is null. If false then the value will be passed to other functions, this may cause error else where.

Parameters:

b -