com.hp.hpl.jena.graph.query
Interface Expression

All Known Implementing Classes:

Dyadic, Expression.Application, Expression.Base, Expression.BoolConstant, Expression.Constant, Expression.Fixed, Expression.Variable, Rewrite.DyadicLiteral

public interface Expression

Expression - the interface for expressions that is expected by Query for constraints. An Expression can be evaluated (given a name->value mapping); it can be prepared into a Valuator (given a name->index mapping); and it can be analysed into its components.

An Expression can be a variable, an application, or a literal value. If an access method (eg getName) is applied to an Expression for which it is not appropriate (eg an application), the result is unspecified; an implementation is free to throw an exception, deliver a null result, deliver a misleading value, whatever is convenient.

The nested class Util provides some expression utility methods, including a generic version of prepare. The nested abstract class Base and its sub-classes Literal, Variable, and Application provide a framework for developers to implement Expressions over.

Author:

kers

Nested Class Summary

static class	Expression.Application An abstract base class for apply nodes; subclasses implement getFun(), argCount(), and getArg().
static class	Expression.Base An abstract base class for Expressions; over-ride as appropriate.
static class	Expression.BoolConstant Base class used to implement TRUE and FALSE.
static class	Expression.Constant An abstract base class for literal nodes; subclasses implement getValue().
static class	Expression.Fixed A concrete class for representing fixed constants; each instance can hold a separate value and its valuator returns that value.
static class	Expression.Util Utility methods for Expressions, captured in a class because they can't be written directly in the interface.
static class	Expression.Valof Valof provides an implementation of VariableValues which composes the "compile-time" VariableIndexes map with the "run-time" IndexValues map to produce a VariableValues map.
static class	Expression.Variable An abstract base class for variable nodes; subclasses implement getName().

Field Summary

static Expression	FALSE An Expression which always evaluates to false.
static Expression	TRUE An Expression which always evaluates to true.

Method Summary

int	argCount() If this Expression is an application, answer the number of arguments that it has.
Expression	getArg(int i) If this Expression is an application, and 0 <= i < argCount(), answer the ith argument.
java.lang.String	getFun() If this Expression is an application, return the string identifying the function, which should be a URI.
java.lang.String	getName() If this Expression is a variable, answer a [non-null] String which is its name.
java.lang.Object	getValue() If this Expression is a literal, answer the value of that literal.
boolean	isApply() Answer true iff this Expression represents the application of some function [or operator] to some arguments [or operands].
boolean	isConstant() Answer true iff this Expression represents a literal [Java object] value.
boolean	isVariable() Answer true iff this Expression represents a variable.
Valuator	prepare(VariableIndexes vi) Answer a Valuator which, when run with a set of index-to-value bindings, evaluates this expression in the light of the given variable-to-index bindings [ie as though the variables were bound to the corresponding values]

Field Detail

TRUE

static final Expression TRUE

An Expression which always evaluates to true.

FALSE

static final Expression FALSE

An Expression which always evaluates to false.

Method Detail

prepare

Valuator prepare(VariableIndexes vi)

Answer a Valuator which, when run with a set of index-to-value bindings, evaluates this expression in the light of the given variable-to-index bindings [ie as though the variables were bound to the corresponding values]

isVariable

boolean isVariable()

Answer true iff this Expression represents a variable.

getName

java.lang.String getName()

If this Expression is a variable, answer a [non-null] String which is its name. Otherwise the behaviour is unspecified.

isConstant

boolean isConstant()

Answer true iff this Expression represents a literal [Java object] value.

getValue

java.lang.Object getValue()

If this Expression is a literal, answer the value of that literal. Otherwise the behaviour is unspecified.

isApply

boolean isApply()

Answer true iff this Expression represents the application of some function [or operator] to some arguments [or operands].

getFun

java.lang.String getFun()

If this Expression is an application, return the string identifying the function, which should be a URI. Otherwise the behaviour is unspecified.

argCount

int argCount()

If this Expression is an application, answer the number of arguments that it has. Otherwise the behaviour is unspecified.

getArg

Expression getArg(int i)

If this Expression is an application, and 0 <= i < argCount(), answer the ith argument. Otherwise the behaviour is unspecified.