java class hierarchy

[Thomas]

Hello I 'm writing a simple ONP expression evaluator with functions in java.
The constrains about the class hierarchy are like that :

abstract Symbol implements Evaluate (which contains
method evaluate)
/ \
/ \
/ \
abstract Function abstract Operand
/ | \ / |
\
Plus Minus Ln Literal Const Variable

The evaluation process should be : after parsing the input we put all those
object into the queue. By reading the queue and using the stack to keep the
temporary values (both of them I implement on my own) i have to do like that
:
1) if it is an Operand put in on stack
2) if it is an function and
2)a we have enought arguments on stack we valuate the function
2)b we throw exepction
(i can distinguish beetwen this two types and here i don't need help,
because of the diffrent number of arguments they take and exceptions)

In the main loop I have to keep the refrence to the object taken from FIFO,
and here is the problem : what type of refrence it should be ? Not a symbol
because it is abstract, but it implements the Evaluate interface. I
confused. Any suggestions, please ?

 

[Joshua Cranmer]

In the main loop I have to keep the refrence to the object taken from
FIFO, and here is the problem : what type of refrence it should be ? Not
a symbol because it is abstract, but it implements the Evaluate
interface. I confused. Any suggestions, please ?

What do you mean, "Not a symbol because it is abstract"? Any type can be
used for reference, including abstract classes and interfaces. This would
be partial code for what I think you want to do:

LinkedList<Symbol> stack = new LinkedList<Symbol>();
Queue<Symbol> queue = new LinkedList<Symbol>();
// ... Fill queue ...
while (!queue.isEmpty()) {
Symbol head = queue.remove();
if (head instanceof Operand) {
stack.addLast(head);
} else if (head instanceof Function) {
// ... manipulate stack, etc.
}
}

 

[Oliver Wong]

Hello I 'm writing a simple ONP expression evaluator with functions in
java.
The constrains about the class hierarchy are like that :

abstract Symbol implements Evaluate (which contains
method evaluate)
/ \
/ \
/ \
abstract Function abstract Operand
/ | \ / |
\
Plus Minus Ln Literal Const Variable

The evaluation process should be : after parsing the input we put all
those
object into the queue. By reading the queue and using the stack to keep
the
temporary values (both of them I implement on my own) i have to do like
that
:
1) if it is an Operand put in on stack
2) if it is an function and
2)a we have enought arguments on stack we valuate the function
2)b we throw exepction
(i can distinguish beetwen this two types and here i don't need help,
because of the diffrent number of arguments they take and exceptions)

In the main loop I have to keep the refrence to the object taken from
FIFO,
and here is the problem : what type of refrence it should be ? Not a
symbol
because it is abstract, but it implements the Evaluate interface. I
confused. Any suggestions, please ?

There's no hard and fast rule, but as a rule of thumb, in this
situation you would use the least general type which is a supertype of all
the possible types you might get from your FIFO queue.

The issue is that you will probably want to do something with each
element you take out of the queue. Usually, to "do something" involves
invoking a method. Which means the method you want to invoke must be part
of the type of the reference (or you could use casting, but that gets
messy).

So say if you wanted to call the .doSomething() method on each
element, the type of the reference would need to be such that the
..doSomething() method is defined on that type. If the .doSomething()
method is defined in Symbol, then use Symbol as the type of the reference.
If the method is defined in Evaluate, then use Evaluate. If it's defined
in both, then it doesn't matter too much which one you use.

- Oliver

 

[Stefan Ram]

In the main loop I have to keep the refrence to the object
taken from FIFO, and here is the problem : what type of
refrence it should be ? Not a symbol because it is abstract,
but it implements the Evaluate interface. I confused. Any
suggestions, please ?

I have not fully understood your design, maybe I read not
careful enough.

I am also writing something like this, but have not yet
published the full code. At least, I can try to quote relevant
parts of the code here.

Since I have not looked at this code for several month, it
looks strange to me, too, but at least i already have written
some documentation earlier.

I am working with two stacks: on operator stack and one
value stack (the »designation stack«). Operators and Values
from the source code are considered to be »Tokens«. And
a token is expected to know how to »enroll« itself to the
stacks. This is what happens when parsing the source code
to an internal representation.

The interface »Token«:

interface Token extends de.dclj.ram.notation.programming.Designation
{ /** Enroll this token to the parser.
Every token knows how to enroll itself to the parser.
The stacks might be somewhat reduced, and
then the token might push itself on one of the stacks.
@param operatorStack the operator stack to be used for enrolling
@param designationStack the designation stack to be used for enrolling */
public void enroll
( final PlacementHint placementHint,
final OperatorStack operatorStack,
final DesignationStack designationStack );
/** Whether this token is a Prefix context.
+1 means that the next operator must be prefix.
This is the case:
At the start of a full expression.
After opening parentheses (like, for example, »(«, »[«, »{«).
After prefix or infix operators (like, for example, »+«, »*«, »/«, »-«, »,«, »;«)
After prefixed function symbols (like, for example, »SIN«, »LOG«)
0 means that the next operator must be infix.
This is the case:
After a numeral (like, for example, »0«)
After a closing parentheses (like, for example »)«, »]«, »}«) */
public boolean isPrefixContext(); }

When the token is an operator, it usually is enrolled as follows.

The method »acceptOperator«:

/** An operator is accepted by a stack pair given a placement hint.
The operator is pushed on the operator stack.
Possibly the stacks will be reduced before this happens
(depending on the priority of the given operators).
@param operatorToken an operator to be accepted
@param placementHint an instance of OperatorMustBePrefixPlacementHint if this operator
token appears in a context where a prefix operator is expected
@param operatorStack a stack of operators
@param designationStack a stack of designations */
public static void acceptOperator
( final OperatorToken operatorToken,
final PlacementHint placementHint,
final OperatorStack operatorStack,
final DesignationStack designationStack )
{ int rightPriority;
if( placementHint instanceof OperatorMustBePrefixPlacementHint )
{ rightPriority = getOperatorRightPriorityInPrefixContext( operatorToken ); }
else
{ rightPriority = getOperatorRightPriorityInInfixContext( operatorToken ); }
while( isReducible( operatorStack, rightPriority ))
{ OperatorToken operator = operatorStack.pop();
operator.reduce( placementHint, operatorStack, designationStack ); }
operatorStack.accept( operatorToken ); }

This takes care of the fact that an operator character, like
»-« might denote both a prefix operator and an infix operator.
Therefore, context is used to figure out which to assume and
then the appropriate priority is retrieved.

The above while loops reduces the stack as long as indicated
by the priority of the operators seen. Eventually, the operator
then is pushed onto the operator stack by the final »accept«.

A value, like a literal, is just pushed onto the operand stack:

The methode »acceptLiteral«:

public static void acceptLiteral
( final LiteralToken literalToken,
final PlacementHint placementHint,
final OperatorStack operatorStack,
final DesignationStack designationStack )
{ designationStack.accept( literalToken.value() ); }

Some parameters are not used above, but given for consistency
with the »acceptOperator« call.

So, back to the »reduce« operation: Every operator token knows
how to reduce itself to a designation of its operation (this
is not the evaluation yet, but an internal representation of
this application of this oprator including its operands):

The interface »OperatorToken«:

/** An operator token. There will be several
kinds of operator tokens. */
interface OperatorToken extends Token
{ /** Reduces the stack by this operator token.
It is assume that this token already has been removed
from the operator stack. Now it will usually pop some
designations from the designation stack, possibly evaluate
some of them (»strict« arguments) and then build a
reduction result and push it onto the designation stack.
For example, »+« might take two designations, which might
might be the number 2 and the number 3 and then push
the number 5 on the designation stack.
@param placementHint A placement hint indicates whether
the reduction takes place in a context where certain
types of operators are required or permissible
see {@link PlacementHint}.
*/
/* todoc: @params */
public void reduce
( final PlacementHint placementHint,
final OperatorStack operatorStack,
final DesignationStack designationStack ); }

An example is the »PRINT« keyword in a BASIC-like language.

It takes its operand from the designation stack and
pushes a new »print designation« on the designation stack.

The method »reduce«:

public void reduce
( final PlacementHint placementHint,
final OperatorStack operatorStack,
final DesignationStack designationStack )
{ designationStack.accept
( new PrintDesignation
( designationStack.popOrProduceNull() )); }

The »print designation« is an object representing the
print operation including its argument.

It has an »eval« operation, that can be called to
actually do the printing:

The method »eval«:

public de.dclj.ram.notation.programming.EvaluableDesignation
eval( de.dclj.ram.notation.programming.World world )
{ Value value =( Value )Evaluator.completeEvaluation
( argumentDesignation, world );
if( value instanceof NumericValue )
{ java.lang.System.out.println
( value.getPrintRepresentation() + " " ); }
else
{ java.lang.System.out.println
( value.getPrintRepresentation() ); }
return new Ok(); }

The result is a new designation indicating the success
of the operation.

In my implementation everything is considered to be built
of operators and operands. Each operator is defined by
two classes. The intention is to make extensions to the
language easy by requiring them just to add two new classes
per operator.

For example, »if ... then ...« also would be considered to
be an operator.

For example, the operator »-« is being specified as follows.

The class pair for the operator »-«:

/** A minus-operator token represents the operator "-". */
class MinusOperatorToken
extends DefaultInfixOperatorToken
implements PrefixOrInfixOperatorToken
{ public java.lang.String fixtext(){ return "-"; }
public int infixLeftPriority(){ return 640; }
public int infixRightPriority(){ return 639; }
public int prefixLeftPriority(){ return 680; }
public int prefixRightPriority(){ return 999 /* 681 */; }
public boolean isPrefix(){ return this.isPrefix; }
public void setPrefix(){ this.isPrefix = true; }
public void reduce
( final PlacementHint placementHint,
final OperatorStack operatorStack,
final DesignationStack designationStack )
{ de.dclj.ram.notation.programming.EvaluableDesignation
rightDesignation = designationStack.pop();
if( placementHint instanceof OperatorMustBePrefixPlacementHint ||
this.isPrefix() )
{ designationStack.accept
( new DifferenceDesignation
( new NumericValue( "0" ), rightDesignation )); }
else
{ de.dclj.ram.notation.programming.EvaluableDesignation
leftDesignation = designationStack.pop();
designationStack.accept
( new DifferenceDesignation
( leftDesignation, rightDesignation )); }}
public MinusOperatorToken newInstance(){ return new MinusOperatorToken(); }
private boolean isPrefix = false;
public java.lang.String toString()
{ return this.getClass().getName(); }}

/** A difference designation designates the difference of two designations. */
class DifferenceDesignation
implements de.dclj.ram.notation.programming.EvaluableDesignation
{ final de.dclj.ram.notation.programming.EvaluableDesignation leftDesignation;
final de.dclj.ram.notation.programming.EvaluableDesignation rightDesignation;
public DifferenceDesignation
( final de.dclj.ram.notation.programming.EvaluableDesignation leftDesignation,
final de.dclj.ram.notation.programming.EvaluableDesignation rightDesignation )
{ this.leftDesignation = leftDesignation;
this.rightDesignation = rightDesignation; }
public de.dclj.ram.notation.programming.EvaluableDesignation
eval( de.dclj.ram.notation.programming.World world )
{ final java.lang.Object l =
Evaluator.completeEvaluation( leftDesignation, world );
final java.lang.Object r =
Evaluator.completeEvaluation( rightDesignation, world );
final java.math.BigDecimal left =
new java.math.BigDecimal((( NumericValue )l ).asDecimal(),
(( BasicWorld )world ).config().getMathContext() );
final java.math.BigDecimal right =
new java.math.BigDecimal((( NumericValue )r ).asDecimal(),
(( BasicWorld )world ).config().getMathContext() );
final java.math.BigDecimal difference = left.subtract
( right, (( BasicWorld )world ).config().getMathContext() );
return new NumericValue( difference.toString() ); }
public de.dclj.ram.notation.programming.EvaluableDesignation
continue_( final de.dclj.ram.notation.programming.World world )
{ return null; }
public java.lang.String toString()
{ return "DIFFERENCE-DESIGNATION( \"" +
leftDesignation + "\",\"" + rightDesignation + "\")" ; }}