operators
Class PSExch

java.lang.Object
  operators.PSExch

All Implemented Interfaces:

PSObject, PSOperator, PSStackOperator

public class PSExch

extends java.lang.Object

implements PSStackOperator

Implements PSStackOperator interface.

Methods such as getOperatorToken() are inherited by PSStackOperator interface which in turn, are inherited by PSOperator.This concrete class is responsible for exchanging the two top elements of stack.

Constructor Summary

PSExch()

Method Summary

void	action(PSStack s) Exchanges the two top literals of stack.
java.lang.String	getOperatorToken() Returns the token identifying this operator.
boolean	isValid(PSStack s) Checks if it valid to use this operator now.

Methods inherited from class java.lang.Object

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

Constructor Detail

PSExch

public PSExch()

Method Detail

getOperatorToken

public java.lang.String getOperatorToken()

Returns the token identifying this operator.

Returns the name of the operand that this concrete class simulates,here "exch".

Precondition : A valid operator class has been created.
Postcondition : A string is returned,labeling the semantics of this class.

Specified by:

getOperatorToken in interface PSOperator

Returns:

The token identifying this operator.

isValid

public boolean isValid(PSStack s)
                throws PSSyntaxException

Checks if it valid to use this operator now.

Checks if the exchange of the two toppest elements on stack can be made.

Precondition : A valid instance of PSStack is given.
Postcondition : A reply to whether the elements of the stack can be exchanged.

Specified by:

isValid in interface PSOperator

Parameters:

s - A reference to a PSStack instance.

Returns:

True if the operation can be performed,false otherwise.

Throws:

PSSyntaxException - The syntax of the token is wrong.

action

public void action(PSStack s)

Exchanges the two top literals of stack.

Precondition : A valid stack is given to method.
Postcondition : Exchanges the two top literals of stack.

Specified by:

action in interface PSStackOperator

Parameters:

s - The Run-time Stack where the elements are stored.