operators
Class PSSub

java.lang.Object
  operators.PSSub

All Implemented Interfaces:

PSArithmeticOperator, PSObject, PSOperator

public class PSSub

extends java.lang.Object

implements PSArithmeticOperator

Implements PSArithmeticOperator interface.

Methods such as getOperatorToken() are inherited by PSArithmenticOperator interface which in turn, are inherited by PSOperator.This concrete class is responsible for the sutraction of two numbers found on the top of the stack.

Before the subtraction,action() checks : First,there have to be at least two elements inside the stack. Second, both operands must be numbers,ie both are PSNumber's.Last,the addition is performed.

Constructor Summary

PSSub()

Method Summary

void	action(PSStack s) Performs subtraction on two operands.
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

PSSub

public PSSub()

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 "sub".

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.

The subtraction can occur only if the stack contains two objects,both being of type PSNumber.

Precondition : A valid instance of PSStack is given.
Postcondition : A reply to the question whether the addition can occur.

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)

Performs subtraction on two operands.

Performs the subtraction upon the top two elements of the stack .

Precondition : A valid stack is given to method with at least two PSNumber operands.
Postcondition : Subtraction will be performed and the result will be stored back to the stack.

Specified by:

action in interface PSArithmeticOperator

Parameters:

s - The Run-time Stack where the operands (here literals) are retrieved and stored.