Errata for Pragmatic Unit Testing in Java with JUnit

#46928: In the description of the invariant for the MyStack class the authors write "However we manipulate the index variable next_index, one thing is supposed to be always true: (next_index >= 0 && next_index < stack.length)."

The upper bound of this condition is off by one, the condition should be: (... && next_index <= stack.length).

next_index is 0 when the stack is empty and is one larger than the index of the top element otherwise (its value could be used to represent the number of items on the stack). When the stack has 99 elements in it, next_index is 99. When the client pushes another element on to the stack, "stack[next_index++] = aString;" puts the 100th value at index 99 and increments next_index to 100 (stack.length). This is a valid state for the stack to be in, and it should be allowed by the invariant. In this example, however, the authors' invariant fails.

The same correction should be made to implementation of the checkInvariant method in the text, as well as in the MyStack.java file in code on the web site.

The following code sample illustrates the error:
public class Driver {
public static void main(String[] args) throws InvariantException {
MyStack s = new MyStack();

for (int i = 0; i < 100; ++i) {
s.push(Integer.toString(i));
s.checkInvariant();
}
}
}

The code produces:
Exception in thread "main" InvariantException: next_index out of range: 100 for stack length 100
at MyStack.checkInvariant(MyStack.java:38)
at Driver.main(Driver.java:7)--Andrew R. Dalton

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