Stacks - Data Structures and Abstractions with Java - page 113

top = stringStack.pop(); // removes and returns "Jane"
System.out.println(top + " is removed from the stack.");

Parts a through e of Figure 5-2 show five additions to the stack. At this point, the stack contains—
from top to bottom—the strings Joe , Jane , Jill , Jess , and Jim . The string at the top of the stack is Joe ;
peek retrieves it. The method pop retrieves Joe again and then removes it (Figure 5-2f ). A subsequent
call to peek retrieves Jane . Then pop retrieves Jane and removes it (Figure 5-2g).
Three more calls to pop would remove Jill , Jess , and Jim , leaving the stack empty. A subse-
quent call to either pop or peek would return null .

FIGURE 5-2

A stack of strings after (a) push adds Jim ; (b) push adds Jess ;
(c) push adds Jill ; (d) push adds Jane ; (e) push adds Joe ; (f ) pop
retrieves and removes Joe ; (g) pop retrieves and removes Jane

Joe

Jane

Jane Jane

Jill
Jess

Jill Jill Jill Jill

Jess Jess

Jess Jess Jess

Jim Jim Jim Jim Jim Jim Jim

(a) (b) (c) (d) (e) (f)

(g)

Programming Tip: Methods such as peek and pop must behave reasonably when the
stack is empty. Here, we specify that they return null . Another possibility is to have them
throw an exception.

Question 1 After the following statements execute, what string is at the top of the stack
and what string is at the bottom?

StackInterface<String> stringStack = new OurStack<String>();
stringStack.push("Jim");
stringStack.push("Jess");
stringStack.pop();
stringStack.push("Jill");
stringStack.push("Jane");
stringStack.pop();

Question 2 Consider the stack that was created in Question 1, and define a new empty
stack nameStack .

a. Write a loop that pops the strings from stringStack and pushes them onto nameStack .

b. Describe the contents of the stacks stringStack and nameStack when the loop that you

just wrote completes its execution.