Stacks and Queues - Advanced Topics in Java: Core Concepts in Data Structures - page 112

public void push(int n) {
if (top == MaxStack - 1) {
System.out.printf("\nStack Overflow\n");
System.exit(1);
}
++top;
ST[top] = n;
} //end push

To illustrate, after the numbers 36, 15, 52, and 23 have been pushed onto S , our picture in memory looks like
Figure 4-2 .

S

3

top

36 15 52 23

ST

0 1 2 3 4 5

Figure 4-2. Stack view after pushing 36, 15, 52, and 23

Finally, to pop an item off the stack, we return the value in location top and decrease top by 1 . The basic idea is
as follows:

set hold to ST[top]
subtract 1 from top
return hold

Again, we must guard against trying to take something off an empty stack. What should we do if the stack is empty
and pop is called? We could simply report an error and halt the program. However, it might be better to return some
“rogue” value, indicating that the stack is empty. We take the latter approach in our function pop . Here is the instance
method pop in the Stack class:

public int pop() {
if (this.empty())return RogueValue; //a symbolic constant
int hold = ST[top];
--top;
return hold;
}

Note that even though we have written pop to do something reasonable if it is called and the stack is empty, it is
better if the programmer establishes that the stack is not empty (using the empty function) before calling pop .
Given the class Stack , we can now write Program P4.1, which reads some numbers, terminated by 0 , and prints
them in reverse order. Note that the word public is dropped from the class Stack in order to store the entire program
in one file, StackTest.java .