CSC 205 – Java Programming II Lecture 30 April 3, 2002

Stack A stack is an abstract data type (ADT) that contains a sequence of elements and honors the last-in-first-out (LIFO) rule. Conceptually, the only element that can be removed, accessed, modified is the one which was most recently inserted. Pop and push are the two basic stack operations

Stack PUSH top POP last-in-first-out

Examples Green Red Yellow Red Yellow Green Red Yellow pop push green

Java Stack Class The Java Stack class represents a stack of objects. It outdated the Java Collections Framework Available since JDK1.0 It extends class Vector with 5 additional operations. With its top at the index size()-1 top

Java Stack API boolean empty() Tests if this stack is empty. Object peek()           Looks at the object at the top of this stack without removing it from the stack. Object pop()           Removes the object at the top of this stack and returns that object as the value of this function. Object push(Object item)           Pushes an item onto the top of this stack. int search(Object o)           Returns the 1-based position where an object is on this stack.

Example Checking for balanced braces The braces are balanced if abc{defg{ijk}{l{mn}}op}qr balanced abc{def}}ghij{kl}m not balanced The braces are balanced if Each time you encounter a }, it matches an already encountered { When you reach the end of the string, you have matched each {

Pseudocode Solution A simplified solution in pseudocode while (not at the end of the string) { if (the next character is a ‘{’) { stack.push(‘{’) } else if (the character is a ‘}’) openBrace = stack.pop()

Examples {a{b}c} {a{bc} {ab}c} { { { { { { { push “}” pop Stack empty balanced {a{b}c} { { { {a{bc} push “}” pop Stack not empty not balanced { push “}” pop Stack empty balanced {ab}c}

Problems w/ The Stack Class All the Vector methods can be used For examples myBookStack.get(7); myBookStack.remove(0); //the element right at the “bottom” can be removed!

Convenient or Risky? myBookStack.get(7); myBookStack.remove(0);

Alternative Designs Implement a RealStack class Use a LinkedList object as its only field Provide push and pop methods public Object push(Object o) { list.add(o); return o; } public Object pop(Object o) { return list.removeLast(o);

Activation Stack Stacks are used almost exclusively by computer systems An activation stack is used by a compiler /interpreter to save the method information when a method is called The stack is part of the main memory. Other parts include a heap to hold data

Activation Records Each activation record includes The return address The value of each argument The values of the method’s other local variables

Decimal To Binary public void processInput(String s) { writeBinary(Integer.parseInt(s)); //RA1 } public void writeBinary(int n) { if (n<=1) throw new IllegalArgumentException(); gui.println(n); else { writeBinary(n/2); //RA2 gui.println(n%2);

Example RA2 1 RA2 RA2 RA2 3 3 3 RA1 RA1 RA1 RA1 RA1 6 6 6 6 6