Data structures / collections

Data structures / collections

Collections • Sometimes we want to group data together into collections so we can deal with related data all at once rather that each component individually. • In computing, collections are often referred to as data structures. • You must give the collection a name • There are two basic types of collections: • homogeneous: where all elements in the collection must be of the same type or class. • heterogeneous: where individual members can be of different types or classes CS 1

Collections Some examples of collections: • a group of playing cards that make up a pile in a card game • lists of names and addresses for a mailing list • parts that make up an automobile • lines of text and figures that make up a document CS 1

Data structures • For any data structure you need to be able to refer to the whole structure, and/or its individual elements. • Each data structure will also have a set of operations you want to perform on it, such as: • create a structure • add a new element • access values • make a copy • Some operations affect the entire structure, others just specific members. CS 1

Data structures • Sometimes it is important that the elements of a data structure be in some sort of order. • Ordering doesn't change the data just the way it is kept. • When performing operations on an ordered structure the order must be maintained. • Data may be ordering in • ascending order (values are increasing) • descending order (values are decreasing) • What does it mean for characters? (‘C’ < ‘c’ < ‘d’) CS 1

Data structures - other kinds of ordering • First In First Out (FIFO) • Last In First Out (LIFO) • Priority CS 1

Data Structures • Data structures permit the storage of related data for use in your program. • Dynamic data structures hold an unknown number of elements. • Arrays hold a specified number of elements. • Generic type data structures hold data elements of different data types. • Arrays hold data of the same data type. CS 1

Data Structures • Types of dynamic generic data structures are: • Stacks • Queues • Lists • Trees CS 1

What is a Stack? • A stack is similar in concept to a pile of plates, books, blocks, boxes, etc. • The first item put on the stack is on the bottom of the stack. • All items added to the stack are placed on top of the previous item. • The last item put on the stack is on the top of the stack. CS 1

Stacks • Stacks are called Last-in First-out (LIFO) data structures. • The last plate put on the top of the stack is the first plate removed from the stack. • The first plate put on the top of the stack is the last plate removed from the stack. CS 1

Characteristics of Stacks • Data can only be place on the top of the stack. • Data can only be removed from the top of the stack. • Data can only be removed from the bottom of the stack if there is only one item on the stack. • Data can not be removed from the middle of the stack without first removing all items from the top. CS 1

Stack Example Create a stack with the following information: • Place the following numbers in a stack in this order: 5, 8, 10, 2, 4, 12 • Remove number 10 from the stack. • Put 3, 7, 1 on the stack. • Now remove 5 from the bottom of the stack and put it on the top of the stack. CS 1

Stack Behaviors • new • create the stack • push • add an element to the top of the stack • pop • remove an element from the top of the stack • empty • tests if this stack is empty. • peek • looks at the object at the top of this stack without removing it from the stack CS 1

Example • What stack is exists after executing the following commands? • push(3) • push(6) • push(8) • push(1) • pop( ) • pop( ) • push(14) CS 1

Stack Implementation • Java has a Stack class designed to hold Objects. • BUT, you could implement a stack using a list, a queue, or an array. CS 1

Stack Implementation • A Stack class can be written using arrays that will simulate a stack in your programs. • A restriction of using arrays to implement a stack is that the total size of the stack is limited. • Why? • Can you define a method that will resize the array to hold a stack of any size? CS 1

Stack Implementation • Create a Stack class to hold Strings. • Assume that the first element of the array is the bottom element on the stack. • How do we know the number of items on the stack? • How do we know if there is anything in the stack? CS 1

Stack Implementation import java.io.*; class MyStack { private String [] stack; private int index; public int size; public MyStack() { this.stack = new String[10]; this.size = 10; this.index = 0; } public MyStack(int size) { stack = new String[size]; this.size = size; index = 0; } public void push(String s) { if (index < size) stack[index++] = s; else System.out.println("Overflow!!!"); } public String pop() { String r = ""; if (index > 0) r = stack[--index]; else r = “Underflow!!!"; return (r); } public boolean empty() { return (index == 0); } public String peek() { String r = ""; if (index > 0) r = stack[index-1]; return (r); } } Note: Not 100% correct. We should handle overflow and underflow better !!! CS 1

Stack Implementation • Now that we have implemented the Stack class, how can we use it? import MyStack; class MyStackTest { public static void main(String[ ] args) { MyStack s = new MyStack(); System.out.println( s.size ); s.push("a"); s.push("b"); s.push("c"); s.push("d"); System.out.println( "------------" ); while (! s.empty() ) System.out.println( s.pop() ); System.out.println( s.pop() ); // Let’s cause an underflow! } } C:\>Test\java MyStackTest 10 ------------ d c b a Underflow!!! CS 1

Stack Implementation • How do we store data of any type into our stack? CS 1

Object Class • Every class in Java is a child of the Object class. • The Object class is the most generic class in Java. • Therefore, we should be able to create a stack of type Object to store any data into the stack. • This is not strictly true. Primitive data types are not descendents of type Object. CS 1

Type Wrapper Classes • Each primitive data type has a type wrapperclass. • The type wrapper class for: • double is Double, • int is Integer, • float is Float, • boolean is Boolean. CS 1

Type Wrapper Classes • Type wrapper classes allow you to manipulate primitive data types as objects of class Object. • The type wrapper class contains functions that manipulate the data as a class. CS 1

Type Wrapper Classes • The primitive data type is not automatically a type wrapper version of the data type. • To define a version of the primitive data type as a type wrapper you need to create a new instance of the type wrapper. Integer newInteger = new Integer(98); Double newDouble = new Double(986.3); CS 1

Stack Implementation • How do we redefine our Stackclass to hold data of any data type? CS 1

Stack Implementation using Objects import java.io.*; class MyOStack { private Object [] stack; private int index; public int size; public MyOStack() { stack = new Object[10]; size = 10; index = 0; } public MyOStack(int size) { stack = new Object[size]; this.size = size; index = 0; } public void push(Object s) { if (index < size) stack[index++] = s; else System.out.println("Overflow!!!"); } public Object pop() { Object r = null; if (index > 0) r = stack[--index]; else r = Underflow!!!"; return (r); } public boolean empty() { return (index == 0); } public Object peek() { Object r = ""; if (index > 0) r = stack[index]; return (r); } } CS 1

Vectors • The Vector class implements a growable array of objects. • Like an array, it contains components that can be accessed using an integer index. • The size of a Vector can grow or shrink as needed to accommodate adding and removing items after the Vector has been created. CS 1

Vectors • Each vector tries to optimize storage management by maintaining a capacity and a capacityIncrement. • The capacity is always at least as large as the vector size; it is usually larger because as components are added to the vector, the vector's storage increases in chunks the size of capacityIncrement. • An application can increase the capacity of a vector before inserting a large number of components; this reduces the amount of incremental reallocation. CS 1

Vectors The Vector class is defined in this package. Adds a new element to the end of the vector This creates a vector whose internal data array has the default capacity of 10. Print vector’s last element. Reset vector’s size. import java.util. *; class VectorExample { publicstaticvoid fillVector(Vector vector) { vector.addElement(“Acura"); vector.addElement(“Audi"); vector.addElement(“BMW"); vector.addElement(“Lexus"); } publicstaticvoid main(String[] args) { Vector vector = new Vector(); if ( vector.isEmpty() ) System.out.println("Vector is empty"); fillVector(vector); System.out.println(vector.size() ); System.out.println(vector.lastElement() ); vector.setSize(3); System.out.println(vector.lastElement() ); } } CS 1

Vectors import java.util. *; class VectorExample { publicstaticvoid fillVector(Vector cars) { cars.addElement(“Acura"); cars.addElement(“Audi"); cars.addElement(“BMW"); cars.addElement(“Lexus"); } publicstaticvoid main(String[ ] args) { Vector carList = new Vector( ); if ( carList.isEmpty( ) ) System.out.println("Vector is empty"); fillVector(carList); System.out.println(carList.size( ) ); System.out.println(carList.lastElement( ) ); carList.setSize(3); System.out.println(carList.size( ) ); System.out.println(carList.lastElement( ) ); } } Output: Vector is empty 4 Lexus 3 BMW CS 1

Vector constructors Note: no type for objects is specified ! • Vector() • Constructs an empty vector so that its internal data array has size 10 and its standard capacity increment is zero. • Vector(int initialCapacity) • Constructs an empty vector with the specified initial capacity and with its capacity increment equal to zero. • Vector(int initialCapacity, int capacityIncrement) • Constructs an empty vector with the specified initial capacity and capacity increment. • Vector(Collection c) • Constructs a vector containing the elements of the specified collection, in the order they are returned by the collection's iterator. At this time, do not bother about this one! CS 1

Vectors Type casting is required. String myCar = (String) carList.elementAt( i ); • Objects we store in a vector can be accessed by giving their index position in the vector, much like the way we did with the array. • We need to type cast the element to a String, because the elements inside a vector can be an instance of any class. • A compile-time error will occur if we forget to type-cast the returned element. CS 1

Vectors Car myCar; int limit = carList.size( ); for (int k=0; k < limit; k++) { myCar = (Car) carList.elementAt( k ); System.out.println( myCar.maker()); } • Here is a code to display the name of all cars in the carList vector. • Vector elements are always objects. They cannot be primitive types. CS 1

Data structures / collections