chris kiekintveld cs 2401 (fall 2010) elementary data structures and algorithms vectors, strings,...

Chris KiekintveldCS 2401 (Fall 2010)

Elementary Data Structures and Algorithms

Vectors, Strings, and Enumeration Data Types

A Weakness of Arrays

Suppose we declare an array of “Student” objects:

Student[] students = new Student[10];

What if a new student joins the class? The size of an array cannot be increased after it is

instantiated

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Resizing an Array (the hard way)

Student[] students = new Student[10];

// do some stuff…

// now we need to add student 11

Student[] students2 = new Student[11];

for(int i = 0; i < students.length; i++) {

students2[i] = students[i];

}

student[10] = new Student();

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class Vector

The class Vector can be used to implement a list, replacing a simple array

The size of a Vector object can grow/shrink during program execution

The Vector will automatically grow to accommodate the number of elements you put in it

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class Vector (continued)

The class Vector is contained in the package java.util

Programs must include either: import java.util.*; import java.util.Vector;

Vector Declaration

Declare/initialize

Vector<Student> students = new Vector<Student>();

The syntax <…> is used to declare the type of object that will be stored in the Vector

If you add a different type of object, an exception is thrown Not strictly necessary, but highly recommended (compiler

warning)

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Vector Size/Capacity

The size of a vector is the number of elements The capacity of a vector is the maximum number of elements

before more memory is needed If size exceeds capacity when adding an element, the capacity

is automatically increased Declares a larger storage array Copies existing elements, if necessary Growing the capacity is expensive!

By default, the capacity doubles each time

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Setting Initial Capacity

If you know you will need a large vector, it may be faster to set the initial capacity to something large

Vector<Student> students = new Vector<Student>(1000);

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Size and capacity

Get the current size and capacity:Vector<Student> students = new Vector<Student>(1000);

students.size(); // returns 0

students.capacity(); // returns 1000

Setting size and capacity:// adds null elements or deletes elements if necessary

students.setSize(10);

// increases capacity if necessary

students.ensureCapacity(10000);

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Vector<String> stringList = new Vector<String>();

stringList.add("Spring");stringList.add("Summer");stringList.addElement("Fall");stringList.addElement("Winter");

Adding Elements

add and addElementhave identical functionality

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stringList.get(0); // “Spring”stringList.get(3); // “Winter”stringList.get(4); // ArrayIndexOutOfBounds Exception

Accessing Elements

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Primitive Data Types and the class Vector

Every component of a Vector object is a reference Primitive data types are not objects

Corresponding to each primitive data type, Java provides a wrapper class

JDK 5.0 provides autoboxing and auto-unboxing of primitive data types

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Primitive Data Types and the class Vector (continued)

Creating a Vector of Integer objects

Vector<Integer> list = new Vector<Integer>();

list.add(13); // with autoboxinglist.add(new Integer(25)); // without autoboxing

int tmp = list.get(0); // with autounboxingint tmp2 = list.get(0).intValue() //without

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Vector and the foreach loop

Each Vector object is a collection of elements You can use a foreach loop to process its elements Exactly like using a foreach loop with an array

Syntax:for (type identifier : vectorObject)

statements

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Members of the class Vector

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Members of the class Vector (continued)

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Members of the class Vector (continued)

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Members of the class Vector (continued)

Chris KiekintveldCS 2401 (Fall 2010)

Elementary Data Structures and Algorithms

Vectors, Strings, and Enumeration Data Types

(continued)

Exercise

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Vector<Integer> a = new Vector<Integer>();

a.add(4);

a.add(7);

a.add(10);

a.set(1, 5);

int tmp = a.remove(0);

System.out.println(tmp);

System.out.println(a.indexOf(new Integer(10));

System.out.println(a.toString());

a.clear();

System.out.println(a.isEmpty());

Multi-dimensional Vectors

Can we have a 2d vector? Yes! Just like a 2d array, but notation is more cumbersome

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Multi-dimensional Vectors

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Vector<Vector<Integer>> a = new Vector<Vector<Integer>>();

for (int i = 0; i < 4; i++) {

Vector<Integer> tmp = new Vector<Integer>();

for (int j = 0; j < 4; j++) {

tmp.add(i+j);

}

a.add(tmp);

}

System.out.println(a.get(2).get(3));

System.out.println(a.get(0).get(2));

Vector vs. ArrayList

Java has another class called ArrayList This class is almost identical in function to Vector,

and has most of the same methods ArrayList is typically faster ArrayList should *not* be used if your code is

multi-threaded (i.e., if you allow parallel execution)

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Enumeration Types

Enumeration or enum types User-defined data types User specifies the values of that data type Defined using the key word enum

Syntax example: enum Grades {A, B, C, D, F};

The values are identifiers Called enumeration or enum constants Must be unique within an enum type

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Enumeration Types (continued)

Each enum type is a special type of class Values are (special types of) objects of that class

Using an enum typeGrades myGrade;

myGrade = Grades.B;

System.out.println (“myGrade: ” + myGrade);

Each enum constant has an ordinal value Ordinal value of the first enum constant is 0

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Enumeration Types (continued)

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Enumeration Types (continued)

Because each enum type is a class, it can contain Constructors, (private) data members, and methods

enum type considerations Defined using enum rather than class enum types are implicitly final enum constants are implicitly static You cannot instantiate objects using the operator new Constructors are implicitly private You cannot create new classes from an enum type

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Enumeration Types (continued)

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public enum Directions{North, South, East, West};

public int xPos = 0;

public int yPos = 0;

public void move(Directions dir) {

switch(dir) {

case Directions.North:

yPos++;

break;

case Directions.South:

yPos--;

break;

case Directions.East:

xPos++;

break;

case Directions.West:

xPos--;

break;

case default:

System.out.println(“Invalid direction!”);

}

}

Enumeration Types

See this site for more discussion and examples: http://download.oracle.com/javase/1.5.0/docs/guide/language/enums.html

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Strings

Strings are essentially arrays of characters The string class provides many functions for

manipulating strings Searching/matching operations Replacing characters Finding characters Trimming whitespace Etc.

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class String (Revisited)

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class String (Revisited) (continued)

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class String (Revisited) (continued)

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class String (Revisited) (continued)