Transcript Chapter 10: The Classes Vector and String, and Enumeration Types

Vectors, Strings, and
Enumeration Data Types
Chris Kiekintveld
CS 2401 (Fall 2010)
Elementary Data Structures and Algorithms
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;
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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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Adding Elements
Vector<String> stringList = new Vector<String>();
stringList.add("Spring");
stringList.add("Summer");
stringList.addElement("Fall");
stringList.addElement("Winter");
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add and addElement
have identical functionality
10
Accessing Elements
stringList.get(0);
stringList.get(3);
stringList.get(4);
// “Spring”
// “Winter”
// ArrayIndexOutOfBounds Exception
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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 autoboxing
list.add(new Integer(25)); // without autoboxing
int tmp = list.get(0);
// with autounboxing
int 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)
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Vectors, Strings, and
Enumeration Data Types
(continued)
Chris Kiekintveld
CS 2401 (Fall 2010)
Elementary Data Structures and Algorithms
Exercise
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());
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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
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));
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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 type
Grades 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
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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!”);
}
}
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Enumeration Types
 See this site for more discussion and examples:
http://download.oracle.com/javase/1.5.0/docs/guide/l
anguage/enums.html
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Strings
 Strings are essentially arrays of characters
 The string class provides many functions for
manipulating strings
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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)
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