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Introduction
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Must maintain references to multiple objects
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E.g., 10 cars, 10 songs, 10 newspaper articles, 10 course descriptions
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Could use files:
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Keep all objects in a file
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Find object in the file
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But files are too slow! Why? I/O is expensive!
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Objective: Read in data once from file into program (memory) - Perform
search in memory. Much faster. Easier to manipulate data.
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How? Requires collection:
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a group of objects
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is also a Java object
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individual objects are elements
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We've seen search as an example. Other examples? Alphabetize a list.
Compute averages. Then count number above average, number below. Etc.
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Collections of Objects
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Overview:
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Create a collection (constructor)
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Add an object to the collection
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Process the objects in the collection
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java.util.Vector class
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One way to create collections in Java (others too!)
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Constructor - Vector () - creates a Vector object
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Method - addElement (object reference)
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Example:
import java.util.*;
...
Vector messages = new Vector ();
String aSingleMessage = "Enjoy spring break";
messages.addElement (aSingleMessage);
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Using this, can we write a program that reads in data lines from a file?
Example:
import cucs.*;
import java.util.*;
...
Vector messages = new Vector ();
BasicDataReader inFile = new BasicDataReader ("mydata.txt");
while (!inFile.eof ())
messages.addElement (inFile.readLine());
How can we access elements? Stay tuned
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Subclasses
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Note that all objects in Java (i.e., instances of classes) can be treated
as an instance of the Object class.
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So, a String, a Date, a Thermometer, etc. are also Objects
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Why is this important?
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Vectors are collections of Objects. Can enter any instance of any Java
class into a vector.
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Accessing Objects in a Vector
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Vector defines the methods:
int size () - number of elements
Object elementAt (int index) - return ith element
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Example:
import cucs.*;
import java.util.*;
...
Vector messages = new Vector ();
BasicDataReader inFile = new BasicDataReader ("mydata.txt");
while (!inFile.eof ())
messages.addElement (inFile.readLine());
...
String tmp;
int s = messages.size();
int i = 1;
while (i <= s) {
tmp = (String) messages.elementAt (i);
System.out.println (tmp);
}
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Collections of primitives
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What about collections of ints, doubles, floats, etc?
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Stay tuned.
