Representing information: binary, hex, ascii Corresponding Reading
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UNDERSTANDING CLASSES
CMSC 150: Lecture 17
String Literals
"Wilco"
String literal: a sequence of characters flanked by "
Want to operate on the sequence
E.g.,
pull out substrings, locate characters, etc.
No primitive data type directly handles such
sequences
Primitives
Primitives are only for storing information
int
myInteger
= 5;
boolean myBoolean
= true;
char
myCharacter = 'c';
No associated methods
myInteger.getValue();
// NOT VALID!!
So a primitive type to hold strings wouldn't help
Let's Write Our Own Class!
public class String
{
// instance variables
private char[] data;
// constructor
public String(char[] value)
{ … }
// some useful methods
public char
charAt(int index) { … }
public int
indexOf(char ch)
{ … }
public String substring(int beginIndex) { … }
…
}
Our String Class
We now have a new class named String
Stores data
Provides useful methods
Create new objects of this class type:
String myString = new String("Wilco");
String ourString = new String("Son Volt");
Use methods we wrote:
int index = ourString.indexOf(' ');
String firstWord = ourString.substring(0, index);
Use String Class in Other Classes
public class FancyEmailReader {
…
public FancyEmailReader()
{
…
String userHost = accountField.getText();
int index = userHost.indexOf('@');
String user = userHost.substring(0, index);
String host = userHost.substring(index + 1);
…
}
}
In Memory
String myString;
127
"myString" 128
Enough space to hold a
memory address
129
130
131
132
0
In Memory
String myString;
myString =
new String("Wilco");
127
"myString" 128
129
Enough space to hold an
object of our class type
130
131
132
130
In Memory
String myString;
myString =
new String("Wilco");
127
"myString" 128
Data
129
130
131
132
130
In Memory
String myString;
myString =
new String("Wilco");
127
"myString" 128
129
Methods
130
131
132
130
The String Class
Gives us a new data type
Allows us to meaningfully store info
And have methods to operate on that info
Use String whenever it is useful
Back to Last Lab…
public class IMClient {
…
String lineFromServer = connection.in.nextLine()
…
}
linefromServer contains info like
"IM_IN2:Lilly:T:T:<HTML><BODY>…"
Back to Last Lab…
public class IMClient {
…
String lineFromServer = connection.in.nextLine()
…
}
linefromServer contains info like
"IM_IN2:Lilly:T:T:<HTML><BODY>…"
Want methods to pull out buddy, status, msg
String class doesn't provide such methods
Let's Write Our Own!
public class TOCServerPacket {
String contents;
public TOCServerPacket(String packetContents) { … }
public boolean isError()
{ … }
public boolean isBuddyUpdate() { … }
public boolean isIncomingIM() { … }
public
public
public
public
String
String
String
String
getBuddyName()
getBuddyStatus()
getErrorCode()
getMessage()
{
{
{
{
…
…
…
…
}
}
}
}
private String getPrefix(String uncutPacket) { … }
private String getSuffix(String uncutPacket, int colonNumber) { … }
}
Let's Write Our Own!
public class TOCServerPacket {
String contents;
public TOCServerPacket(String packetContents) { … }
public boolean isError()
{ … }
public boolean isBuddyUpdate() { … }
public boolean isIncomingIM() { … }
public
public
public
public
String
String
String
String
getBuddyName()
getBuddyStatus()
getErrorCode()
getMessage()
{
{
{
{
…
…
…
…
}
}
}
}
private String getPrefix(String uncutPacket) { … }
private String getSuffix(String uncutPacket, int colonNumber) { … }
}
Similar to the String Class
class gives us a new data type
TocServerPacket
Allows us to meaningfully store info
The
And have methods to operate on that info
String returned from the server
isIncomingIM(), getBuddyName(), getBuddyStatus()
Use TocServerPacket whenever it is useful
Use TOCServerPacket in Other Classes
public class IMControl {
…
public void dataAvailable()
{
…
String lineFromServer = connection.in.nextPacket();
// get the buddy name
int firstIndex = lineFromServer.indexOf(':');
int secondIndex = lineFromServer.indexOf(':', firstIndex + 1);
String buddy =
lineFromServer.substring(firstIndex + 1, secondIndex);
// pull out message
while ( … ) { … }
}
}
Use TOCServerPacket in Other Classes
public class IMControl {
…
Use a TOCServerPacket!
public void dataAvailable()
{
…
String lineFromServer = connection.in.nextPacket();
// get the buddy name
int firstIndex = lineFromServer.indexOf(':');
int secondIndex = lineFromServer.indexOf(':', firstIndex + 1);
String buddy =
lineFromServer.substring(firstIndex + 1, secondIndex);
// pull out message
while ( … ) { … }
}
}
Use TOCServerPacket in Other Classes
public class IMControl {
…
Use a TOCServerPacket!
public void dataAvailable()
{
…
String lineFromServer = connection.in.nextPacket();
TOCServerPacket packet =
new TOCServerPacket(lineFromServer);
String buddy = packet.getBuddyName();
String message = packet.getMessage();
}
}
In Memory
String line =
conn.in.nextPacket();
TOCServerPacket packet;
127
"packet"
128
129
Enough space to hold a memory
address
130
131
132
0
In Memory
String line =
conn.in.nextPacket();
TOCServerPacket packet;
127
packet = new
TOCServerPacket(lineFromServer);
"packet"
128
129
130
131
132
130
In Memory
String line =
conn.in.nextPacket();
TOCServerPacket packet;
127
packet = new
TOCServerPacket(lineFromServer);
"packet"
128
Data
129
130
131
132
130
In Memory
String line =
conn.in.nextPacket();
TOCServerPacket packet;
127
packet = new
TOCServerPacket(lineFromServer);
"packet"
128
129
Methods
130
131
132
130
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