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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.
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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 • intmyInteger = 5; • booleanmyBoolean = 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 intindexOf(charch) { … } public String substring(intbeginIndex) { … } … }
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; • Enough space to hold a memory address 127 "myString" 128 0 129 130 131 132
In Memory • String myString; • myString = new String("Wilco"); • Enough space to hold an object of our class type 127 "myString" 128 130 129 130 "Wilco" charAt() indexOf() Substring() … 131 132
In Memory • String myString; • myString = new String("Wilco"); 127 "myString" 128 130 129 Data 130 "Wilco" charAt() indexOf() Substring() … 131 132
In Memory • String myString; • myString = new String("Wilco"); 127 "myString" 128 130 129 130 "Wilco" Methods charAt() indexOf() Substring() … 131 132
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() … } • linefromServercontains info like "IM_IN2:Lilly:T:T:<HTML><BODY>…"
Back to Last Lab… • linefromServercontains info like "IM_IN2:Lilly:T:T:<HTML><BODY>…" • Want methods to pull out buddy, status, msg • String class doesn't provide such methods public class IMClient { … String lineFromServer = connection.in.nextLine() … }
Let's Write Our Own! public class TOCServerPacket { String contents; public TOCServerPacket(StringpacketContents) { … } public booleanisError() { … } public booleanisBuddyUpdate() { … } public booleanisIncomingIM() { … } public String getBuddyName() { … } public String getBuddyStatus() { … } public String getErrorCode() { … } public String getMessage() { … } private String getPrefix(StringuncutPacket) { … } private String getSuffix(StringuncutPacket, intcolonNumber) { … } }
Let's Write Our Own! public class TOCServerPacket { String contents; public TOCServerPacket(StringpacketContents) { … } public booleanisError() { … } public booleanisBuddyUpdate() { … } public booleanisIncomingIM() { … } public String getBuddyName() { … } public String getBuddyStatus() { … } public String getErrorCode() { … } public String getMessage() { … } private String getPrefix(StringuncutPacket) { … } private String getSuffix(StringuncutPacket, intcolonNumber) { … } }
Similar to the String Class • TocServerPacket class gives us a new data type • Allows us to meaningfully store info • TheString returned from the server • And have methods to operate on that info • 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 intfirstIndex = lineFromServer.indexOf(':'); intsecondIndex = lineFromServer.indexOf(':', firstIndex + 1); String buddy = lineFromServer.substring(firstIndex + 1, secondIndex); // pull out message while ( … ) { … } } }
Use TOCServerPacket in Other Classes public class IMControl { … public void dataAvailable() { … String lineFromServer = connection.in.nextPacket(); // get the buddy name intfirstIndex = lineFromServer.indexOf(':'); intsecondIndex = lineFromServer.indexOf(':', firstIndex + 1); String buddy = lineFromServer.substring(firstIndex + 1, secondIndex); // pull out message while ( … ) { … } } } Use a TOCServerPacket!
Use TOCServerPacket in Other Classes public class IMControl { … public void dataAvailable() { … String lineFromServer = connection.in.nextPacket(); TOCServerPacket packet = new TOCServerPacket(lineFromServer); String buddy = packet.getBuddyName(); String message = packet.getMessage(); } } Use a TOCServerPacket!
In Memory • String line = conn.in.nextPacket(); • TOCServerPacket packet; • Enough space to hold a memory address 127 "packet" 128 0 129 130 131 132
In Memory • String line = conn.in.nextPacket(); • TOCServerPacket packet; • packet = new TOCServerPacket(lineFromServer); 127 "packet" 128 130 129 130 "IM_IN2:Lilly:T:…" isIncomingIM() getBuddyName() getBuddyStatus() … 131 132
In Memory • String line = conn.in.nextPacket(); • TOCServerPacket packet; • packet = new TOCServerPacket(lineFromServer); 127 "packet" 128 130 129 Data 130 "IM_IN2:Lilly:T:…" isIncomingIM() getBuddyName() getBuddyStatus() … 131 132
In Memory • String line = conn.in.nextPacket(); • TOCServerPacket packet; • packet = new TOCServerPacket(lineFromServer); 127 "packet" 128 130 129 130 "IM_IN2:Lilly:T:…" Methods isIncomingIM() getBuddyName() getBuddyStatus() … 131 132
