CS110 Lecture 22
Tuesday, April 20, 2004
• Announcements
– hw10 due Thursday, April 22
– exam next Tuesday, April 27
• Agenda
– Questions
– Error handling (finally)
– What’s on the exam?
Lecture 22
1
RumpelStiltskin
• Exception examples in a short standalone program
• The fairy tale: guess my name
• examples/RumpelStiltskin.java
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Java
Java
Java
Java
RumpelStiltskin
RumpelStiltskin foo
RumpelStiltskin foo bar
RumpelStiltskin RumpelStiltskin
Lecture 22
2
Java RumpelStiltskin
• Design (pseudocode)
If there is no command line argument
print usage message
end the program
• Two possible implementation strategies
– test for args[0], proceed based on test result
– assume args[0] is there, catch Exception if not
Lecture 22
3
Test first strategy
if (args.length == 0 ) {
System.out.println(
"usage: java RumpelStiltskin guess");
System.exit(0); // leave program gracefully
}
// continue normal processing
Lecture 22
4
Exception strategy
try {
System.out.println(”Are you " + args[0] +'?');
rumpelstiltskin.guessName(args[0]);
System.out.println("Yes! How did you guess?");
System.exit(0); // leave program gracefully
}
// come here right away if there is no args[0]
catch (IndexOutOfBoundsException e) {
System.out.println(
"usage: java RumpelStiltskin guess");
System.exit(0); // leave program gracefully
}
Lecture 22
5
java RumpelStiltskin foo
sorry - foo is not my name
Intentionally generate a NullPointerException,
see what the Exception's toString method returns
java.lang.NullPointerException
Experiment with the printStackTrace() method:
BadGuessException
at java.lang.Throwable.<init>(Compiled Code)
at java.lang.Exception.<init>(Compiled Code)
at BadGuessException.<init>(Compiled Code)
at Wizard.guessName(Compiled Code)
at Wizard.makeMischief(Compiled Code)
at RumpelStiltskin.main(Compiled Code)
Look for a second command line argument,
see what happens if it's not there:
java.lang.ArrayIndexOutOfBoundsException: 1
at RumpelStiltskin.main(Compiled Code)
Lecture 22
6
Equality
• In Java, “==” means “two variables have same value”
• Box and arrow pictures help:
– same value for primitive types is just what you expect
– same value for reference types: arrow points to the same Object
• In Object
public boolean equals(Object o) {
return this == o;
}
• Override equals when you have a better idea about what
equality should mean
Lecture 22
7
Equals in AbstractList.java
public boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof List))
return false;
ListIterator e1 = listIterator();
ListIterator e2 = ((List) o).listIterator();
while(e1.hasNext() && e2.hasNext()) {
Object o1 = e1.next();
Object o2 = e2.next();
if (!(o1==null ? o2==null : o1.equals(o2)))
return false;
}
return !(e1.hasNext() || e2.hasNext());
}
Lecture 22
8
How ArrayLists work
private Object elementData[];
private int size;
public ArrayList() {
this(10);
}
public boolean add(Object o) {
ensureCapacity(size + 1);
elementData[size++] = o;
return true;
}
Lecture 22
9
How ArrayLists grow
public void ensureCapacity(int minCapacity) {
int oldCapacity = elementData.length;
if (minCapacity > oldCapacity) {
Object oldData[] = elementData;
int newCapacity = (oldCapacity * 3)/2 + 1;
if (newCapacity < minCapacity)
newCapacity = minCapacity;
elementData = new Object[newCapacity];
System.arraycopy(oldData, 0, elementData,
0, size);
}
}
Lecture 22
10
hw10
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Test scripts
Error handling for all shell commands
Error handling to register and login
Count fraction of Juno that’s there for error
handling (class Profile)
Lecture 22
11
Characters
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Type char is primitive in Java
A char is really an int
In the old days characters were just small integers
The ASCII character set contains 128 characters
numbered 0-127
• one byte, 8 bits: 00000000 to 11111111 in binary
(0-255 decimal)
• ascii codes are the bytes with the high bit 0
• Googling for ASCII code will find lots of
information
Lecture 22
12
Characters (continued)
• Printable characters are 32-126 (decimal) – other
bytes are
– visible in emacs (look at a class file)
– used for emacs commands, like ^S
• To represent them in Java use escape character: \
• ‘\ddd’ // ddd is base 8 number < 256
• System.out.println(‘\007’); //ring bell
• ‘\n’, ‘\b’, ‘\t’, ‘\”’, ‘\\’
• See Escape.java in joi/examples/
Lecture 22
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Unicode
• Unicode extends character set to 16 bits (0 to 216-1)
for kanji, Arabic, Hebrew, mathematics, …
• Type char in Java really is a 16 bit int
• We usually write these values as hexadecimal strings:
16 bits is four hex digits
• ‘\uXXXX’ (X = 0, 1, …, 9, A, … , F)
• Internationalization (I18N)
– locale
– collation sequence
– time, date, number format
Lecture 22
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class Character
• Wrapper class for primitive type char
• Static methods to process char values
• Use Character to save char in a Collection
• Character(char ch) // constructor
• public char charValue()
• static int getNumericValue(char ch)
// unicode value
• static boolean isDigit(char ch)
• static char toUpperCase(char ch)
• … see API for more
Lecture 22
15
Strings ...
• The String API - lots there to use when needed
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constructors
equality
comparisons
substrings
character contents
changing String contents (not)
• Read (some of) String.java
Lecture 22
16
String constructors
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String s;
s = “hello”; //common and convenient
s = new String(“hello”);
char[ ] charArray = {‘O’, ‘K’} ;
s = new String( charArray );
• String t = new String(s);
Lecture 22
17
String matches and searches
boolean equals(String anotherString);
boolean equalsIgnoreCase(String anotherString);
int compareTo(String anotherString); // +,-,0
boolean startsWith(String prefix);
boolean endsWith(String suffix);
int
int
int
int
indexOf(int ch);
indexOf(String str);
indexOf(..., int fromIndex);
lastIndexOf(...);
Lecture 22
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