n n n n n n n
Date
Calendar
Math
Random
String
StringBuffer
Parsing strings
Department of Computer Science 1
• Java gives us a number of small utility classes that can be useful in a variety of situations
• We’ll take a look at a subset of these
– Some live in java.util, others in java.lang
• Recall in lecture 18 we saw that a utility class often groups together a set of related static methods and constants
Department of Computer Science 2
• Represents an instance in time with millisecond precision
– long value = # milliseconds since Jan 1, 1970 ( epoch )
• Some details of Date : class Date { public Date() // current time and date!
public Date(int y, int m, int d, int h, int m, int s)
Public int getDate() // day of month, 1-31 public int getMonth() // 0-11 public int getSeconds() // 0-59 public int getDay() // day of the week, 1-6 public int getYear() // year – 1900 public long getTime() // milliseconds since epoch public void setMonth(int m), .., setSeconds(int s)
import java.util.*; public class DateTest {
public static void main(String [] args) {
Date s = new Date();
System.out.println("s toString = "
+ s.toString()); int j = 0; for (int i=0; i<100000; i++) {j = j + i};
Date e = new Date();
System.out.println("That took " +
(e.getTime() - s.getTime()) + " msecs");
}
}
• Some methods in Date have been deprecated since the introduction of Java1.2
– Reduce the connection between dates and the western style
Gregorian calendar
• Use Calendar and Date for performing certain functions
– Convert between a Date object and a set of integer fields such as YEAR, MONTH, HOUR etc.
• Calendar is an abstract base class
– Concrete subclasses provide locale sensitive implementations that interpret a Date according to the rules of a specific calendar system
Department of Computer Science 5
Date now = new Date();
// get a calandar for the default time zone
// and locale
Calendar c = Calendar.getInstance(); c.setTime(now);
System.out.println(“Day of month: “
+ c.get(Calendar.DAY_OF_MONTH));
Department of Computer Science 6

• Supplies static constants and methods public static final double E // = 2.71828
public static final double PI // = 3.1415926
– Trigonometric ops (double ‡ double): sin, cos, tan, asin, acos, atan, atan2
– Rounding ops: ceil, floor, rint, round
– Exponentials: exp, pow, log, sqrt
– Other: abs, max, min, random
Department of Computer Science 7
static public int weightedDistribution (int[ ] weights){ int sum = 0; // sum of weights for(int i = 0; i < weights.length; i++) {
sum += weights[i];
}
// compute random value less than sum int val = (int) Math.floor(Math.random() * sum + 1);
// find point in distribution for(int i = 0; i < weights.length; i++) {
val -= weights[i];
if (val < 0) { return i; }
} return 0; // should never happen
}
Ë weights (1,3,2) will yield p(0)=1/6, p(1)=1/2, p(2)=1/3
• The method java.util.Math.random() can be used to generate random floating point numbers uniformly distributed between 0 and 1.0
• The class java.util.Random provides more general facilities
– Random integers
– Seed value (for repeatable sequences of pseudo-random numbers)
– etc.
Department of Computer Science 9
class Random {
boolean nextBoolean() // uniformly distributed boolean
void nextBytes(byte [] bytes) // random bytes into array
double nextDouble()
float nextFloat()
double nextGaussian() // normally distrib. m =0, s =1.0
int nextInt()
long nextLong()
setSeed(long seed) // set the random seed
}
Department of Computer Science 10
• In Java a String is an immutable value
– Once constructed its contents never change
– Has only accessor methods and no side effects
• Constructing Strings
– With a literal: String name = “John Smith”
– With various constructors in the String class
• String name = new String(“John Smith”);
• Char [] data = {‘a’, ‘b’, ‘c’};
String abc = new String(data);
Department of Computer Science 11
• The + operator is overloaded and can be used to concatenate two strings
– If either the left or right expression is a String , then the other is automatically converted to a String as well
– System.out.println(“Answer = ”+answer)
• + groups from left to right:
– System.out.println(“Catch-”+2+2) ‡ “Catch22”
– System.out.println(2+2+“warned”) ‡ “4warned”
– System.out.println(“”+2+2+“warned”) ‡
“22warned” the old “leading empty string” trick
Department of Computer Science 12

• String methods
– Because strings are immutable, methods that operate on strings return new String objects
• Some methods
– concat, length, replace (characters), (retrieve) substring, toLowerCase, toUpperCase, trim
(whitespace), valueOf, compareTo, equalsIgnoreCase, endsWith, startsWith, indexOf, lastIndexOf
Department of Computer Science 13
• The indexOf method can be used to search for the occurrence of a substring
String mainString = “Hello there world!”; if (mainString.indexOf(“there world”) != -1) {
System.out.println(“Found a match!”);
}
• The substring method computes substrings of a String
String tail = mainString.substring(12, mainString.length()-1);
// tail = “world!”
public static String valueOf(Object o) { return (o == null) ? “null” : o.toString();
}
• valueOf safer than toString
– valueOf is a polymorphic method—its argument can be of any type of object, including a null object
Shape aShape = null;
…
String a = String.valueOf(aShape); // “null”
String b = aShape.toString();
// NullPointerException
Department of Computer Science 15
• == on Strings
– String constructor makes a copy
– valueOf method returns a reference
String one = “One”;
String two = new String(one); // copy of one
String three = String.valueOf(one); // ref to one
System.out.println((one == two)); // “false”
System.out.println((one == three)); // “true”
Department of Computer Science 16
• StringBuffer implements a mutable sequence of characters
– i.e. it is like a String , but can be modified
– is similar to the C language concept of a string as an array of characters
– At any point in time it contains some sequence of characters, but the length and content of the sequence can be changed through method calls
Department of Computer Science 17
• The principal operations on a StringBuffer are append and insert
– Are overloaded methods that accept data of any type—convert given datum to String and then inserts or appends the characters of the string to the
StringBuffer
• Constructors:
– StringBuffer() — allocates enough space to initially hold 16 characters
– StringBuffer(String str) — makes a StringBuffer that holds the contents of str
+ initial extra space for 16 more characters
– StringBuffer(int capacity) — allocates enough space for capacity number of characters

• append, insert, reverse —all modify the buffer
• setCharAt, charAt
• length, setLength (can be used to allocate more space or truncate the string buffer)
• ensureCapacity — takes a min capacity argument and sets the size of the string buffer to the larger of min capacity or twice the old capacity + 2
• substring, toString
Department of Computer Science 19
• Forming new strings by concatenating many smaller strings is inefficient
– Each intermediate String is a new object that is used only once
– StringBuffer’s append, insert and reverse methods return the implicit parameter (i.e. this ) rather than a new object (like String does)
– Therefore, string buffers are used by the compiler to implement
+ for strings x = "a" + 4 + "c”; gets compiled to: x = new StringBuffer().append("a").
append(4).append("c");
Department of Computer Science 20
• Breaks a String into a sequence of tokens
• Tokens are defined by delimiters (e.g. space)
• Tokenization process is much simpler than that used by the StreamTokenizer
– Doesn’t distinguish among words, numbers, quoted strings, comments etc
• Implements the Enumeration protocol
// uses default delims - "!\t\n\r\f" public StringTokenizer(String s) public StringTokenizer(String s, String delims) public boolean hasMoreElements() // Enum. interface
Public boolean hasMoreTokens() public Object nextElement() public String nextToken() public int countTokens() // remaining tokens
public void readLines(BufferedReader r) throws IOException {
String delims = " \t\n.,!?;:";
String line;
while ((line = r.readLine()) != null) {
line = line.toLowerCase();
StringTokenizer tok = new StringTokenizer(line, delims);
while (tok.hasMoreTokens()) {
System.out.println(tok.nextToken());
}
}
}
• Unlike StreamTokenizer ,
StringTokenizer does not parse numbers for us
• If a token represents a primitive value, we can parse it using a wrapper class:
String dstr = “23.7”;
Double dval = Double.parseDouble(dstr);
• Similar for int, float, boolean etc
Department of Computer Science 23
• Write a program Palindrome in two ways:
– First, using StringBuffer (and reverse)
• public StringBuffer reverse( )
– Second, using String
• public char charAt(int index)
• Write a program that takes the name of a text file as an argument and
– Counts the number of words in the file
– Computes letter frequencies
Department of Computer Science 24

• Budd chapter 17
Department of Computer Science 25