Utility Classes. Date Test. java.util.date. java.util.calendar. java.util.calendar. Department of Computer Science 1

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1 COMP209 Object Oriented Programming Utility Classes Mark Hall Date Calendar Math Random String StringBuffer Parsing strings Utility Classes 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 1 Department of Computer Science 2 java.util.date 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.*; Date Test 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"); java.util.calendar 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 java.util.calendar 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 Department of Computer Science 1

2 Supplies static constants and methods java.util.math public static final double E // = public static final double PI // = 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 Example: Draw from weighted distribution 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 java.util.random java.util.random 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. 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 9 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( warned ) 22warned the old leading empty string trick Department of Computer Science 12 Department of Computer Science 2

3 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 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! Department of Computer Science 13 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 Buffer 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 Buffer 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 Department of Computer Science 17 Department of Computer Science 3

4 Buffer 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 Buffer 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" "c ; gets compiled to: x = new StringBuffer().append("a"). append(4).append("c"); Department of Computer Science 20 java.util.stringtokenizer 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 StringTokenizer Example 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()); Parsing String Values 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 Write a program Palindrome in two ways: First, using StringBuffer (and reverse) public StringBuffer reverse( ) Second, using String public char charat(int index) Exercises 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 23 Department of Computer Science 24 Department of Computer Science 4

5 Readings Budd chapter 17 Department of Computer Science 25 Department of Computer Science 5