COMP201 Java Programming

COMP201 Java Programming Topic 2: Java Basics Readings: Chapter 3

Objective and Outline • Objective • Show basic programming concepts • Outline • What do java programs look like? • Basic ingredients • Java primitive types • Variables and constants • Operators and control flow • Simple commonly used built-ins. • Simple input/output • Arrays and Strings

What do java program look like? public class MyProgram { public static void main(String args[]) { System.out.println(“Hello world!”); } } //File: MyProgram.java main: method of the wrapping class public:Access modifier class: everything is inside a class Compilation and run: javac MyProgram.java => MyProgram.class java MyProgram • MyProgram: class name. • matches file name. • Case sensitive

Objective and Outline • Outline • What do java programs look like? • Basic ingredients • Java primitive types • Variables and constants • Operators and control flow • Simple commonly used built-ins. • Simple input/output • Arrays and Strings

Basic Ingredients/Primitive Types • Integers • byte(1 byte, -128 to 127) • short (2 bytes) • int (4 bytes) • long (8 bytes) • Floating-point types • float (4 bytes, 6-7 significant decimal digits) • double (8 bytes, 15 significant decimal digits) • char(2 byte, Unicode) (ASCII 1 byte) • boolean(true or false) • Those are all the primitive types in Java. Everything else is an object. For really large numbers useBigInteger and BigDecimalclasses

char bytes short int long float double Basic Ingredients/Primitive Types • Legal conversions between numeric types • Arrows indicate direction of legal and automatic conversion • double x = 123; • long x = 123456789; float y = x; • Solid arrow: no loss of precision • Dotted arrow: might lose precision • z=1.234567E8

char bytes short int long float double Basic Ingredients/Primitive Types • Conversion in the opposite direction required explicit cast. • double x = 9.997; • int num = (int) x; • int num = x; // does not compile Can easily lead to the loss of precision (round-up errors) • Cannot convert between boolean and numerical values.

Basic Ingredients/Variables and Constants • Variables can be declared anywhere for (int i = 0; i < 20; i++) { System.out.println(“Hi”); char ch = ‘A’; } double pi = 3.14159; • Java compilers require initialization of local variables before use public void someMethod() { … int x; // does not compile } • Instance variables of class automatically initialized.

Basic Ingredients/Variables and Constants • final marks a variable “read-only” • Variable is assigned once and cannot be changed public void someMethod() { final double pi = 3.14159; .. .. .. pi = 3.14; // illegal } • Use static final to define constants which are available to multiple methods inside a single class public class Time { static finalint MinHour = 0; static finalint MaxHour = 23; private int hour, minute; // these properties are set to 0 // unless overwritten by constructor … }

Basic Ingredients/Operators • Basically the same as C++: +, -, *, /, %, ++, --, <, <=, >, >=, ==, !=, !, &&, || =, +=, -=, *=, /=, %=, • User cannot “overload” operators; although + is overloaded to do string concatenation • In C++: • Int x=0; x += 1; • String& String::operator+=( const String &s).. • Note that methods can be overloaded

Basic Ingredients /Operators No Pointers! • No explicit pointer types (although objects are implemented as references) • No & or * operators • In C++: • int *i = (int *) malloc( 3 * sizeof(int)); • (i+1)* = 2; • int& y = &i; • No pointer arithmetic • No function pointers

Basic Ingredients/Control flow Basically the same as C++: if (boolean-expr) statement; (has optional else) if ( x = 0 ) //leads to compiling error for (expr; boolean-expr; expr) statement; while (boolean-expr) statement; (do-while variant also) switch (integer-expr) case constant: statement; break;

Basic Ingredients/Control flow Labeled break; Int n; read_data: while (…) { … for (…) { String input = JOptionPane.showInputDialog (“Enter a number >=0”); N = Integer.parseInt(input); if ( n < 0 ) break read_data; } } // moves to here when n<0. • No explicit goto (no union, no struct); Colon

Basic Ingredients/Control flow Recursion Basically the same as C++: public class Factorial { public staticint factorial( int n ) { if ( n <=1 ) return 1; return factorial( n-1 ) * n; } public static void main(String args[]) { System.out.println( factorial ( 4 ) ); } }//Factorial.java

Simple Input/Output • Contents • Writing to standard output • Reading keyboard input via dialog box • Formatting output • We discuss I/O in more detail later.

Simple Input/Output • It is easy to print output to the “standard output device (the console window) by using the predefined Stream objects out System.out.print(“Your name is “ + name + “ and you are “ + num + “ years old.”);

Simple Input/Output • It is a bit more complex to read input from the “standard input device” using Stream. • However it is easy to supply a dialog box for keyboard input: JOptionPane.showInputDialog(promptString) The return value is the string that the user typed • Example: InputTest.java

Simple Input/Output • Need to include this statement: import javax.swing.*; //JOptionPane class is defined in that package • For example: you can query name of the user by: String name=JOptionPane.showInputDialog(“Your name:”); • To read in a number, use the Integer.parseInt or Double.parseDouble method to convert the string to its numeric value. For example, String input=JOptionPane.showInputDialog(“Your age:”); int age = Integer.parseInt(input); • End the program with the method call: System.exit(0);

Simple Input/Output • Use string formatters provided in java.text.NumberFormat to format output: NumberFormat.getNumberInstance() // for numbers NumberFormat.getCurrencyInstance()// for currency values NumberFormat.getPercentInstance()// for percentage values • For Example: double x = 10000.0 / 3.0; NumberFormat nf = NumberFormat.getNumberInstance(); nf.setMaximumFractionDigits(4); nf.setMinimumIntegerDigits(6); System.out.println(nf.format(x)); //003,333.3333

Arrays • Contents • Arrays are objects • Arrays are implemented as references • Multidimensional arrays

Arrays • Arrays are objects of class java.lang.reflect.Array • Can’t specify size when declaring array int arr[3]; // not legal in Java! int arr[]; // okay int[] arr; // okay (same as previous line) • Arrays (as all objects) are dynamically allocated int[] arr = newint[3]; • Before allocation, array variable is null • All elements are zeroed when array allocated • Destroyed automatically by garbage collector. No delete operator • Shorthand to declare, allocate, and initialize int[] arr = { 5, 10, 15, 20};

Arrays • Java array (object) always knows its own length int[] arr = {5, 20, 15, 10}; System.out.println(“Length is ” + arr.length); • Elements indexed from 0 to length-1, like C++ • Raises exception for “ArrayIndexOutOfBounds” • Length is fixed when allocated; create new array and copy over to change length

Arrays • Used in similar way as ordinary arrays int sum(int[] arr) { int i, sum = 0; for (i=0; i < arr.length; i++) sum += arr[i]; return sum; }

Arrays are references • Arrays are objects, hence, are implemented as references (reference is a pointer in disguise) int[] arr = {5, 20, 15, 10}; int[] b = arr; b[0] = 3; // arr[0] also becomes 3! • Array copying (java.lang.System) System.arraycopy(from, fromIndex, to, toindex, count); int[] c; System.arraycopy(arr, 0, c, 0, 4); • Array sorting (java.util.Arrays) Arrays.sort(arr) //use a tuned QuickSort

Array Examples public class ArrayRef { public static void main( String[] args ) { int [] a = {0, 1, 2, 3, 4}; int [] b = {10, 11, 12, 13, 14}; a = b; b = newint[] { 20, 21, 22, 23, 24 }; for (int i=0; i<a.length && i<b.length; i++) { System.out.println( a[i]+ ” ”+ b[i] ); } } } // ArrayRef.java //what is the output ? Notice this quote

public class Swapping { public static void main(String argv[]) { int[] a = {1,2,3}; for (int i=0; i<3; i++) System.out.print(a[i]+" "); System.out.print(“\n”); swap(a,0,2); for (int i=0; i<3; i++) System.out.print(a[i]+" "); System.out.print(“\n”); } public static void swap(int[] a, int i, int j) { int temp = a[j]; a[j]=a[i]; a[i]=temp; } }//Swapping.java

Multidimensional Arrays int[][] matrix = newint[5][10]; int[] firstRow = matrix[0]; //ref to 1st row int[] secondRow = matrix[1]; //ref to 2nd row int firstElem = matrix[0][0]; firstElem = firstRow[0];

Strings • Contents • Strings are objects, immutable, differ from arrays • Basic methods on Strings • Convert String representation of numbers into numbers • StringBuffer, mutable version of Strings

String • Java.lang.String • Java.lang.StringBuffer • String is an object • Creating a String • form string literal between double quotes String s = “Hello, World!”; • by using the new keyword String s = newString(“Java”);

Accessor Methods • String.length() • obtain the length of the string • String.charAt(int n) • obtain the character at the nth position

Strings • String is a class (java.lang.String)offering methods for almost anything String s = “a string literal”; • + is concatenation, when you concatenate a string with a value that is not a string, the latter is converted to a string s = “The year is ” + 2002 + “!”; • Everything can be converted to a string representation including primitive types and objects (topic 3, class object)

Strings • A String is not an array. It is immutable. You cannot change a String • but you can change the contents of a String variable and make it refer to a different String. String s = “Hello”; s[2]=‘a’; // Illegal s = “Bye”; //Legal • Equality test: s.equals(t)// determines whether s and t are same s == t// determines whether s and t stored at same location

String Example public class StringExample { public static void main(String argv[]) { String h = “hello”; String w = “world”; System.out.println(h + “ “ +w); w = h.substring(1,3); w += "binky"; for (int i = 0; i < w.length(); i++) System.out.println(w.charAt(i)); int pos = w.indexOf("in"); System.out.println("Starting position of \"in\ " in string \" " + w + " \" is " + pos);

String Example if ( h=="hello" ) System.out.println( "String h == \"hello\" "); if ( "hello".equals(h) ) System.out.println("\"hello\" == string h "); } } }//StringExample.java

Example - String • To print a string in reverse order class ReverseString { public static void reverseIt(String source) { int i, len = source.length(); for (i = (len - 1); i >= 0; i--) { System.out.print(source.charAt(i)); } } }

Convert String to number • String class itself does not provide such a conversion • Type wrapper classes (Integer, Double, Float and Long) provide a method valueOf to do the job String piStr = “3.14159”; Float pi = Float.valueOf(piStr);

StringBuffer • The String class is used for constant strings • While StringBuffer is for strings that can change • StringBuffer contains a method tostring() which returns the string value being held • Since String is immutable, it is “cheaper”!

StringBuffer class ReverseString { public static String reverseIt(String source) { int i, len = source.length(); StringBuffer dest = new StringBuffer(len); for (i = (len - 1); i >= 0; i--) { dest.append(source.charAt(i)); } return dest.toString(); } }

StringBuffer • StringBuffer(int length) • leaving the length undetermined is less efficient • length, charAt, capacity • append, insert, substring • toString

StringBuffer StringBuffer sb = newStringBuffer("Drink Java!"); StringBuffer prev_sb = sb; sb.insert(6, "Hot "); sb.append(" Cheer!"); System.out.println(sb.toString() + " : " + sb.capacity()); System.out.println("prev_sb becomes " + prev_sb ); **** output ***** Drink Hot Java! Cheer! : 27 (initial size + 16) prev_sb becomes Drink Hot Java! Cheer!

COMP201 Java Programming