COMP201 Java Programming

Topic 2: Java Basics

COMP201 Topic 2 / Slide 2

Objective and Outline 

Objective 

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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

COMP201 Topic 2 / Slide 3

What do java program look like? public class MyProgram { public static void main(String args[]) { System.out.println(“Hello world!”); } } //File: MyProgram.java 

public: Access modifier

class: everything is inside a class  MyProgram: class name. 

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matches file name.

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Case sensitive

main: method of the wrapping class 

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Compilation and run: javac MyProgram.java => MyProgram.class java MyProgram

COMP201 Topic 2 / Slide 4

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

COMP201 Topic 2 / Slide 5

Basic Ingredients/Primitive Types 

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  

Integers  byte (1 byte, -128 to 127)  short (2 bytes) For really large numbers use  int (4 bytes) BigInteger and BigDecimal  long (8 bytes) classes 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.

COMP201 Topic 2 / Slide 6

Basic Ingredients/Primitive Types 

Legal conversions between numeric types char bytes

short

int

float 

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double

Arrows indicate direction of legal and automatic conversion – double x = 123; – long x = 123456789; float y = x;

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long

Solid arrow: no loss of precision Dotted arrow: might lose precision – z=1.234567E8

COMP201 Topic 2 / Slide 7

Basic Ingredients/Primitive Types char bytes

short

int

float 

double

Conversion in the opposite direction required explicit cast.  





long

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.

COMP201 Topic 2 / Slide 8

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

COMP201 Topic 2 / Slide 9

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;

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Java compilers require initialization of local variables before use public void someMethod() { … int x; // does not compile System.out.println(“The value of x is “ + x); }

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Instance variables of class automatically initialized.

COMP201 Topic 2 / Slide 10

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 objects of the same class public class Time { static final int MinHour = 0; static final int MaxHour = 23; private int hour, minute; // these properties are set to 0 // unless overwritten by constructor … }

COMP201 Topic 2 / Slide 11

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

COMP201 Topic 2 / Slide 12

Basic Ingredients/Operators 

Basically the same as C++: +, -, *, /, %, ++, --, <, <=, >, >=, ==, !=, !, &&, || =, +=, -=, *=, /=, %=,

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User cannot “overload” operators; although + is overloaded to do string concatenation 

In C++: – Int x=0; x += 1; – String& String::operator+=( const String &s)..

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Note that methods can be overloaded

COMP201 Topic 2 / Slide 13

Basic Ingredients /Operators No Pointers! 

No explicit pointer types (although objects are implemented as references)

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No & or * operators 

In C++:   

 

int *i = (int *) malloc( 3 * sizeof(int)); *(i+1) = 2; int& y = i;

No pointer arithmetic No function pointers

COMP201 Topic 2 / Slide 14

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;

COMP201 Topic 2 / Slide 15

Basic Ingredients/Control flow Recursion Basically the same as C++: public class Factorial { public static int 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

COMP201 Topic 2 / Slide 16

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

COMP201 Topic 2 / Slide 17

Simple Input/Output 

Contents   



Writing to standard output Reading keyboard input via dialog box Formatting output

We discuss I/O in more detail later.

COMP201 Topic 2 / Slide 18

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.”);

COMP201 Topic 2 / Slide 19

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

COMP201 Topic 2 / Slide 20

Simple Input/Output 

Need to include this statement: import javax.swing.*; //JOptionPane class is defined in that package

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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);

COMP201 Topic 2 / Slide 21

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

COMP201 Topic 2 / Slide 22

Arrays 

Contents   

Arrays are objects Arrays are implemented as references Multidimensional arrays

COMP201 Topic 2 / Slide 23

Arrays  

Arrays are objects of class java.lang.reflect.Array Can’t specify size when declaring array int arr[3]; int arr[]; int[] arr;

  

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// not legal in Java! // okay // okay (same as previous line)

Arrays (as all objects) are dynamically allocated int[] arr = new int[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};

COMP201 Topic 2 / Slide 24

Arrays 

Java array (object) always knows its own length int[] arr = {5, 20, 15, 10}; System.out.println(“Length is ” + arr.length);

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Elements indexed from 0 to length-1, like C++

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Raises exception for “ArrayIndexOutOfBounds”

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Length is fixed when allocated; create new array and copy over to change length

COMP201 Topic 2 / Slide 25

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; }

COMP201 Topic 2 / Slide 26

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!

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Array copying (java.lang.System) System.arraycopy(from, fromIndex, to, toindex, count); int[] c; System.arraycopy(arr, 0, c, 0, 4);

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Array sorting (java.util.Arrays) Arrays.sort(arr)

//use a tuned QuickSort

COMP201 Topic 2 / Slide 27

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 = new int[] { 20, 21, 22, 23, 24 }; for (int i=0; i
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

COMP201 Topic 2 / Slide 29

Multidimensional Arrays int[][] matrix = new int[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];

COMP201 Topic 2 / Slide 30

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

COMP201 Topic 2 / Slide 31

String    

Java.lang.String Java.lang.StringBuffer String is an object Creating a String 

form string literal between double quotes String s = “Hello, World!”;

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by using the new keyword String s = new String(“Java”);

COMP201 Topic 2 / Slide 32

Accessor Methods 

String.length() 

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obtain the length of the string

String.charAt(int n) 

obtain the character at the nth position

COMP201 Topic 2 / Slide 33

Strings 

String is a class (java.lang.String)offering methods for almost anything String s = “a string literal”;

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+ 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 + “!”;

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Everything can be converted to a string representation including primitive types and objects (topic 3, class object)

COMP201 Topic 2 / Slide 34

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

COMP201 Topic 2 / Slide 35

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);

COMP201 Topic 2 / Slide 36

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

COMP201 Topic 2 / Slide 37

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)); } } }

COMP201 Topic 2 / Slide 38

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);

COMP201 Topic 2 / Slide 39

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”!

COMP201 Topic 2 / Slide 40

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(); } }

COMP201 Topic 2 / Slide 41

StringBuffer 

StringBuffer(int length) 



 

leaving the length undetermined is less efficient

length, charAt, capacity append, insert, substring toString

COMP201 Topic 2 / Slide 42

StringBuffer StringBuffer sb = new StringBuffer("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!