COMP201 Topic 2 / Slide 2
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
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.
matches file name.
Case sensitive
main: method of the wrapping class
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
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 use BigInteger and BigDecimal classes
COMP201 Topic 2 / Slide 6
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
bytes short
double
int long
float
char
COMP201 Topic 2 / Slide 7
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.
bytes short
double
int long
float
char
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;
Java compilers require initialization of local variables before use public void someMethod(){ …
int x; // does not compile}
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 changedpublic 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 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++: +, -, *, /, %, ++, --, <, <=, >, >=, ==, !=,
!, &&, || =, +=, -=, *=, /=, %=,
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
COMP201 Topic 2 / Slide 13
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
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
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);
Basic Ingredients/Control flow
Colon
COMP201 Topic 2 / Slide 16
Basic Ingredients/Control flow
RecursionBasically 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 17
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 18
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 19
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 20
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
COMP201 Topic 2 / Slide 21
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);
COMP201 Topic 2 / Slide 22
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
COMP201 Topic 2 / Slide 23
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 24
Arrays
Contents Arrays are objects Arrays are implemented as references Multidimensional arrays
COMP201 Topic 2 / Slide 25
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[]; // okayint[] arr; // okay (same as previous line)
Arrays (as all objects) are dynamically allocatedint[] 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 26
Java array (object) always knows its own lengthint[] 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
COMP201 Topic 2 / Slide 27
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
COMP201 Topic 2 / Slide 28
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
COMP201 Topic 2 / Slide 29
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<a.length && i<b.length; i++) { System.out.println( a[i]+ ” ”+ b[i] ); }
}
}
// ArrayRef.java
//what is the output ?
Array Examples
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
COMP201 Topic 2 / Slide 31
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];
Multidimensional Arrays
COMP201 Topic 2 / Slide 32
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 33
String
Java.lang.String Java.lang.StringBuffer String is an object Creating a String
form string literal between double quotesString s = “Hello, World!”;
by using the new keywordString s = new String(“Java”);
COMP201 Topic 2 / Slide 34
Accessor Methods
String.length() obtain the length of the string
String.charAt(int n) obtain the character at the nth position
COMP201 Topic 2 / Slide 35
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)
COMP201 Topic 2 / Slide 36
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 37
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
COMP201 Topic 2 / Slide 38
if ( h=="hello" )
System.out.println(
"String h == \"hello\" ");
if ( "hello".equals(h) )
System.out.println("\"hello\"
== string h ");
} }
} //StringExample.java
String Example
COMP201 Topic 2 / Slide 39
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 40
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 jobString piStr = “3.14159”;
Float pi = Float.valueOf(piStr);
COMP201 Topic 2 / Slide 41
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 42
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 43
StringBuffer StringBuffer(int length)
leaving the length undetermined is less efficient
length, charAt, capacity append, insert, substring toString
COMP201 Topic 2 / Slide 44
StringBufferStringBuffer 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!