STRINGSSTRINGS

CSC 171 FALL 2004

LECTURE 17

chars & Stringschars & StringsComputers process information

Not just numerical information

Human beings deal with “letters” and “words”

Java codes “letters” as the “char” type

Java codes “words” as the “String” type

ASCII – one byte (8 bits) per character

UniCode – two bytes (16 bits) per character

Under the HoodUnder the Hood

In the computer’s memory we only have bits

0000000001000001

We can interpret (semantics) the bits any way we like

The same bit pattern (syntax) can have multiple meanings

public class StringChar { public static void main(String [] args) {

char c1 = 'A';int i1 = (int) c1;System.out.println("c1 (char) == " + (char) c1);System.out.println("c1 (int) == " + (int) c1);System.out.println("i1 (char) == " + (char) i1);System.out.println("i1 (int) == " + (int) i1);

}

c1 (char) == Ac1 (int) == 65i1 (char) == Ai1 (int) == 65

public static void prnt1(char c1){for (int i = 0 ; i<10;i++) System.out.println(c1 + " + " + i +

" == " + (c1 + i )); }// output for prnt1(‘A’); ??

A + 0 == 65A + 1 == 66A + 2 == 67A + 3 == 68A + 4 == 69A + 5 == 70A + 6 == 71A + 7 == 72A + 8 == 73A + 9 == 74

public static void prnt1(char c1){for (int i = 0 ; i<10;i++) System.out.println(c1 + " + " + i +

" == " + (char) (c1 + i )); } // output for prnt1(‘A’); ??

A + 0 == AA + 1 == BA + 2 == CA + 3 == DA + 4 == EA + 5 == FA + 6 == GA + 7 == HA + 8 == IA + 9 == J

Strings – groups of charsStrings – groups of chars

String s1 = “Hello CSC 171”;

String s2 = new String(“Hello CSC 171”);

Strings as ArraysStrings as Arrays

String name = “Teddy”;

T e d d y

0 1 2 3 4

2352

2354

2356

2358

2360

name

String lengthString lengthString name = “Teddy”,

str1 = “Ted”,

str2 = new String(),

str3;

name.length();

str1.length();

str2.length();

str3.length();

53

0

Error – str3 is null

Strings are objectsStrings are objects

String st1; // declares a reference

st1 = new String(“Teddy”);

//constructs a new object

// sets st1 to refer to the new object

st1 String

“Teddy”

Comparing StringsComparing Strings

Is one string equal to another?

It is import to know what we are comparing

“= =“ compares the reference

“equals” compares the contents of the objects

The “reference”The “reference”

String st1; // declares a reference

st1 = new String(“Teddy”);

//constructs a new object

// sets st1 to refer to the new object

st1 String

“Teddy”

Memory address

String st1;

st1 = new String(“Teddy”);

String st2;

st2 = new String(“Teddy”);st1 String

“Teddy”

st2 String

“Teddy”

String st1; st1 = new String(“Teddy”);

String st2; st2 = new String(“Teddy”);

boolean b1 = (st1 = = st2);

// false because different locst1 String

“Teddy”

st2 String

“Teddy”

String st1; st1 = new String(“Teddy”);

String st2; st2 = st1;

boolean b1 = (st1 = = st2);

// true because same loc

st1 String

“Teddy”

st2

String st1; st1 = new String(“Teddy”);

String st2; st2 = new String(“Teddy”);

boolean b1 = (st1.equals(st2));

// true because contentsst1 String

“Teddy”

st2 String

“Teddy”

String st1; st1 = new String(“Teddy”);

String st2; st2 = new String(“Ted”);

boolean b1 = (st1.equals(st2));

// false because contentsst1 String

“Teddy”

st2 String

“Ted”

Primitive & ReferencePrimitive & Reference

Primitive

int

byte

char

short

long

float

double

boolean

Reference

Object

String

Applet

myRational

Passing Objects to MethodsPassing Objects to Methods

public class myClass { public int value1; }

public class myClassTest {

public static void main(String[] args) {

myClass mc1 = new myClass();

mc1.value1 = 3;

int x1 = 3;intChange(x1);

System.out.println("x1 == " + x1);

public static void intChange(int intVal){

intVal = 5;

}

myClass mc1 = new myClass();mc1.value1 = 3;myClassRefChange(mc1);System.out.println("mc1.value1 == " +mc1.value1);

public static void myClassRefChange(myClass myClassRef){ myClassRef = new myClass();

myClassRef.value1 = 7;}

myClassValChange(mc1);System.out.println("mc1.value1 == " +

mc1.value1);

public static void myClassValChange(myClass myClassRef){myClassRef.value1 = 9;

}

mc1 = myClassRefReturn(mc1);System.out.println("mc1.value1 == " + mc1.value1);

public static myClass myClassRefReturn(myClass myClassRef){myClassRef = new myClass();myClassRef.value1 = 11;return myClassRef ;

}

Technical AsideTechnical Aside

FORMATTING NUBMERS

Formatting NumbersFormatting Numbers

int quarters = 2;

int dollars = 3;

double total = dollars + quarters * 0.25;

final double TAX_RATE = 8.5;

double tax = total * TAX_RATE / 100;

System.out.println("Total: $"+total);

System.out.println("Tax : $"+tax);

Total: $3.5Tax : $0.2975

Formatting NumbersFormatting Numbers

NumberFormat formatter =

NumberFormat.getNumberInstance();

formatter.setMaximumFractionDigits(2);

System.out.println("Total: $"+total);

System.out.println("Tax : $"+

formatter.format(tax));

Total: $3.5Tax : $0.3

Formatting NumbersFormatting Numbers

NumberFormat formatter =

NumberFormat.getNumberInstance();

formatter.setMaximumFractionDigits(2);

formatter.setMinimumFractionDigits(2);

System.out.println("Total: $"+total);

System.out.println("Tax : $"+

formatter.format(tax));Total: $3.5Tax : $0.30

Strings are immutableStrings are immutable

They don’t change

We simulate change by making new strings

StringBuffers are used to change

SubString – creates newSubString – creates new

Two ways of getting substrings

String letters = “abcdefghijklmabcdefghijklm”;

System.out.println(letters.substring(20));

// form index 20 to end “hijklm”

System.out.println(letters.substring(0,6));

// form index 0 up to 6 “abcdef”

FRAME WINDOWSFRAME WINDOWSApplets are programs that run inside a

browser.– Limited access to local machine– Good graphics/UI support

Applications run outside of a browser– Access to local machine– Often (so far) console/text based

Frame based applications– Access to local machine– Good graphics/UI

import javax.swing.ImageIcon;import javax.swing.JFrame;import javax.swing.JLabel;import javax.swing.JPanel;

public class FrameTest {

public static void main(String [] args) { JFrame frame = new JFrame(); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); JLabel iconLabel = new JLabel(new ImageIcon ("sierpins.gif")); JLabel textLabel = new JLabel("Hello, World!");

JPanel panel = new JPanel(); panel.add(iconLabel); panel.add(textLabel); frame.setContentPane(panel); frame.pack(); frame.show(); }}

Better DesignBetter Design

public class FrameAloneTest {

public static void main(String[] args) { FrameAlone myFrame = new FrameAlone(); myFrame.runFrameAlone();

}}

FrameAloneFrameAlone

import javax.swing.ImageIcon;import javax.swing.JFrame;import javax.swing.JLabel;import javax.swing.JPanel;

public class FrameAlone extends JFrame{

public FrameAlone() { this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); JLabel iconLabel = new JLabel(new ImageIcon ("sierpins.gif")); JLabel textLabel = new JLabel("Hello, World!");

JPanel panel = new JPanel(); panel.add(iconLabel); panel.add(textLabel); this.setContentPane(panel); } public void runFrameAlone(){ this.pack(); this.show(); }}