STRINGS

1
STRINGS

• CSC 171 FALL 2004
• LECTURE 17

2
chars Strings

• Computers 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

3
Under the Hood

• In the computers 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

4

• 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) A c1 (int) 65 i1 (char) A i1
(int) 65
5

• public static void prnt1(char c1)
• for (int i 0 ilt10i)
• System.out.println(c1 " " i
• " " (c1 i ))
• // output for prnt1(A) ??

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

• public static void prnt1(char c1)
• for (int i 0 ilt10i)
• System.out.println(c1 " " i
• " " (char) (c1 i ))
• // output for prnt1(A) ??

A 0 A A 1 B A 2 C A 3 D A 4
E A 5 F A 6 G A 7 H A 8
I A 9 J
7
Strings groups of chars

• String s1 Hello CSC 171
• String s2 new String(Hello CSC 171)

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Strings as Arrays

• String name Teddy

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String length

• String name Teddy,
• str1 Ted,
• str2 new String(),
• str3
• name.length()
• str1.length()
• str2.length()
• str3.length()

5
3
0
Error str3 is null
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Strings are objects

• String st1 // declares a reference
• st1 new String(Teddy)
• //constructs a new object
• // sets st1 to refer to the new object

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Comparing 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

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The reference

• String st1 // declares a reference
• st1 new String(Teddy)
• //constructs a new object
• // sets st1 to refer to the new object

Memory address
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• String st1
• st1 new String(Teddy)
• String st2
• st2 new String(Teddy)

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• String st1 st1 new String(Teddy)
• String st2 st2 new String(Teddy)
• boolean b1 (st1 st2)
• // false because different loc

15

• String st1 st1 new String(Teddy)
• String st2 st2 st1
• boolean b1 (st1 st2)
• // true because same loc

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• String st1 st1 new String(Teddy)
• String st2 st2 new String(Teddy)
• boolean b1 (st1.equals(st2))
• // true because contents

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• String st1 st1 new String(Teddy)
• String st2 st2 new String(Ted)
• boolean b1 (st1.equals(st2))
• // false because contents

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Primitive Reference

• Primitive
• int
• byte
• char
• short
• long
• float
• double
• boolean

• Reference
• Object
• String
• Applet
• myRational

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Passing Objects to Methods

• public class myClass public int value1

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• public class myClassTest
• public static void main(String args)
• myClass mc1 new myClass()
• mc1.value1 3

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• int x1 3
• intChange(x1)
• System.out.println("x1 " x1)
• public static void intChange(int intVal)
• intVal 5

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• 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

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• myClassValChange(mc1)
• System.out.println("mc1.value1 "
  mc1.value1)
• public static void myClassValChange(myClass
  myClassRef)
• myClassRef.value1 9

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• mc1 myClassRefReturn(mc1)
• System.out.println("mc1.value1 "
  mc1.value1)
• public static myClass myClassRefReturn(myClass
  myClassRef)
• myClassRef new myClass()
• myClassRef.value1 11
• return myClassRef

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Technical Aside

• FORMATTING NUBMERS

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Formatting 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 3.5 Tax 0.2975
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Formatting Numbers

• NumberFormat formatter
• NumberFormat.getNumberInstance()
• formatter.setMaximumFractionDigits(2)
• System.out.println("Total "total)
• System.out.println("Tax "
• formatter.format(tax))

Total 3.5 Tax 0.3
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Formatting Numbers

• NumberFormat formatter
• NumberFormat.getNumberInstance()
• formatter.setMaximumFractionDigits(2)
• formatter.setMinimumFractionDigits(2)
• System.out.println("Total "total)
• System.out.println("Tax "
• formatter.format(tax))

Total 3.5 Tax 0.30
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Strings are immutable

• They dont change
• We simulate change by making new strings
• StringBuffers are used to change

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SubString 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

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FRAME WINDOWS

• Applets 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

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• 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!")

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• JPanel panel new JPanel()
• panel.add(iconLabel)
• panel.add(textLabel)
• frame.setContentPane(panel)
• frame.pack()
• frame.show()

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Better Design

• public class FrameAloneTest
• public static void main(String args)
• FrameAlone myFrame new FrameAlone()
• myFrame.runFrameAlone()

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FrameAlone

• 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_O
  N_CLOSE)
• JLabel iconLabel new JLabel(new
  ImageIcon ("sierpins.gif"))
• JLabel textLabel new JLabel("Hello,
  World!")

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• JPanel panel new JPanel()
• panel.add(iconLabel)
• panel.add(textLabel)
• this.setContentPane(panel)
• public void runFrameAlone()
• this.pack()
• this.show()