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1 C. A. R. Hoare, The Emperor s Old Clothes Java C++ Objective C Eiffel Smalltalk Mesa Lisp Java Java Java C++ Java 10 Item 1 Item 2 String.equals() == 1

2 1 Item 3 Java C++ Java Item 4 Java Item 5 Java Item 6 Java this Item 7 Item 8 Java Item 9 C++ Java Java Item 10 Java short-circuit 2 Java 3 3 Java Item 5 Item 1 Item 5

3 Java Item 1: 01: class Super 03: static String greeting() 04: { 05: return "Goodnight"; 06: } 07: 08: String name() 09: { 10: return "Richard"; 11: } 12: } 01: class Sub extends Super 03: static String greeting() 04: { 05: return "Hello"; 06: } 07: 08: String name() 09: { 10: return "Dick"; 11: } 12: } 01: class Test 03: public static void main(string[] args) 04: { 05: Super s = new Sub(); 06: System.out.println(s.greeting() + ", " + s.name()); 07: } 08: } Test Goodnight, Dick Super greeting name Sub Super greeting name Test main 3

4 1 Test 5 Sub s Super Sub s Super Sub ( 6 ) s.greeting() s.name() Super Sub name() name() Sub Super name() Sub name() Super name() s Sub s.name() Dick greeting() Super Sub greeting() Sub greeting() Super greeting() s Super Sub greeting() Super greeting() s.greeting() Goodnight Super/Sub s Sub Sub greeting() Super s Super greeting() s Sub Super name() Java Sub/Super 4

5 Item 2: String.equals( ) == final 1 C++ = = string String.equals(...) = = 01: public class StringExample 03: public static void main (String args[]) 04: { 05: String s0 = "Programming"; 06: String s1 = new String ("Programming"); 07: String s2 = "Program" + "ming"; 08: 09: System.out.println("s0.equals(s1): " + (s0.equals(s1))); 10: System.out.println("s0.equals(s2): " + (s0.equals(s2))); 11: System.out.println("s0 == s1: " + (s0 == s1)); 12: System.out.println("s0 == s2: " + (s0 == s2)); 13: } 14: } 3 String Programming Programming String equals(...) = = s0.equals(s1): true s0.equals(s2): true s0 == s1: false s0 == s2: true 1 5

6 1 String.equals() equals(...) true s0 s1 s2 true == String s0 s1 String s0 s2 s0 s2 Java String Literals Programming Program 1 ming s2 Java Programming Program + ming Java constant pool Java.class JVM.class s0 s2 JVM constant pool resolution 3 JVM 5.4 constant pool entry CONSTANT_String 2 Unicode String intern() Unicode String String String CONSTANT_String Unicode String Java String String JVM 6 Programming String s0 s1 false s0==s1 s0.equals(s1) s0==s1 s0.equals(s1) 1 Java 0 escape sequence 2.class 6

7 Item 2: String.equals( ) ==.class JVM intern() String String intern() intern() 3 01: import java.io.*; 02: 03: public class StringExample2 04: { 05: public static void main (String args[]) 06: { 07: String sfilename = "test.txt"; 08: String s0 = readstringfromfile(sfilename); 09: String s1 = readstringfromfile(sfilename); 10: 11: System.out.println("s0 == s1: " + (s0 == s1)); 12: System.out.println("s0.equals(s1): " + (s0.equals(s1))); 13: 14: s0.intern(); 15: s1.intern(); 16: 17: System.out.println("s0 == s1: " + (s0 == s1)); 18: System.out.println("s0 == s1.intern(): " + 19: (s0 == s1.intern())); 20: } 21: 22: private static String readstringfromfile (String sfilename) 23: { 24: // read string from file 25: } 26: } s0 s1 readstringfromfile(...) 9 String

8 1 s0 == s1: false s0.equals(s1): true s0 == s1: false s0 == s1.intern(): true 14 String s0 15 s1.intern() s s0 s1 String s0==s1 false s1.intern() s0 s0==s1.intern() true s1 s0 null garbage collector s0 String s0 s1.intern() equality comparison String.equals(...) == intern() n m String n.equals(m) n.intern() == m.intern() String.intern() Java Java primitive type Java strongly type building blocks 8 Java Java Java JVM boolean C C++ Java boolean boolean 0 false 0 true 8

9 Item 3: Java C value = get_value(); if (value) do_something; Java boolean value = getvalue(); if (value!= null) dosomething; Java boolean loss of precision Java arithmetic operation accidental truncation Truncation : public class Truncation 03: static void printfloat (float f) 04: { 05: System.out.println ("f = " + f); 06: } 07: 08: public static void main (String[] args) 09: { 10: printfloat (12 / 5); // data lost! 11: printfloat ((float) 12 / 5); 12: printfloat (12 / 5.0f); 13: } 14: } 12 5 integer 10 integer printfloat float float promote float

10 1 int byte int long float long double -46 public class LostPrecision { public static void main (String[] args) { int orig = ; float approx = orig; int rounded = (int) approx; // lost precision System.out.println (orig - rounded); } } Java narrowing 1.1 float double short int long overflow long short cast 10

11 Item 3: Java byte short char int TypeConversion byte : public class TypeConversion 03: static void convertarg (byte b) { } 04: 05: public static void main (String[] args) 06: { 07: byte b1 = 127; 08: // byte b2 = 128; // won't compile 09: 10: // convertarg (127); // won't compile 11: convertarg ((byte)127); 12: 13: byte c1 = b1; 14: // byte c2 = -b1; // won't compile 15: int i = ++b1; // overflow 16: System.out.println ("i = " + i); 17: } 18: } 3 Math.pow() double int int float short double 11

12 1 14 byte short char int 14 TypeConversion.java:14: possible loss of precision found : int required: byte b1 int int byte 15 byte int 16 i = 128 i = Bug Bug Bug 01: public class IntAdder 03: private int x; 04: private int y; 05: private int z; 06: 07: public void IntAdder() 08: { 09: x = 39; 10: y = 54; 11: z = x + y; 12: } 13: 14: public void printresults() 15: { 16: System.out.println("The value of 'z' is '" + z + "'"); 17: } 18: 19: public static void main (String[] args) 20: { 21: IntAdder ia = new IntAdder(); 22: ia.printresults(); 23: } 24: }

13 Item 4: IntAdder 3 private x y z printresults main 21 ia IntAdder 7 x y z 22 printresults z IntAdder The value of 'z' is '93' Bug The value of 'zé is '0' 21 IntAdder ia 7 IntAdder x y z 7 void IntAdder IntAdder void 7 IntAdder ia Java IntAdder public IntAdder() { } 21 ia x y z 0 22 printresults The value of 'z' is '0'. Bug IntAdder Java

14 1 Java Java Java RTF DocumentManager Document Document HTML HTML Document HTMLDocument Document Document DocumentManager HTMLDocument HTMLDocument HTML HTMLDocument Java HTMLDocument Doucment spellcheck() Document Document spellcheck() HTMLDocument Document Document Document Document spellcheck() Document Java

15 Item 5: super DocumentManager HTMLDocument HTMLDocument HTMLDocument spellcheck() Document spellcheck() super 01: class DocumentManager 03: public Document newdocument() 04: { 05: return (new HTMLDocument()); 06: } 07: } 08: 09: class Document 10: { 11: public boolean spellcheck() 12: { 13: return (true); 14: } 15: } 16: 17: class HTMLDocument extends Document 18: { 19: public boolean spellcheck() 20: { 21: System.out.println("Trouble checking these darn hyperlinks!"); 22: return (false); 23: } 24: } 25: 26: public class OverridingInstanceApp 27: { 28: public static void main (String args[]) 29: { 30: DocumentManager dm = new DocumentManager(); 15

16 1 31: Document d = dm.newdocument(); 32: boolean spellchecksuccessful = d.spellcheck(); 33: if (spellchecksuccessful) 34: System.out.println("No spelling errors where found."); 35: else 36: System.out.println("Document has spelling errors."); 37: } 38: } 32 Document spellcheck() HTMLDocument Trouble checking these darn hyperlinks! Document has spelling errors. Item spellcheck() public static boolean spellcheck(document d) 32 boolean spellchecksuccessful = d.spellcheck(d); No spelling errors where found. JavaSoft Java 2D Graphics2D JDK Graphics AWT paint(...) update(...) JDK Graphics2D Graphics2D Graphics draw3drect() Graphics2D draw3drect() Bug Graphics draw3drect() draw3drect() Graphics2D 16

17 Item 6: getclass().getname() Java 01: public class Wealthy 03: public String answer = "Yes!"; 04: public void wantmoney() 05: { 06: System.out.println("Would you like $1,000,000? > "+ answer); 07: } 08: public static void main(string[] args) 09: { 10: Wealthy w = new Wealthy(); 11: w.wantmoney(); 12: } 13: } Would you like $1,000,000? > Yes! Wealthy answer wantmoney main main Wealthy w w wantmoney answer 17

18 1 01: public class Poor 03: public String answer = "Yes!"; 04: public void wantmoney() 05: { 06: String answer = "No!"; // hides instance variable answer 07: System.out.println("Would you like $1,000,000? > " + answer); 08: } 09: public static void main(string[] args) 10: { 11: Poor p = new Poor(); 12: p.wantmoney(); 13: } 14: } Would you like $1,000,000? > No! No answer answer answer answer Java Java 18 Java 6

19 Item 6: try catch for 01: public class Types 03: int x; // instance variable 04: static int y; // class variable 05: public Types(String s) // s is a constructor parameter 06: { 07: // constructor code f. 08: } 09: public createurl(string urlstring) //urlstring is a method parameter 10: { 11: String name = "example"; // name is a local variable 12: try 13: { 14: URL url = new URL(urlString); 15: } 16: catch(exception e) // e is a exception-handler parameter 17: { 18: // handle exception 19: } 20: } 21: } 3 x x x y Types catch name 11 createurl createurl 19

20 1 01: class Hidden 03: public static void main(string[] args) 04: { 05: int args = 0; // illegal results in a compiler error 06: String s = "string"; 07: int s = 10; // illegal results in a compiler error 08: } 09: } 5 args 7 s catch 01: public class Bike 03: String type; 04: public Bike(String type) 05: { 06: System.out.println("type =" + type); 07: } 08: } 20

21 Item 6: type type System.out.println type type type 01: public class Bike 03: String type = "generic"; 04: } 01: public class MountainBike extends Bike 03: String type = "All terrain"; 04: } Bike type MoutainByte type 01: public interface Stretchable 03: int y 04: } 01: public class Line 03: int x; 04: } 01: public class MultiLine extends Line implements Stretchable 03: public MultiLine() 04: { 05: System.out.println("x = " + x); 06: } 07: } Strectchable x MultiLine x 21

22 1 this super. 01: public class Wealthy 03: public String answer = "Yes!"; 04: public void wantmoney() 05: { 06: String answer = "No!"; 07: System.out.println("Do you want to give me $1,000,000? > " + 08: answer); 09: System.out.println("Would you like $1,000,000? > " + 10: this.answer); 11: } 12: public static void main(string[] args) 13: { 14: Wealthy w = new Wealthy(); 15: w.wantmoney(); 16: } 17: } Do you want to give me $1,000,000 > No! Would you like $1,000,000? > Yes! Wealthy answer wantmoney answer wantmoney answer answer this answer answer answer answer this 01: public class StillWealthy extends Wealthy 03: public String answer = "No!"; 22

23 Item 6: 04: public void wantmoney() 05: { 06: String answer = "maybe?"; 07: System.out.println("Did you see that henway? > " + answer); 08: System.out.println("Do you want to give me $1,000,000? > " + 09: this.answer); 10: System.out.println("Would you like $1,000,000? > " + super.answer); 11: } 12: public static void main(string[] args) 13: { 14: Wealthy w = new Wealthy(); 15: w.wantmoney(); 16: } 17: } Did you see that henway? > maybe? Do you want to give me $1,000,000 > No! Would you like $1,000,000? > Yes! 7 answer 8 StillWealthy this 10 Wealthy super 01: public class Wealthier extends Wealthy 03: public void wantmoney() 04: { 05: System.out.println("Would you like $2,000,000? > " + answer); 06: } 07: public static void main(string[] args) 08: { 09: Wealthier w = new Wealthier(); 10: w.wantmoney(); 11: ((Wealthy)w).wantMoney(); 12: } 13: } 23

24 1 Would you like $2,000,000? > Yes! Would you like $2,000,000? > Yes! Wealthier Wealthy wantmoney main Wealthier w wantmoney() main w Wealthy wantmoney() 01: public class Poorer extends Wealthier 03: String answer = "No!"; 04: public void wantmoney() 05: { 06: System.out.println("Would you like $3,000,000? > " + answer); 07: } 08: public static void main(string[] args) 09: { 10: Poorer p = new Poorer(); 11: ((Wealthier)p).wantMoney(); 12: System.out.println("Are you sure? > " + ((Wealthier)p).answer); 13: } 14: } Do you want $3,000,000?? No! Are you sure? > Yes! Poorer Wealthier main Poorer p p wantmoney wantmoney 11 Poorer wantmoney No! main Are you sure?> 24

25 Item 7: JVM Java 8.5 1: public class ForwardReference 2: { 3: int first = second; // this will fail to compile 4: int second = 2; 5: } ForwardReference.java:3: Can't make forward reference to second in class ForwardReference. first second Java Java 01: public class ForwardReferenceViaMethod 03: static int first = accesstoosoon(); 04: static int second = 1; 05: 06: static int accesstoosoon() 07: { 25

26 1 08: return (second); 09: } 10: 11: public static void main (String[] args) 12: { 13: System.out.println ("first = " + first); 14: } 15: } second accesstoosoon second 0 first 0 1 Java 01: import java.awt.*; 02: import java.awt.event.*; 03: import java.io.*; 04: import java.util.*; 26

27 Item 8: 05: 06: import javax.swing.*; 07: import javax.swing.event.*; 08: 09: public class ListDialog extends JDialog 10: implements ActionListener, ListSelectionListener 11: { 12: JList model; 13: JButton selectbutton; 14: LDListener listener; 15: Object[] selections; 16: 17: public ListDialog (String title, 18: String[] items, 19: LDListener listener) 20: { 21: super ((Frame)null, title); 22: 23: JPanel buttonpane = new JPanel (); 24: selectbutton = new JButton ("SELECT"); 25: selectbutton.addactionlistener (this); 26: selectbutton.setenabled (false);//nothing selected yet 27: buttonpane.add (selectbutton); 28: 29: JButton cancelbutton = new JButton ("CANCEL"); 30: cancelbutton.addactionlistener (this); 31: buttonpane.add (cancelbutton); 32: 33: this.getcontentpane().add (buttonpane, BorderLayout.SOUTH); 34: 35: this.listener = listener; 36: setmodel (items); 37: } 38: 39: void setmodel (String[] items) 40: { 41: if (this.model!= null) 42: this.model.removelistselectionlistener (this); 43: this.model = new JList (items); 44: model.addlistselectionlistener (this); 45: 46: JScrollPane scroll = new JScrollPane (model); 47: this.getcontentpane().add (scroll, BorderLayout.CENTER); 48: this.pack(); 49: } 50: 51: /** Implement ListSelectionListener. Track user selections. */ 52: 53: public void valuechanged (ListSelectionEvent e) 54: { 55: selections = model.getselectedvalues(); 27

28 1 56: if (selections.length > 0) 57: selectbutton.setenabled (true); 58: } 59: 60: /** Implement ActionListener. Called when the user picks the 61: * SELECT or CANCEL button. Generates the LDEvent. */ 62: 63: public void actionperformed (ActionEvent e) 64: { 65: this.setvisible (false); 66: String buttonlabel = e.getactioncommand(); 67: if (buttonlabel.equals ("CANCEL")) 68: selections = null; 69: if (listener!= null) 70: { 71: LDEvent lde = new LDEvent (this, selections); 72: listener.listdialogselection (lde); 73: } 74: } 75: 76: public static void main (String[] args) // self-testing code 77: { 78: String[] items = (new String[] 79: {"Forest", "Island", "Mountain", "Plains", "Swamp"}); 80: LDListener listener = 81: new LDListener() 82: { 83: public void listdialogselection (LDEvent e) 84: { 85: Object[] selected = e.getselection(); 86: if (selected!= null) // null if user cancels 87: for (int i = 0; i < selected.length; i++) 88: System.out.println (selected[i].tostring()); 89: System.exit (0); 90: } 91: }; 92: 93: ListDialog dialog = 94: new ListDialog ("ListDialog", items, listener); 95: dialog.show(); 96: } 97: } LDListener LDEvent 01: public interface LDListener 03: public void listdialogselection (LDEvent e); 04: } 01: import java.util.eventobject; 28

29 Item 8: 02: 03: public class LDEvent extends java.util.eventobject 04: { 05: Object source; 06: Object[] selections; 07: 08: public LDEvent (Object source, Object[] selections) 09: { 10: super (source); 11: this.selections = selections; 12: } 13: 14: public Object[] getselection() 15: { 16: return (selections); 17: } 18: } ListDialog API ListDialog ListSelectionEvents 44 addlistselectionlistener(...) model private ListDialog 17 String super(...) ListDialog Javadoc setmodel(...) items ListDialog setmodel(...) 01: public SoundDialog (String title, LDListener listener, 02: String path) 03: { 04: super (title, null, listener); 29

30 1 05: String[] items = getitems (path); 06: setmodel (items); 07: model.addlistselectionlistener (this); 08: model.setselectionmode (ListSelectionModel.SINGLE_SELECTION); 09: } Exception occurred during event dispatching: java.lang.nullpointerexception... at java.awt.window.pack(window.java:259) at ListDialog.setModel(ListDialog.java:48) at ListDialog.<init>(ListDialog.java:36) at SoundDialog.<init>(SoundDialog.java:18) at SoundDialog.main(SoundDialog.java:89) ListDialog setmodel(...) NullPointerException model item JList pack() ListDialog setmodel(...) private 01: import java.applet.*; 02: import java.awt.*; 03: import java.io.*; 04: import java.net.*; 05: 06: import javax.swing.*; 07: import javax.swing.event.*; 08: 09: public class SoundDialog extends ListDialog 10: implements FilenameFilter, ListSelectionListener 11: { 12: String selection; 13: 14: public SoundDialog (String title, LDListener ldl, String path) 15: { 16: super (title, null, ldl); 17: String[] items = getitems (path); 18: setmodel (items); 19: } 30

31 20: 21: public void setmodel (String[] items) 22: { 23: if (items!= null) 24: { 25: super.setmodel (items); 26: model.addlistselectionlistener (this); 27: model.setselectionmode 28: (ListSelectionModel.SINGLE_SELECTION); 29: } 30: } 31: 32: public String[] getitems (String path) 33: { 34: File file = new File (path); 35: File soundfiles[] = file.listfiles (this); 36: String[] items = new String [soundfiles.length]; 37: for (int i = 0; i < soundfiles.length; i++) 38: items = soundfiles.getname(); 39: return (items); 40: } 41: 42: // implement FilenameFilter 43: public boolean accept (File dir, String name) 44: { 45: return (name.endswith (".aiff") 46: name.endswith (".au") 47: name.endswith (".midi") 48: name.endswith (".rmf") 49: name.endswith (".wav")); 50: } 51: 52: // implement ListSelectionListener 53: public void valuechanged (ListSelectionEvent e) 54: { 55: super.valuechanged (e); 6: JList items = (JList) e.getsource(); 57: String filename = items.getselectedvalue().tostring(); 58: if (!filename.equals (selection)) 59: { 60: selection = filename; 61: play (selection); 62: } 63: } 64: 65: private void play (String filename) 66: { 67: try 68: { 69: File file = new File (filename); 70: URL url = new URL ("file://" + file.getabsolutepath()); Item 8: 31

32 1 71: AudioClip audioclip = Applet.newAudioClip (url); 72: if (audioclip!= null) 73: audioclip.play(); 74: } 75: catch (MalformedURLException e) 76: { 77: System.err.println (e + ": " + e.getmessage()); 78: } 79: } 80: 81: public static void main (String[] args) // self-test 82: { 83: LDListener listener = 84: new LDListener() 85: { 86: public void listdialogselection (LDEvent e) 87: { 88: Object[] selected = e.getselection(); 89: if (selected!= null) // null if user cancels 90: for (int i = 0; i < selected.length; i++) 91: System.out.println (selected[i].tostring()); 92: System.exit (0); 93: } 94: }; 95: SoundDialog dialog = 96: new SoundDialog ("SoundDialog", listener, "."); 97: dialog.show(); 98: } 99: } 32 ListDialog private public

33 Item 9: Swing JList JList JList setlistdata(...) setmodel(...) Swing C C++ Java Java pointer arithmetic Java Java Java Java Java boolean byte short int long char float double Hashtable PassPrimitiveByReference1 Hashtable.put() getpersoninfo() 33

34 1 Java getpersoninfo() 01: import java.util.hashtable; 02: 03: public class PassPrimitiveByReference1 04: { 05: public static void main (String args[]) 06: { 07: String name = null; 08: int age = 0; 09: float weight = 0f; 10: boolean ismarried = false; 11: 12: getpersoninfo(name, age, weight, ismarried); 13: 14: System.out.println("Name: " + name + 15: "\nage: " + age + 16: "\nweight: " + weight + 17: "\nis Married: " + ismarried); 18: 19: storepersoninfo(name, age, weight, ismarried); 20: } 21: 22: private static void getpersoninfo(string name, int age, 23: float weight, boolean ismarried) 24: { 25: name = "Robert Smith"; 26: age = 26; 27: weight = 182.7f; 28: ismarried = true; 29: } 30: 31: private static void storepersoninfo (String name, int age, 32: float weight, boolean ismarried) 33: { 34: Hashtable h = new Hashtable(); 35: h.put("name", name); 36: h.put("age", age); // produces compile time error 37: h.put("weight", weight); // produces compile time error 38: h.put("ismarried", ismarried); // produces compile time error 39: } 40: } PassPrimitiveByReference2 PassPrimitiveByReference1 Java 34

35 Item 9: Hashtable.put() java.lang int Integer float Float boolean Boolean set(...) 1 getpersoninfo() 01: import java.util.hashtable; 02: 03: public class PassPrimitiveByReference2 04: { 05: public static void main (String args[]) 06: { 07: String[] name = new String[1]; 08: int[] age = new int[1]; 09: float[] weight = new float[1]; 10: boolean[] ismarried = new boolean[1]; 11: 12: getpersoninfo(name, age, weight, ismarried); 13: 14: System.out.println("Name: " + name[0] + 15: "\nage: " + age[0] + 16: "\nweight: " + weight[0] + 17: "\nis Married: " + ismarried[0]); 18: 19: String name2 = name[0]; 20: Integer age2 = new Integer(age[0]); 21: Float weight2 = new Float(weight[0]); 22: Boolean ismarried2 = new Boolean(isMarried[0]); 23: 24: storepersoninfo(name2, age2, weight2, ismarried2); 25: } 26: 27: private static void getpersoninfo (String[] name, int[] age, 28: float[] weight, boolean[] ismarried) 29: { 30: name[0] = "Robert Smith"; 31: age[0] = 26; 32: weight[0] = 182.7f; 33: ismarried[0] = true; 34: } 35

36 1 35: 36: private static void storepersoninfo (String name, Integer age, 37: Float weight, Boolean ismarried) 38: { 39: Hashtable h = new Hashtable(); 40: h.put("name", name); 41: h.put("age", age); 42: h.put("weight", weight); 43: h.put("ismarried", ismarried); 44: } 45: } PassPrimitiveByReference2 Name: Robert Smith Age: 26 Weight: Is Married: true Java C++ C Java & C++ Java & && C++ if (ptr!= null & ptr->count > 1) // wrong operator! Gnu C warning: suggest parentheses around comparison in operand of & ptr null 36

37 Item 10: Java boolean & Java Vector if ((v!= null) & (v.size() > 0)) // wrong operator! v null NullPointerException short-circuit && if ((v!= null) && (v.size() > 0)) true && & 37

Swing-02.pdf

Swing-02.pdf 2 J B u t t o n J T e x t F i e l d J L i s t B u t t o n T e x t F i e l d L i s t J F r a m e 21 2 2 Swing C a n v a s C o m p o n e n t J B u t t o n AWT // ToolbarFrame1.java // java.awt.button //