20070329 Java Programing Tips

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20070329 Java Programing Tips

  1. 1. Java TIPS
  2. 2.     
  3. 3. String str1 = new String(“hoge”); String str2 = “hoge”; System.out.println(str1 == str2); System.out.println(str1.equals(str2)); False True
  4. 4. == equals char[] value str1 str1 int length boolean equals(Object obj) char[] value str2 str2 int length boolean equals(Object obj) str1 == str2 str1 str2 char[] value str1.equals(str2)
  5. 5. java.lang.String public final class String implements java.io.Serializable, Comparable<String>, CharSequence{ private final char value[]; private final int count; public boolean equals(Object anObject) { if (this == anObject) { return true; } if (anObject instanceof String) { String anotherString = (String)anObject; int n = count; if (n == anotherString.count) { char v1[] = value; char v2[] = anotherString.value; int i = offset; int j = anotherString.offset; while (n-- != 0) { if (v1[i++] != v2[j++]) return false; } return true; } } return false; }
  6. 6. equals True False Account equals Account equals
  7. 7. public abstract class Account{ private char[] custmorCode = new char[4]; Account(char[] custmorCode){ this.custmorCode = custmorCode; } public char[] getCustmorCode(){ return this.custmorCode; } @Override public boolean equals(Object obj){ // } }
  8. 8. SavingAccount a1 = new SavingAccount(new char[]{‘0’,’1’,’2’,’3’}); SavingAccount a2 = new SavingAccount(new char[]{‘9’,’1’,’2’,’3’}); TimeDepositAccount a3 = new TimeDepositAccount(new char[]{‘0’,’1’,’2’,’3’}); TimeDepositAccount a4 = new TimeDepositAccount(new char[]{‘9’,’1’,’2’,’3’}); System.out.println(a1.equals(a3)); System.out.println(a2.equals(a4)); System.out.println(a1.equals(a2)); System.out.println(a2.equals(a3)); True True False False
  9. 9. getClass instanceof boolean equals(Object obj) Object ClassCastException instanceof Class getClass() → true → true → false → true → false
  10. 10. public abstract class Account{ // @Override public boolean equals(Object obj){ if(obj instanceof Account){ //if(getClass() == obj.getClass()){ char[] compareCode = ((Account)obj).getCustmorCode(); if(custmorCode.length != compareCode.length){ return false; } for(int i = 0; i<custmorCode.length; i++){ if(custmorCode[i] != compareCode[i]){ return false; } } return true; } else { return false; } } }
  11. 11. equals java.lang.Object String char[] value equals Instanceof getClass equals
  12. 12. CASE:
  13. 13. Inspector Inspector 100ms
  14. 14. Inspector Inspector 100ms
  15. 15. Inspector Account  100ms  100ms  100ms  100ms  100ms CPU
  16. 16. Inspector Inspector Inspector
  17. 17. public class Account{ private byte[] lock = new byte[0]; private ArrayList<Inspector> targets = new ArrayList<Inspector>(); public int get(int amount){ synchronized(lock){ if(this.amount < amount){ return 0; } for(Inspector i : targets){ i.inspect(); } this.amount = this.amount - amount; return amount; } } public void addInspector(Inspector insp){ this.targets.add(insp); }  addInspector  Inspector
  18. 18. CPU CPU
  19. 19. String public class Main{ public static void main(String[] args){ String className = "RefrectionTest"; RefrectionTest rt = null; try{ rt = (RefrectionTest)Class .forName(className).newInstance(); } catch(Exception e) { } rt.dosth(); } } public class RefrectionTest{ public void dosth(){ System.out.println("Do Something"); } }
  20. 20. String public class Main{ public static void main(String[] args){ String className = "RefrectionTest"; RefrectionTest rt = null; try{ rt = (RefrectionTest)Class.forName(className).newInstance(); } catch(Exception e) { } String methodName = "dosth"; Method method = null; try{ method = (Method)Class.forName(className) .getMethod(methodName,null); } catch(Exception e) { } method.invoke(Class.forName(className).newInstance(),null); } }
  21. 21. import java.io.*; public class Main{ public static void main(String[] args){ Account a1 = new Account("hoge",5000); a1.get(100); try{ ObjectOutputStream oos = new ObjectOutputStream( new FileOutputStream("hoge.ser")); oos.writeObject(a1); oos.close(); } catch(Exception e) {} System.out.println(a1); Account ser = null; try{ ObjectInputStream ois = new ObjectInputStream( new FileInputStream("hoge.ser")); ser = (Account)ois.readObject(); ois.close(); } catch(Exception e) {} ser.get(100); System.out.println(ser); } } public class Account implements Serializable{ // }
  22. 22. 1 2 100 100 0 0 100 100   1 2
  23. 23. synchronized public class Account{ private int amount; private byte[] lock = new byte[0]; Account(int amount){ this.amount = amount; } public int get(int amount){ synchronized(lock){ this.amount = this.amount - amount; return amount; } } public void put(int amount){ synchronized(lock){ this.amount = this.amount + amount; } } }  Synchonized:
  24. 24. synchronized 1 2 100 100 0 100 0 200  2 1
  25. 25. volatile public class Account{ private volatile int amount; Account(int amount){ this.amount = amount; } public int getAmount(){ return this.amount; } }   syncronized  sysnronized
  26. 26. volatile 1 2 100 0 0 100 100  1  2
  27. 27. Java.util.concurrent import java.util.concurrent.atomic.*; public class Account{ private AtomicInteger amount; Account(int amount){ this.amount = new AtomicInteger(amount); } public int get(int amount){ if(this.amount.get() < amount){ return 0; } do{ if(this.amount.get() < amount){ break; } } while(!this.amount.compareAndSet(this.amount.get(), this.amount.get() - amount)); return amount; } public void put(int amount){ do{ if(amount < this.amount.get()){ break; } } while(!this.amount.compareAndSet(this.amount.get(), this.amount.get() + amount)); } public int getAmount(){ return amount.get(); } }
  28. 28. AtomicInteger public class AtomicInteger extends Number implements java.io.Serializable { private volatile int value; public AtomicInteger(int initialValue) { value = initialValue; } public final int get() { return value; } public final void set(int newValue) { value = newValue; } public final int getAndSet(int newValue) { for (;;) { int current = get(); if (compareAndSet(current, newValue)) return current; } } public final boolean compareAndSet(int expect, int update) { return unsafe.compareAndSwapInt(this, valueOffset, expect, update); } }  CAS  CAS  Int CAS
  29. 29. CAS Compare And Swap 1 2 100 0 0 100 CAS    2 0 CAS 100
  30. 30. synchonize volatile JDK5.0 java.util.concurrent Java

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