Java Iterators

1. Java Iterators interface Collection { … Iterator iterator(); … } interface Set extends Collection { … Iterator iterator(); … } interface List extends Collection { … Iterator iterator(); ListIterator listIterator(); ListIterator listIterator(int index); … } Interface Iterator { boolean hasNext() ; Object next() ; void remove() ; } Interface ListIterator extends Iterator { boolean hasNext() ; Object next() ; boolean hasPrevious() ; Object previous() ; int nextIndex() ; int previousIndex() ; void remove() ; void set(Object o) ; void add(Object o) ; }

2. Java Iterator import java.util.*; public class IteratorExample { public static void main(String[] args) { List ints = new ArrayList(); for(int i = 0; i < 10; i++) ints.add(new Integer(i)); Iterator e = ints.iterator(); while(e.hasNext()) System.out.println( ((Integer)e.next()).intValue() ) ; } }

3. Object Serialization

4. Object Serialization To represent an object in a byte-encoded format that can be stored and passed to a stream, and in need can be reconstructed. Live Object Frozen Object Stream Live Object Serialize DeSerialize

5. Serialization • ObjectOutputStream & ObjectInputStream • Works like other input-output streams • They can write and read Objects. • ObjectOutputStream: Serializes Java Objects into a byte-encoded format, and writes them onto an OutputStream. • ObjectInputStream: Reads and reconstructs Java Objects from a byte-encoded format read from InputStream. • Serialization can be used in. • Remote Method Invocation (RMI), communication between objects via sockets. (Marshaling and unmarshaling objects) • Archival of an object for use in a later invocation of the same program. • Objects to be serialized • Must implement Serializable interface • Non-persistent fields can be marked with transient keyword • The following is written and read during serialization • Class of the object • Class signature • Values of all non-transient and non-static members

6. Serialization • To Write into an ObjectOutputStream • FileOutputStream out = new FileOutputStream(“afile”) ;ObjectOutputStream oos = new ObjectOutputStream(out) ;oos.writeObject(“Today”) ;oos.writeObject(new Date()) ;oos.flush() ; • To Read from an ObjectInputStream • FileInputStream in = new FileInputStream(“afile”) ;ObjectInputStream ois = new ObjectInputStream(in) ;String today = (String) ois.readObject() ;Date date = (Date) ois.readObject() ;

7. Serialization • ObjectOutputStream.writeObject(Object) traverses all the internal references of the object recursively and writes all of them. • ObjectOutputStream implements DataOutput interface to write primitive data types. • writeInt(…), writeFloat(…), writeUTF(…), etc. • ObjectInputStream implements DataInput interface ro read primitive data types. • readInt(), readFloat(), readUTF(), etc. • writeObject(Object) throws NotSerializableException if Object does not implement Serializable interface

8. Object Serialization Example import java.io.* ; import java.util.* ; class A implements Serializable { public int i = 5 ; public String str = "Hi" ; public List l = new ArrayList() ; } public class ObjSerTest { public static void main(String[]args) { A a = new A() ; a.i = 10 ; a.str = "Hello" ; a.l.add("One") ; a.l.add("Two") ; serialize(a) ; } private static void serialize(A a) { System.out.println("Serializing..."); try { FileOutputStream fos = new FileOutputStream("test.out") ; ObjectOutputStream oos = new ObjectOutputStream(fos) ; oos.writeObject(a) ; } catch (Exception e) { System.err.println("Problem: "+e) ; } } }

9. Object De-serialization Example import java.io.* ; import java.util.* ; class A implements Serializable { public int i = 5 ; public String str = "Hi" ; public List l = new ArrayList() ; } public class ObjDeSerTest { public static void main(String[]args) { A a = deserialize() ; System.out.println(a.i) ; System.out.println(a.str) ; System.out.println(a.l) ; } private static A deserialize() { System.out.println("DeSerializing..."); try { FileInputStream fis = new FileInputStream("test.out") ; ObjectInputStream iis = new ObjectInputStream(fis) ; return (A) iis.readObject() ; } catch (Exception e) { System.err.println("Problem: "+e) ; } return null ; } }

10. Customizing Serialization • To define writeObject() and readObject() to append additional information. • private void writeObject(ObjectOutputStream oos) throws IOException { • oos.defaultWriteObject() ; • // customized serialization code • } • private void readObject(ObjectInputStream ois) throws IOException { • ois.defaultReadObject() ; • // customized deserialization code • // if necessary, must include code to update the object • }

11. Externalizable interface • To control the serialization process explicitly, Externalizable interface must be implemented. • Externalizable interface • public interface Externalizable extends Serializable { • public void writeExternal(ObjectOutput out) throws IOException ; • public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException ; • } • writeExternal and readExternal must save/load the state of the object. They must explicitly coordinate with its supertype to save its state.

12. General Programming Tips

13. General Programming Tips • Try to write self-documented programs: • Use logical variable names and function names : not : int x; but : int studentCount; • Comments • Use comments whenever something is unclear. Think that you are telling some other person about your code. (After a long time, this person might be you) • Coding Conventions • Class names start with Capital • class MyClass { • Instance names, fields, function names start with lower, etc. • Object myObject ; • int myInt ; • void myFunc() { … • Static Final variables are All-Capitalized • static final MAXVALUE = 200 ; • Necessary Tabs and spaces in Blocks. (Theoretically you can write a long program in a single line)

14. General Programming Tips • Use getter, setter, and is methods. • int getCount() ; • int setCount() ; • isEmpty() ; • Self-checking classes • Use main() almost in every class to check class functionality. • You can make an expected output check, in order to verify changes in the class.

15. General Programming Tips • In highly coupled classes, consider to make inner classes. • Keep method definitions and scopes as short as possible. (For easy visualization and readability.) • Compiler-time errors are better then run time errors. • Prefer Interface definition to Abstract Class • Not : abstract class X { abstract void f() ; } • But : interface X { void f() ; • Spelling problems in overriding Classes • class X { void encode() { … } } • class Y extends X { void Encode() { … } } // Does not overload the method • Use Java Container Library classes • List l = new ArrayList() ; • Map hm = new HashMap() ; • Set s = new HashSet() ; • List myStack = new LinkedList() ;

16. Debugging a program • Handle Exceptions in their proper level. try { … } catch (Throwable) { … } // Bad • Don’t output errors to System.out, but to System.err • For Debugging code use a single global boolean variable showing the Debug mode. bDebug = true ; … if (bDebug) { // Print some informationCare must be taken not to change any state inside. if (bDebug) { if (i++==0) … }