Java 1'5 and Advanced Features

1
Java 1.5 and Advanced Features

• CS2335
• Fall 2005

2
Generics

• Problem How can we have type-safe data
  structures and methods, when we do not know what
  types programmers might invent later?
• Solution Use placeholders for types that get
  filled in and expanded at compile time.

3
Generics Example

• Old Way
• Method 1
• List myList new ArrayList()
• What can I put in myList? Any Object!!!!
• Player p (Player) myList.get()
• Method 2 (Try to make safe)
• public class PlayerList
• List players
• Player get( )
• return (Player) players.get( )

4
Generic Collections

• New
• ListltPlayergt players new ArrayListltPlayergt()
• Now we can only put players in the list.
• players.add(new Player()) //OK
• players.add(new String()) //Compile Error
• Player p players.get( ) //no cast needed!

5
Can create our own generics
public class PairltX, Ygt private X first
private Y second public Pair (X fst, Y
scnd) first fst second scnd public
X getFirst( ) return first public Y
getSecond() return second PairltString,
intgt pair new PairltString, intgt(Bob, 34)
6
Improved For loop

• To iterate over collections required us to use a
  formulaic for loop or while loop that might be
  error prone.
• Old way
• Iterator iter players.iterator()
• while (iter.hasNext())
• Player p (Player) iter.next()
• for (int i 0 i lt array.length i)

7
Improved For Loop
for (Player p players )
p.doSomething() Works for arrays
too!!! int scores new int20 for ( int i
scores) total i
8
Enumerated Types

• We have constants and special categories of
  things, and we want to treat them in a type safe
  manner, not just as integer codes. We want to
  stop an invalid code from being accepted.
• Old Way
• class Furnace
• public static final int AUTOMATIC 1
• public static final int OFF 2
• public static final int MANUAL 3
• public static final int FAULT 4
• private int mode OFF

9
Enumerated Types

• public enum FurnaceMode
• OFF, AUTOMATIC, MANUAL, FAULT
• class Furnace
• private FurnaceMode mode
• public void doSomething()
• switch(mode)
• case AUTOMATIC

10
But Wait, there is more

• We can put behavior IN the enumerated type
• public enum FurnaceMode
• OFF (1),
• AUTOMATIC (2),
• MANUAL (3),
• FAULT(4)
• private int errorCode
• public FurnaceMode (int code) errorCode
  code
• public int value()
• switch (this)
• case OFF return errorCode

11
Static Initializer Blocks

• We need to be able to initialize static data, but
  it requires more complex initialization than a
  simple assignment.
• Old Way
• public static final int RATE MyClass.initRate()

12
Static Initialization Blocks

• New Way
• public MyClass
• public static final int RATE
• static
• TaxTable tt new TaxTable()
• for (int i0 i lt 100 i)
• tt.iterate()
• RATE tt.getResult()

13
Instance Initializer

• I need to initialize an instance variable, and I
  would like to put the code near the variable, not
  off in a constructor. Used most often to
  initialize arrays.

private int values new int13 for (int i
0 i lt values.length i) valuesi i
14
Reflection

• Sometimes we need to manipulate objects and
  classes at runtime rather than at compile time.
  What if I know a game is called VideoPoker,
  and I want to make a class using that string, at
  Runtime?