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Interfaces, Arrays, Exceptions

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Title: Interfaces, Arrays, Exceptions

1
Interfaces, Arrays, Exceptions

Clone
Checked Exceptions

2
Interface

An interface is an abstract class. Implicitly
Members are public.
Fields are final and static (with initializers).
Methods are abstract.
Behavior may be informally specified.
Java supports single inheritance of classes
(fields and methods), but multiple inheritance of
interfaces (constants and method signatures).
An interface can extend another interface.
A class can implement an interface.

3
Multiple Inheritance Pros and Cons

Tree-structured class hierarchy leads to needless
code duplication when several abstractions are
specializations of other orthogonal abstractions.
Independent sets of classes A,B and C,D can
result in four new classes using multiple
inheritance. In contrast, single inheritance
requires four new class definitions by copying.

4
(contd)

DAG class structure facilitates code sharing and
reduces code duplication.
Unfortunately
There is no uniform application-independent way
to deal with repeated inheritance of fields in
diamond net. (To share or to duplicate?)
There is no uniform strategy to resolve method
conflicts or to combine method impls.
Reuse of source code possible, but not object
code.

5
Java Design Decisions

Simplify the language (compiler) by banning
multiple inheritance of classes.
Support multiple inheritance of interfaces (to
share method signatures and to document
behavior).
Code duplication can be minimized by
approximating multiple inheritance using
composition and delegation.

6
(contd)

From orthogonal classes A,B and C,D, generate
subclasses AC (AD, BC, BD).
These extend A (A, B, B) with a new field of
type C (D, C, D).
Then, delegate the call to a method in C
(D, C, D) on objects of class
AC (AD, BC, BD) to the new field.

7
class C extends A, B

class A ...
interface IB ...
T bm(Fs) ...
class B implements IB
T bm(Fs) ...
class C extends A implements IB
B b
T bm b.bm(Vs)

8
Role of Interfaces in Java

Defining Scalar types (a la C) in Java 1.4
and before
Improves Readability but not Reliability. (Cf.
Ada)
Java 5 natively supports Enumerated types
Enables integration of multiple implementations
of an abstract data type.
Polymorphic data in arrays Dynamic binding.
Serves as a type parameter.
(Cf. Ada Generics, C Templates, MLs Functors,
etc)
In AWT, it provides a convenient, abstract and
reliable means for connecting event generating
graphical/widget objects to application dependent
event handlers.

9
(contd)

Multiple Inheritance of interfaces facilitates
approximating multiple inheritance of classes
for minimizing code duplication. This involves
defining a field for each parent class and
explicitly maintaining consistency among its
fields and dispatching each method incorporating
conflict resolution strategies.

interface I
Vector v new Vector() int i 5
interface J extends I
Vector w (Vector) v.clone()
int i 10 int j i
class Interface implements J
public static void main(String args)
// I.v J.w ERROR
System.out.println(" i " i " j "
j )
I.v.addElement(new Integer(44))
System.out.println(" w " w " v "
v)
System.out.println(" Hidden I.i "
I.i)
i 10 j 10
w 44 v 44

interface I
Vector v new Vector()
String i ABC
interface J extends I
Vector w (Vector) v.clone()
String i new String(pqr ) String j i
XYZ
class InterfaceVectorString implements J
public static void main(String args)
System.out.println(" Hidden I.i "
I.i)
// compile-time constant no interface
initialization
System.out.println(" i " i " j " j
)
// J initialized after I
System.out.println(" w " w " v "
v)
I.v.addElement(new Integer(55))
System.out.println(" w " w " v "
v)

interface I
Vector v new Vector()
String i ABC
interface J extends I
Vector w (Vector) v.clone()
String i PQR String j i XYZ
class InterfaceVectorString1 implements J
public static void main(String args)
System.out.println(" Hidden I.i "
I.i)
System.out.println(" i " i " j " j
)
// compile-time constant no interface
initialization
I.v.addElement(new Integer(55))
System.out.println(" w " w " v "
v)
// J initialized after I

interface I
Vector v new Vector()
String i ABC
interface J extends I
Vector w (Vector) v.clone()
String i new String(pqr ) String j i
XYZ
class InterfaceVectorString2 implements J
public static void main(String args)
System.out.println(" Hidden I.i "
I.i)
// compile-time constant no interface
initialization
System.out.println(" i " i " j " j
)
// J initialized after I Is value is NOT a
compile-time constant
I.v.addElement(new Integer(55))
System.out.println(" w " w " v "
v)

14
Arrays
15
Arrays

An array object contains a number of variables
accessed using a non-negative integer index.
Array Type and Array Variable
int twoD
String items
Array Creation and Initialization
int twoD 1,2, null
String items abc

16
Array Members

public final length
number of elements in the array.
public Object clone () ...
creates a new array object and then initializes
its elements from the source array.
copying is shallow, not recursive.
int ia1 1,2,3
int ia2 (int ) ia1.clone()
Cf. Assignment ia1 ia2
Array indices checked at run-time.
(ArrayStoreException)
Cf. Ada Unconstrained array types

17
Cloning

Cf. copy constructor

18
Cloning

public class Object ...
protected Object clone()
throws CloneNotSupportedException ...
public interface Cloneable
clone() method creates an object from another
object of the same type.
Direct instances of class Object cannot be
cloned.
Cloneable is a marker interface.

19
(contd)

A class may
support clone, by defining a public clone and
implementing Cloneable.
conditionally support clone, if contents
support it. (Collection classes)
allow subclasses to support clone. (E.g., Object)
forbid clone by throwing CloneNotSupportedExceptio
n.

class A
protected Object clone()
throws CloneNotSupportedException
return super.clone() //assume a new defn
class B
public Integer b new Integer(5)
public Object clone()
throws CloneNotSupportedException
return super.clone()
Cannot clone both A- and B-objects.
Subclasses of B must declare clone public.
Used for appropriate clone for descendents.

class C extends B implements Cloneable
public Integer c new Integer(6)
public Object clone()
throws CloneNotSupportedException
return super.clone()
Supports clone but descendents can throw
exception.
class D extends C
public Object clone()
try return super.clone()
catch (CloneNotSupportedException
e)
Descendents must support public clone that does
not throw any exception.

class TestClone
public static void main(String args)
throws CloneNotSupportedException
new Object() . clone() // compile-time
error
new A() . clone() // run-time error
new B() . clone() // run-time error
C cobj new C() C cobjc cobj.clone()
System.out.println( C (cobj cobjc))
// C false
D dobj new D() D dobjc dobj.clone()
System.out.println( D (dobj dobjc))
// D D_at_f3a655dd

23
Recursive types (Cf. Expanded types)

class Problem
Problem p new Problem()
class Infinite
public static void main(String args)
new Problem()

24
Deep Clone?

public class IntStack implements Cloneable
private int buf
private int top
public IntStack(int max)
buf new intmax top -1
public Object clone()
try
IntStack is
(IntStack) super.clone()
is.buf (int) buf.clone()
return is
catch (CloneNotSupportedException e)
throw new InternalError(e.toString())

25
Exceptions
26

Compiler checks syntax, and flags type
mismatches, uninitialized variables, etc.
Semantic constraints of the language or
applications can be violated at run-time.
Instead of aborting a program, or exhibiting
arbitrary behavior when such violations occur,
the exception mechanism enables graceful
degradation, enhancing portability and
robustness.

27
The causes of exceptions in Java

An abnormal execution condition was synchronously
detected by JVM.
Java semantics violation.
Error in linking/loading of a program.
Exceeding resource limitations.
A throw-statement (user-defined exception) was
executed.
An asynchronous exception occurred as
Thread.stop() was invoked.
An internal error occurred in JVM.

28
Construct

try ...
throw new Exception(test)
...
catch (InterruptedException e) ...
catch (Exception e) ...
finally ...

Exceptions are well-integrated into the class
hierarchy.
When an exception is thrown, control is
transferred to the nearest dynamically enclosing
catch-clause of a try-statement that handles the
exception.
Context-sensitive handling
catch-clauses are ordered using exception
specificity.
An exception thrown by the finally-block is
propagated in preference to an exception thrown
by the try-block.

class Exp1
public static void main(String args) throws
Exception
try
throw new Exception("Throw Exception")
catch (Exception e)
throw e
throw new Exception("REThrown Exception")
finally throw new Exception("FINALLY")
No Exception
Exception in thread "main" java.lang.Exception
Throw Exception
at Exp1.main(Exp1.java4)
Exception in thread "main" java.lang.Exception
REThrown Exception
at Exp1.main(Exp1.java7)
Exception in thread "main" java.lang.Exception
FINALLY
at Exp1.main(Exp1.java9)

class Exp2
static String f()
try
throw(new Exception())
return("first-try")
catch (Exception e)
return("second-catch")
finally
return("third-finally")
public static void main(String args)
System.out.println(f())
first-try
//return(first-try) unreachable
second-catch

32
Examples

try ...
CallThatThrowsException(TRY)
...
finally ...
throw new
Exception(FINALLY)
try ...
return (TRY) ...
finally ...
return
(FINALLY)

33
Exception Handlers

Pinpoint location/cause of problem (in a layered
software). Record information for debugging.
Give diagnostic message, and take corrective or
alternate action.
In a real-time system, this may be turning-on
fault indicator lights to turning-off devices.
Restore environment to a well-defined state.
Release resources (locks), save files, etc.

34
Exceptions vs Error codes

Checking and Returning error code to signal
abnormal condition is unsuitable because
Error handling code and normal processing code
mixed up.
A caller can ignore error code.
Not conducive to portability and robustness.
Difficult to pinpoint source/location of
abnormality from results that are only indirectly
related to it.
For a sequence of nested calls, the
intermediate procedures must explicitly check
and propagate codes.
Using global variable or gotos is not
appropriate either.
In Java, the compiler guarantees that checked
exceptions are always handled, and the JVM takes
care of propagating exceptions using the
call-stack.

35
Java Errors and Exceptions

Subclasses of class Throwable
Error
unchecked recovery difficult or impossible
Exception (language defined user-defined)
unchecked too complex for compiler guarantees
too cumbersome for programmer to declare/handle.
(logic errors)
checked requires programmer to provide an
exception handler or propagate exception
explicitly.

class Exp
void p() q(1)
int q(int x) return 1/(x - x)
public static void main(String args)
(new Exp()).p()
gtjava Exp
java.lang.ArithmeticException
/ by zero
at Exp.q(Exp.java3)
at Exp.p(Exp.java2)
at Exp.main(Exp.java6)

class MyExp extends Exception
class TestExp3
static void p() throws MyExp
throw new MyExp()
public static void main(String args)
throws MyExp
p()
Checked Exception is explicitly propagated by
main.

class MyExp extends Exception
class TestExp4
static void p() throws MyExp
throw new MyExp()
public static void main(String args)
try
p()
catch (MyExp e)
Checked Exception is explicitly handled in main.

39
Errors

Loading ClassFormatError, ClassCircularityError,
NoClassDefFoundError.
Linking IllegalAccessError,InstantantiationError,
NoSuchField, NoSuchMethodError.
Verification VerifyError.
Initialization ExceptionInInitializerError.
Resource InternalError, OutOfMemoryError,
StackOverflowError, UnknownError.

40
RuntimeException