Statements and Expressions

1
Statements and Expressions

• Execution

2
Statements

• Essentially C/C syntax
• Dangling-else ambiguity if (C) if (D) S1
  else S2 resolved as if (C) if (D)
  S1 else S2 .
• Compiler checks for
• Uninitialized variables
• int i i is illegal, but int i (i
  3) is legal.
• Unreachable statements
• while (false) i 3 is illegal.
• if (false) i 3 is legal.

3
Expression

• Expression Statements
• Assignment, post/pre increment/decrement.
• Method invocation, Class instance creation.
• Expression evaluation results in a variable, a
  value, or nothing (void).
• For reference type expressions, the actual class
  of referenced object is constrained. The actual
  class of object determines program execution in
• Method invocation, instanceof.
• Cast, Assignment to array components, Exception
  handling.

4

• Operands of operators are evaluated from left to
  right, respecting parentheses and precedence, for
  portability.
• int i 2 i (i 3) i binds 9 to i.
• int i 2 i (i 3) binds 6 to i.
• Assignment operator is right associative.
• Every operand of an operator is evaluated before
  any part of the operation is performed
  (exceptions , , and ?).
• Argument lists in call and array dimension exprs.
  are evaluated from left to right.

5
Execution
6

• Virtual Machine Start-up
• java Test 1 2 abc
• Loading a class
• Class Loader loads binary representation (byte
  code file) of a class, and constructs a Class
  object.
• May throw ClassCircularityError,
  ClassFormatError, NoClassDefFound, etc
• Linking
• Verification, Preparation, Resolution (optional)
• Initialization
• Runs class variable initializers and static
  intializer.
• Interfaces initialized on active use.
• May involve loading, linking, and initializing
  ancestor classes.

7

• Verification of byte codes
• Structural and type correctness of instructions
• Branches to instruction start, etc
• May throw VerifyError
• Run-time stack growth
• Preparation
• Allocation of static storage and internal data
  structures
• Resolving Symbolic References
• Checks symbolic references to guard against
  possible incompatible changes since compilation
• May throw IllegalAccessError, InstantiationError,
  NoSuchFieldError, NoSuchMethodError.

8
Other Activities

• Creation of an instance of a class
• Constructor invocation
• Destruction of an instance
• Finalization (finalize method)
• Destruction of class objects removed in JLS
  update as redundant and rarely used feature.

9
(No Transcript)
10
Finalization
11
Special Method in class Object

• protected void finalize() throws Throwable
• ... super.finalize() ...
• This method is executed to reclaim non-Java
  resources, without waiting until garbage
  collection, to prevent resource leakage.
• It is guaranteed to be called by JVM at most once
  automatically.
• Typically the code for finalize() contains a call
  to finalize() in parent.

12
Typical Use

• public class ProcessFile
• private FileReader file . . .
• public synchronized void close()
• throws IOException
• if (file ! null)
• file.close()
• file null
• protected void finalize() throws Throwable
• try
• close()
• finally
• super.finalize()

13
Example

• class Resurrection
• public static Resurrection rs
• Integer ii
• Resurrection (Integer j)
• ii j
• protected void finalize () throws
  Throwable
• super.finalize()
• rs this
  
• . . .

14

• public static void main ( String args )
  throws Throwable
• new Resurrection(new Integer("666"))
• // f-reachable,
  unfinalized
• Integer m new Integer(666)
• Resurrection rm new Resurrection(m)
• // rm reachable,
  unfinalized
• rm.finalize() rm.finalize() //
  no effect on state
• rm null m null
• // unreachable,
  unfinalized
• // JVM
  finalizer-reachable, finalizable
• System.runFinalization()
• // reachable
  finalizable/finalized
• // unreachable finalized

15
Partition of Instances

• W.r.t Reachability
• Reachable
• from variables in live threads,
• starting from main() or run()
• Finalizer-Reachable
• from instances in finalize-queue,
• eventually reachable from this in finalize()
• Unreachable
• W.r.t Finalize-queue
• Unfinalized ( yet to enter the queue )
• Finalizable ( in the queue )
• Finalized ( off the queue )

16
FSM to model state of an instance

• States Reachable, F-Reachable, Unreachable
  x Unfinalized,
  Finalizable, Finalized
• (Reachable, Unfinalized) Start
• (Unreachable, Finalized) Final
• (Unreachable, Finalizable) Inconsistent
• Transitions-on ( garbage creators )
• Assignment, call to finalize(), method return
  etc.
• Thread death, JVMs action on finalize-queue
  etc.

17
(No Transcript)
18
Extended Example
class P ... class Q extends P ... public
void finalize()
19

• Step I Initially,
• nodes 1-5 (reachable, unfinalized)
• Step II p null
• node 1 (unreachable, unfinalized)
• nodes 2-5 (f-reachable, unfinalized)
• Step III Reclaim node 1
• Step IV Set up to finalize nodes 3 and 4
• node 2 (f-reachable, unfinalized)
• nodes 3-4 (f-reachable, finalizable)
• node 5 (f-reachable, unfinalized)

20

• Step V Run finalizer for node 3
• node 2 (f-reachable, unfinalized)
• node 3 (reachable, finalized)
• node 4 (reachable, finalizable)
• node 5 (reachable, unfinalized)
• Step VI
• node 2 (f-reachable, unfinalized)
• node 3 (f-reachable, finalized)
• node 4 (f-reachable, finalizable)
• node 5 (f-reachable, unfinalized)
• node 3 cannot be collected as it is still linked
  to node 2.

21

• Step VII Set up and run finalizer for node 2
• nodes 2-3 (reachable, finalized)
• node 4 (reachable, finalizable)
• node 5 (reachable, unfinalized)
• Step VIII
• nodes 2-3 (unreachable, finalized)
• node 4 (f-reachable, finalizable)
• node 5 (f-reachable, unfinalized)
• Step IX Reclaim nodes 2 and 3

22

• Step X Run finalizer for node 4
• node 4 (reachable, finalized)
• node 5 (reachable, unfinalized)
• Step XI
• node 4 (unreachable, finalized)
• node 5 (unreachable, unfinalized)
• Step XII Reclaim nodes 4 and 5
• trivial finalizer for node 5 not run

23
General remarks on Transitions

• A reachable object becomes unreachable if there
  is no reference to it from a variable and it has
  no predecessor object. (E, F)
• A (temporarily reachable) object becomes
  f-reachable if its predecessor is
  finalizable. (B, C, D)
• An f-reachable object becomes reachable when its
  finalize() or its predecessors finalize() is
  executed. (L, M, N)

24

• JVM marks unfinalized and not reachable objects
  f-reachable and finalizable. (G, H)
• If an object is unreachable (not f-reachable),
  and finalize() is not overridden, then JVM can
  mark it as finalized. (Optimization)
• JVM invokes finalize() on a finalizable object,
  after marking it finalized. (J, K)
• Unreachable and finalized objects can be garbage
  collected. (I)
• A newly created object can be reachable,
  f-reachable or unreachable. (A, O, I)

25
Odds and Ends
26
Parsing Issues LALR(1)

• Ambiguity (for one-lookahead)
• Cast vs Parenthesized array access
• ( z ) vs ( z 3 )
• Cast vs binary operator expression
• (p) q vs (p) q
• (p) q vs (p)
• Array creation expression
• new int 3 2
• Two-dimensional array new (int 3 2)
• Array access (new int 3) 2

27

• (p) q
• (p) q vs (p)
• C/C parsers resolve ambiguity between type cast
  operator vs operand for binary or unary by
  semantic analysis to determine if p is a type or
  a variable.
• In Java, for / to be unary and numeric, p must
  be a keyword for casting an integer subtype. If
  operators could be overloaded, it would
  complicate matters here.

28
Irregularities in Declarations?

• Local variable vs formal parameter (definitions)
• int i,j int i int j
• f(int i, j) ¹ f(int i, int j)
• Constructor signature vs method signature
• constructor return type omitted, not void
• Multiple declarations (not definitions)
• Import and Inheritance of the same field
  multiple times permitted.
• Repeating an interface name in implement-clause
  illegal.

29
Minor Naming Inconsistencies in APIs

• size() in Vector, ByteArrayOutputStream
• length() in String
• getLength() in DataGramPacket
• countItems() in List
• countTokens() in StringTokenizer
• countComponents() in Container