whileloopsyntax
ReviewforMidterm3
1
Chapter6:Iteration Whilestatements:
while(expression)action Actionisexecutedrepeatedlywhileexpressionistrue Once expression is false, program execution moves
ontonextstatement Actioncanbeasinglestatementorablock Ifexpressionisinitiallyfalse,actionisneverexecuted
Notethatdowhilestatementswillnotbeonthe
exam
2
Chapter6:Iteration forloopsyntax
Forstatements:
for(forinit;forexpression;forupdate)action forinit is executed once only (before the loop starts
thefirsttime) Action is executed repeatedly while forexpression is true After action is executed at the end of each loop, forupdateisexecuted Once forexpression is false, program execution movesontonextstatement Actioncanbeasinglestatementorablock Ifexpressionisinitiallyfalse,actionisneverexecuted
3
Chapter6:Iteration Commonpitfalls
Infiniteloop:aloopwhosetextexpression
neverevaluatestofalse Besurethatyourforloopstartsandends whereyouwantitto
Forexample,inanarrayofsize n,itneedsto
startat0andendatn1 Otherwise,itscalledanoffbyoneerror
Be sure your loop variable initialization is
correct
4
Chapter6:Iteration Fileaccess
JavaprovidestheFileclassforfileI/O
ConstructortakesinthefilenameasaString
A stream is a name for a input or output
method
System.out:outputstream System.err:erroroutputstream System.in:inputstream File:fileinputoroutputstream
5
Chapter6:Iteration Scannermethods
The Scanner class can be initialized with
anFileobject
Scanner
filein = new Scanner (new File (filename));
The
Scanner class has a bunch of methodsusefulinloops:
hasNextInt():tellswhetherthereisanextint hasNextDouble():sameidea,butwithdoubles
6
Chapter7:Methodsandclasses Instancemethodsvs.classmethods
Instance
That
(member) methods modify the stateoftheobject
state can include instance (member) variablesaswellasclassvariables
Class methods do not modify the state of
theobject
Examples:Math.sin(),Math.cos(),etc. Canonlyaccessclassvariables Theyaredeclaredwiththekeywordstatic
7
Chapter7:Methodsandclasses Instancevariablesvs.classvariables
Instance (member) variables are one per
object
Can only be accessed by instance (member)
methods
Classvariablesareonefortheentireclass
Thesingleclassvariableiscommontoallthe
objectsofaclass Can be accessed by both instance (member) methodsandclassmethods
8
Chapter7:Methodsandclasses Parameters
Thevaluespassedintothemethodcallarecalledactual parameters
foo(7);
//7istheactualparameter
The parameter names within the method are called formalparameters
voidfoo(intx){ //xistheformalparameter
Java copies the values of the actual parameters to the formalparameters
That copy is kept in a spot of memory called the activation record Anymodificationsinthemethodaremodificationstothecopy Notethatifaobjectispassedin,theobjectsreferenceiswhatis copied,nottheobjectitself
Thus,theobjectcanbemodified!
9
Chapter7:Methodsandclasses Instancevariables
Instancevariablesarenormallydeclaredprivate
Modificationsisthroughmutatormethods Accessisthroughaccessormethods
Classesshouldusetheirownmutator/accessor
methodstochange/accessthefieldsoftheclass
For mutators, it allows checking to be done when
theyarechanged For accessors, it becomes more important when dealingwithinheritance
10
Chapter7:Methodsandclasses Blocksandscoping
A statement block is a number of statements
withinbraces Anestedblockisoneblockwithinanother
Note
that the braces that enclose a class do not constituteablock!
A local variable is a variable defined within a
block
Youcandefineasmanylocalvariablesineachblock
asyouwant
However,
there cant be variables of the same name declaredwithinthesameblock Example:voidpublicfoo(intx){ doublex=0; 11
Chapter7:Methodsandclasses Overloading
Method
overloading when is there are multiple methods of the same name with differentparameterlists
Java will figure out which one you mean to
call by which methods parameter list best matchestheactualparametersyousupply
12
Chapter7:Methodsandclasses Constructorsandthis
Keyword this references the object being
operatedwithin
Isnotvalidwithinaclassmethod!!!
A
constructor constructor
can
invoke
another
Needstobeatthebeginningofthemethod
Ifyoudontprovideanyconstructors,Java
createsadefaultconstructorforyou
This
default constructor invokes the default constructorofthesuperclass
13
Chapter7:Methodsandclasses Specificmethodsandinstances
All
classes inherit certain methods, and shouldoverridethem
toString() clone() equals()
clone()sreturntypemustbeObject instanceof returns true if the object is an
intanceoftheclass
Example:Strings=foo;
if(sinstanceofObject){
14
Chapter7:Methodsandclasses equals()
equals()shouldhavethefollowingproperties:
Reflexivity:x.equals(x)shouldbetrue Symmetry:ifx.equals(y)theny.equals(x) Transitivity:
x.equals(z) Consistency: x.equals(y) should always return the samevalue(providedxandydontchange) Physicality:x.equals(null)shouldreturnfalse
You
if x.equals(y) and y.equals(z) then
dont have to remember the property names,though
15
Chapter7:Methodsandclasses Notontheexam
Generics
16
Todaysdemotivators
17
Chapter8:Arraysandcollections Arraybasics
Anarrayisanobject
Thus, it is actually a reference to a series of
valuessomewhereinmemory
Theindividualpartsofanarrayarecalled
elements
Elementscanbeaprimitivetypeoranobject
All elements in the array must have the
sametype Multidimensional arrays will not be on the exam
18
Chapter8:Arraysandcollections Arraydeclarations
Therearetwopartstocreatinganarray
Arraydeclaration
int[]array;
Arrayinitialization
Thisdeclaredauninitializedarrayreference! array=newint[10];
Thiscreatesanarrayof10intseachwithvalue0
Canbecombined
Javagivesdefaultvaluestotheelements:null,0,orfalse
Ifdeclaringanarraycandeclarespecificelements: NotethattheintherecouldhavebeenString,etc.
int[]array=newint[10];
int[]array={1,2,3,4,5,6,7,8,9,10};
If an object type, then the array holds references to those objects
19
Chapter8:Arraysa...