SCJP 5.0 Study Notes(1)

Note:   Please feel free to modify, improve, circulate, and repost these notes at will. –  Dave Allen

 

Collections Framework (only Maps, Iterators do not extend java.io.Collection Interface)

Collection interface	add(E) addAll (Collection<e>) remove(Object) removeAll ( Collection<?>) contains(Object) size() iterator () isEmpty () Object[] toArray () – returns Object array, must cast – e.g. String[] str = (String[]) myCollection.toArray (); T[] toArry (T[] a) – returns typecast array – syntax: String[] str = myCollection.toArray (new String[0]);
Map interface, SortedMap interface extends Map	put(keyObject , valueObject ) – inserts key/value pair, will overwrite duplicate keyObject values keySet () – returns set of keys values() – returns collection of values get(Object), remove(Object), size(), clear() entrySet () – returns set of Map.Entry type (key/value pair) Map.Entry – a key/value pair, has getKey () and getValue () methods   Maps care about unique identifiers There is no add() method; Don’t have to override equals(Object) method to compile, but necessary to find objects
HashMap
Hashtable	synchronized version of HashMap
TreeMap	Can have null values, but NO null keys
LinkedHashMap (extends Hashmap )	maintains insertion order, slower updates, faster iteration than HashMap , order remains same if replacing duplicate key, can have null keys or values
Set interface, SortedSet interface extends Set	boolean add() – if element already exists, returns false and element not added Sets care about uniqueness
HashSet
LinkedHashSet (extends HashSet )	fastest iteration through items without duplicates (a Set) that maintains insertion order
TreeSet <T>	new TreeSet (), new TreeSet (Collection<E>), new TreeSet <T>(Comparator<? super E>)
List interface	Lists care about the index
ArrayList	fastest iteration through items with duplicates
Vector	synchronized version of ArrayList ; slower
LinkedList	addFirst (), getFirst ()   doubly linked, ideal for implementing a stack or maintaining an ordered sequence of objects when they are frequently inserted and removed from the middle of the sequence
Queu interface	peek() – returns highest priority entry without removing it poll() – returns highest priority entry and removes it offer() – adds entry
PriorityQue
Collections	Collections.sort (Collection) Collections.sort (Collection, Comparator) Collections.binarySearch (collectionName , pattern) – must be sorted first Collections.binarySearch (collectionName , pattern, Comparator) – must be sorted first Collections.reverse () – reverses order of elements in List, doesn’t have to be sorted first Collections.reverserOrder () – returns Comparator that sorts in reverse, must b sorted first
Arrays	Arrays.sort (arrayToSort ) Arrays.binarySearch (arrayName , pattern), Arrays.binarySearch (collectionName , pattern, Comparator) – returns int index Arrays.asList (arrayName ) – returns List, changes to either returned list or original array will be reflected in the other
Comparable interface	int compareT o (SpecificObject Type ) returns     negative int if this < Obj2                 0 if Objects are equal                 positive int if this > Obj2
Comparator class	int compare(SpecificObject Type1, SpecificObject Type2) – returns int with same meaning as above – substitute “SpecificObjectType1” for “this”
Iterator	boolean hasNext () next() – returns type Object which must be cast if no generic type is specified for iterator , even if Collection that called iterator () method has generics, like List<Integer>   Can use generics syntax to avoid explicit casting, e.g. Iterator <Dog> = dogs.iterator ();

 

 

Dates

java.util.Date	new Date() – date Object with current date new Date(long millis ) – millis since Jan 1, 1970   getTime () – returns long setTime (long millis )   internally stored as primitive long which is milliseconds between date and Jan 1, 1970
Calendar	Calendar.getInstance () – gets default Calendar (almost always GregorianCalendar ) with current time Calendar.getInstance (Locale)   add( intField , intAmount ) – adds or decrements field.  intAmount can be negative roll(intField , intAmount ) – larger parts of date will not be incremented/decremented, only the field specified set(intField , intValue ) get(intField ) – e.g. calendarInstance.get (Calendar.MONTH ); set() – various other set methods setTime (Date) getTime () – returns Date object, useful for passing to DateFormat setTimeInMillis (long millis ) getTimeInMillis () – returns long compareTo (Calendar)   NO new Calendar(), only instantiated using getInstance (), months are 0-based (January is 0)
java.text . NumberFormat	NumberFormat.getInstance (), NumberFormat.getInstance (Locale) – e.g. NumberFormat.getInstance (Locale.US ); NumberFormat.getNumberInstace ( ), NumberFormat.getNumberInstanceLocale () – same as getInstance ()? NumberFormat.getCurrencyInstance (), NumberFormat.getCurrencyInstance (Locale)   format(long), format(double) – takes various numbers to be formatted, returns String parse(String) – takes a String (must be formatted correctly to locale), returns a Number – e.g. parse(“$5,075”); setParseIntegerOnly ( boolean ) – whether to format and display digits after decimal. getMaximumFractionDigits (), getMinimumFractionDigits – get number of decimal places setMaximumFractionDigits (int), setMinimumFractionDigits – set number of decimal places get/setMinimum/MaximumIntegerDigits ()   Should only instantiated using getInstance ()
java.text . DateFormat	getInstance () – returns DateFormat with default Locale, sometimes SHORT style getDateInstance () (has defaults, sometimes MEDIUM style),getDateInstance (styleConstant ), getDateInstance (styleConstant , Locale) parse(String) – converts String to a Date format(Date) – returns String with formatted Date   String format for each style possible default format – 0/8/01 7:46 PM DateFormat.SHORT –  9/8/01 DateFormat.MEDIUM – Sep 8, 2001 DateFormat.LONG – September 8, 2001 DateFormat.FULL – Saturday, September 8, 2001   There is no format( Calendar) method.  Calendar must convert to Date first(using yourCalendar.getTime ()), then use format(Date) DateFormat’s Locale can only be set at instantiation using getInstance () – NO new DateFormat ()
java.util.Locale	Locale.getDefault () – returns default Locale new Locale(stringLanguage ) new Locale(stringLanguage , stringCountry ) – e.g. new Locale(“it”, “IT”)   setDefault (Locale) getDisplayLanguage () – returns String of Language getDisplayLanguage (Locale) – displays language using language of Locale passed in getDisplayCountry () – returns String code getDisplayCountry (Locale) – displays language using language of Locale passed in) Locale.getAvailableLocales () – returns an array of Locales installed   used in Scanner, DateFormat , NumberFormat

 

String Stuff

String	new String(), new String(StringBuilder /Buffer) new String(“StringLiteral ”) new String(ExistingStringObject ) will create a new String object different than ExistingStringObject   length() charAt (int) concat (String) equalsIgnoreCase (String) substring(intIndex ) substring(startIntIndex , endIntIndexExclusive ) replace(char/stringRegex , char/stringReplacement ) replaceAll (char/stringRegex , char/stringReplacement ) replaceFirst (stringRegex , stringReplacement ) split(regex ) – returns String array of tokens, good for just small amount of data split(regex , IntLengthOfArrayReturned ) toLowerCase () toUpperCase () trim() – remove whitespace off ends toString () – returns a new String object   indexOf (char/String) indexOf (char/String, startingIntIndex ) lastIndex (char/String, startingIntIndex ) lastIndex (char/String, startingIntIndex ) contains(charSequence ) matches(regex ) – returns boolean indicating if regex matches string boolean startsWith (String) boolean startsWith (String, offsetIndex ) String format(“formattingString ”, args ) – returns String formatted to specs String format(Locale, “formattingString ”, args )   All methods return a String, do no alter original String NO insert() method (this is in StringBuilder/StringBuffer ) NO length member variable (this is for array)
StringBuilder / StringBuffer	append(xxx) – can append almost any char related type delete(intStartIndex , intEndIndexExclusive ) deleteCharAt () insert(intOffset , String) reverse() toString () length() substring(startIndexInclusive ) substring(startIndex , endIndexProbablyExclusive )   lastIndexOf (String) lastIndexOf (String, startSearchIndex ) replace(intStart , intEnd , String) setCharAt ()   StringBuffer is synchronized and slower. StringBuilder is not synchronized and faster equals() method is not overridden, so it compares references, not meaningful values – same as ==
java.util . Scanner	new Scanner(File), new Scanner(stringToSearchOrTokenize )   useDelimiter (String) – can be regex useLocal (Locale) hasNext (), hasNextInt (), hasNextFloat (), hasNextXxx () – for all primitives EXCEPT char next(), nextInt (), nextFloat (), nextXxx – for all primitives EXCEPT char Scanner.close () – static method that should be used after done with file findInLine (regexString ) – returns String found   match() – returns MatchResult matchResult.groupCount () – method in MatchResult (not Scanner) matchResult.group (groupCountIndexInt ) – method in MatchResult (not Scanner)   Mostly used for tokenizing, but can also find regexes using findInLine () Default delimiter is whitespace
java.util.regex . Pattern	Pattern.compile (regex ) – returns new Pattern matcher(CharSequence ) – returns new Matcher split(regex ) – returns String array of matching patterns found in the CharSequence Pattern.matches (regex , stringtoSearch ) – returns Boolean indicating if string matches regex   /d             digits /s             whitespace /w            word character (letters, digits, or underscores)   [af ]         a or f   [a-f]        a, b, c, d, e, f   [a-dA -D]                A,a,B,b,C,c,D,d .               any character +              greedy one or more (e.g. /d+ means one or more digits) *              greedy zero or more (e.g. filename/d*) ?              greedy zero or more occurrences(e.g. /d?) ^              not (e.g. [^abc ] excludes abc )   String pattern = “\\d” creates string regex with \d.  String pattern = “\\.” creates string regex with .
java.util.regex . Matcher	find() – returns boolean indicating a match’s existence, advances to next index that matches, left to right start() – returns starting index group() – returns string that matches   Probably Not on Exam replaceAll (stringReplacement ) replaceFirst (stringReplacement ) appendReplacement (StringBuffer , stringReplacement ) -- appendTail (StringBuffer ) – to be used after appendReplacement
java.util . Formatter	new Formatter(), new Formatter(Appendable , Locale) – Appendable can be StringBuilder/StringBuffer , BufferedWriter , FileWriter , but NOT String   format(“formattingString ”, args ) – returns formatted String

 

Serialization (package java.io , classes must implement Serializable to be serialized)

FileOutputStream	new FileOutputStream (stringFileName ) new FileOutputStream (File)
ObjectOutputStream	defaultWriteObject () writeObject (Object) writeInt (int), writeFloat (float), writeXxx (xxx)
FileInputStream	new FileInputStream (stringFileName ) new FileInputStream (File)
ObjectInputStream	defaultReadObject () readObject () – returns Object, must be cast before use readInt (), readFloat (), readXxx ()

 

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