SCJP 5.0 Study Notes

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 compareTo(SpecificObjectType) returns negative int if this < Obj2 0 if Objects are equal positive int if this > Obj2
Comparator class	int compare(SpecificObjectType1, SpecificObjectType2) – 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()

Serialization Notes

· For custom serialization, must have methods (EXACTLY as shown except parameter names):

--private void writeObject(ObjectOutputStream oos), call oos.defaultWriteObject() before trying to do anything else in method

--private void readObject(ObjectInputStream ois), call ois.defaultReadObject() before trying to do anything else in method

· If a superclass implements Serializable all subclasses do automatically

· If an object within a object to be serialized is not serializable, code will compile, but a RuntimeException will be thrown

· An object is serializable even if superclass is not. However, the first superclass in the hierarchy that does not implement Serializable must have a no-arg constructor (that will get called). If this is violated, readObject() will produce a java.io.InvalidClassException

· The (no-arg?) contructor of every non-serializable superclass will run when an object is deserialized. However, the deserialized objects’ constructor does not run when it is deserialized.

· Deserialization order is FIFO

· Only member (instance) variables are serialized

· Static and transient member variables are not serialized

· If a non-serializable element is present in a class you are trying to serialize, the code will compile but there will be a runtime exception upon attempt to serialize

· Filter Streams – another name for streams that wrap (get chained to) other streams

IO Stuff (java.io package)

File – can be file or directory	new File(pathNameAndFileString) new File(pathNameDirString, childPathNameString), new File(File pathname, fileORDirFileString) createNewFile() – creates file if doesn’t yet exist, links to existing file, actually creates file on hard drive mkdir() exists() – returns boolean isDirectory(), isFile() – returns boolean delete() – can only delete directories if they are empty list(), list(FilenameFilter) – returns String array renameTo(File) – directory does not have to be empty to rename listFiles(), listFiles(FilenameFilter) – returns File array getName() – returns String getPath() canRead() NO method to change working directory
FileReader	read() No method for reading line at a time, almost always wrapped with BufferedReader
BufferedReader	read(), readline() Usually wraps around FileReader, No method new BufferredReader(File)
FileWriter	new FileWriter(File) new FileWriter(File, appendToEndBoolean) – instead of overwriting new FileWriter(fileNameString) new FileWriter(fileNameString, appendToEndBoolean) – no overwrite write() flush() close() Almost always wrapped by (chained to) BufferedWriter
BufferedWriter	write() newline() flush() close() Usually wraps around FileWriter
PrintWriter	newPrintWriter(File), new PrintWriter(stringFileName), new PrintWriter(Writer), new PrintWriter(OutputStream) write() format()/printf() print()/println() flush(), close() Often wraps around BufferedWriter: PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("foo.out"))); will buffer the PrintWriter's output to the file
FilenameFiler interface	accepts(directoryFile, nameString) – returns boolean indicating whether to file passes through filter or not used by File’s list(FilenameFilter) and listFiles(FilenameFilter) methods
DataInputStream/ DataOutputStream	Not on exam despite what Sun objectives say– don’t study

Other APIs in java.lang

Thread	new Thread(), new Thread(stringName), new Thread(runnableTarget), new Thread(runnableTarget, stringName) Thread.MIN_PRIORITY – usually 1 Thread.NORM_PRIORITY – usually 5 Thread.MAX_PRIORITY – usually 10 run() – can be overridden to make new thread, but not recommended start() – causes thread to move to alive state (available to run) isAlive() – returns boolean Thread.sleep(long millis) – throws InterreptedException join(), join(longMaxTimeToWaitInMillis) – throws InterruptedException Thread.yield() – usually shouldn’t be used, but should return current thread back to runnable thread pool if other threads with same or higher priority exist setName(String) Thread.currentThread().getName() setPriority(int1-10) int getPriority() getState() boolean holdsLock()
Object	wait(), wait(longMaxTimeToWaitInMillis), wait(longMaxTimeToWaitInMillis, intNanosMoreToWait) – throws InterruptedException notify() notifyAll() public boolean equals(mustBeObjectNotSubtype) public int hashCode() public String toString()
java.lang.Math	Math.random() Math.round() Math.ceil() -- Returns the smallest (closest to negative infinity) double value that is greater than or equal to the argument and is equal to a mathematical integer. Math.floor() -- Returns the largest (closest to positive infinity) double value that is less than or equal to the argument and is equal to a mathematical integer. Math.round() Math.sqrt() Math.max(number, number) Math.min(number, number)
java.lang.Enum	ordinal() – prints position (zero-based), similar to index for an array e.g. enumName.ordinal(); Enums in case statement should NOT use class prefix, but everywhere else should have prefix (e.g. Grade.A), unless enum is in the same class
java.lang.System	System.out.println() System.out.print() System.out.format/printf(“string with %below”, argsToString) System.exit(intStatus) System.getenv() – returns Map of all environment variables System.getenv(stringEnvironmentVariable) – returns value for specified environment variable System.getProperty(stringKey) System.getProperties() – returns Property object (subclass of Hashtable) containing all properties System.setProperties(stringKey, stringValue) System.setProperties(PropertyObject) System.gc() – same as Runtime.getRuntime().gc() – requests (but may not obtain) garbage collection For Formatting with format()/printf() %[arg_index$][flags][width][.precision]conversionCharacter -,+,0,,,( b,c,d,f,s,S - Left justify b boolean + Include sign c char 0 Pad with zeros d integer (digit) , Use locale-specific grouping separators f float ( negatives in parentheses s string S CAPITALIZE the string %illigitChar results in java.util.UnknownFormatConversionException Wrong format results in IllegalFormatConversionException Can have more arguments than conversions, but more conversions than arguments results in java.util.Missing FormatArgumentException You can’t use an int literal argument for %f

Primitives, WrappersAssignments, Autoboxing

· new XXX(String), new XXX(primitive) -- wrapper class constructors all take corresponding primitives or strings as arguments (e.g. new Long(“12”) or new Long(12))

· XXX.valueof(String), XXX.valueOf(String, intRadixBase) – static method returns XXX wrapper type

· parseXxx(String) – static method returns primitive of type Xxx

· xxxValue -- converts wrapper to any primitive xxx type, truncates and does NOT round

· table on p. 233

· integer literals are always interpreted as int types by the JVM, number literals with decimals are interpreted as doubles

· byte – 8 bits, short – 16 bits, int – 32 bits, long – 64 bits, float – 32 bits, double – 64 bits, char – 16 bits (but can’t fit into a short because it has a bigger positive range than short since there are no negatives)

· no static member can use a generic type parameter declared by the enclosing class

· Calling run() method will not create a separate thread, and Thread.currentThread().getName() of the thread in run() will be “main”

· Variables and classes can’t be synchronized, but instance methods, static methods, and any code block within a method (enclose in {}) can be synchronized

· Conditional operators (==, <, >) have higher precedence than assignment operators, so “if (b = x == y)” can be a legal statement

· Wrappers with values over 127 are unique, so “new Integer(130) == new Integer(130)” returns false, but “new Integer(120)” == new Integer(120)” returns true

· WrapperClass.equals(AnyOtherClass) = false, String.equals(anyNonString) = false, StringBuffer.equals(AnyOtherObjectRef) = false (StringBuffer does not override equals method)

· 0 % 4 = 0

· new Boolean(“anyMessage”); will create a Boolean with false

· Trying to unbox a null will result in a NullPointerException

· if there are multiple ++x incrementors in a compound expression, each is incremented before that part of the expression?

· For Boolean bool = new Boolean(“TRUE”), in both if (bool) and if (bool == true), bool is unboxed automatically

· Number, StringBuffer/StringBuilder DO NOT override equals() and hashCode() and inherit these from Object (so .equals() for different objects with the same values evaluates to false)

· For array instantiation, array size must be specified (INCORRECT – in myArray[] = new int[])

· Vararg methods CAN be invoked with 0 or more arguments

· Integer can be boxed to int and vice-versa