Here are my study notes for the Java SE 7 Programmer II to get the Oracle Certified Professional, Java SE 7 Programmer certification. This is based on the topics in Oracle published syllabus for the exam.

This page is a work in progress, so come back for more details as I go.

See also my Study Notes for Java SE 7 Programmer I OCA.

Note: I accept no liability for any inaccuracy or mistakes in this document. If you find any errors, please let me know so I can correct it!

Java Class Design

• Use access modifiers:
  • private
    • only accessible within class
  • protected
    • only accessible with package and subclasses
  • public
    • world accessible
  • default
    • package-private
    • accessible within package
• Override methods
  • Don’t override Object.finalize as it’s not guaranteed to run
• Overload constructors and methods
  • Overloaded method use the number and types of arguments to determine which version of the overloaded method to call
  • Return type are not used to determine which overloaded method to call.
  • If a class will be inherited, a no argument constructor should be supplied. This will be called when a subclass is instantiated.
• instanceof operator
  • Comparison between object and specified type.
  • Will return true when an object is:
    • An instance of the specified class
    • An instance of a subclass of the specified class
    • An instance of a class that implements the specified interface
  • Will return false when an object is:
    • An instance of a parent class of the specified class
    • A reference to null
  • Will not compile if comparing neither a subtype or supertype
    • Will produce compilation error: Unresolved compilation problem: Incompatible conditional operand types.
  • Often over-used and can be replaced by polymorphism
• Casting
  • Used with superclass reference types allow access to subclass properties and methods
  • Uses syntax (MyClass)myInstance.
• Virtual method invocation
  • Used when multiple subclasses override the same method
  • The object type’s implementation of the overriden method is used, not the reference type method
  • Important part of polymorphism
• Override methods from Object class:
  • hashCode
    • Must consistently return the same integer for the same object each time invoked, provided information used by equals method is not changed.
    • If two objects are not equal they should not return the same integer value
  • equals
    • Compares two objects
    • Must be reflexive, symmetric, transitive and consistent
    • Comparing to null should return false
  • toString
    • Must return a string that represents the object as text
    • Recommended that all subclasses override this method
• package statement
  • The package statement must be the first executable line of the source file, preceding any import statements.
  • Creates a package, put package statement with the name of the package at the top of every source file that contains types(classes, interfaces, enumerations and annotations) to be part of the package.
  • Without a package statement the type becomes part of an unnamed package.
• import statement
  • Import a package member using import package.Member;
  • Import an entire package by using import package.*;
    • Packages are not hierarchical, importing package.* will not import package.subpackage.*
    • Where members of two or more imported packages have the same name, the fully qualified member name must be used.
  • import static statement allows you to reference constants and static methods from a class without prefixing the class name.

Advanced Class Design

• Abstract classes
  • Cannot be instantiated but can be subclassed
  • May or may not include abstract methods
  • Subclass must provide implementations for all abstract methods of parent, otherwise must also be abstract.
• static keyword
  • Static methods must use object reference to access instance variables or methods.
  • Static methods cannot use this keyword.
  • Static methods can be hidden by a static method with the same signature
    • Differs from overriding because the version of the static method that is invoked depends on the reference type from which it is invoked
  • static keyword is used to declare a class variable, which is part of the class not the object instance
  • Class variables are shared between all instances of the class
• final keyword
  • Indicates that a field cannot change
  • Used with methods to prevent them being overriden
• Create top-level classes
  • Top level classes expected to be inherited by subclasses should have a default no-arg constructor.
  • The default no-arg constructor cannot be used if the class has any constructors defined.
• Create nested classes
  • Static Nested classes
    • Declared using static keyword
    • Do not have access to members of enclosing class
  • Non-static nested classes
    • Known as inner classes
    • Have access to members of enclosing class, including private members
    • Cannot define any static members
  • May be private, public, protected or package-private
• Enumerated types
  • Allows a variable to be set equal to one of a set of predefined constant values.
  • Is a class which extends java.lang.Enum.
  • Constructor must be package-private or private.
  • Java compiler adds a values() method at compile time which returns an array of values in the enum.

Object-Oriented Design Principles

• Interfaces
  • Declaring
  • Implementing
  • Extending
• Interface inheritance or class inheritance
  • Multiple interfaces can be implemented
  • Multiple unrelated classes can inherit interfaces.
• Applying
  • Cohesion
    • The degree to which the elements of a class belong together
    • Cohesion in a class is increased if the functions within a class have a lot in common and perform a small number of related tasks.
    • Advantages:
      • Reduced complexity
      • Increased maintainability
      • Increased reusability
  • Low-coupling
    • The degree to which each component uses and has knowledge of other components.
    • Advantages:
      • Components from a low-coupling system can be replaced with alternative implementations that provide the same functions.
      • Components may be implemented with different platforms, using different languages, operating systems or build environments.
  • IS-A
    • Can be identified by a class using the extends or implements keywords.
  • HAS-A relationship
    • A class uses HAS-A relationship by composition, by having an instance of another class and using it.
• Object composition principles
• Singleton design pattern
  • Only one instance of the object is created.
  • Possible implementations:
    • Class with a private constructor and instance created as a private static member and a static method that returns that instance.
    • An enum.
• Data Access Object(DAO) design pattern
  • Consists of:
    • A Data Access Object Interface
    • A Data Access Object concrete class that implements the DAO interface
    • A Model object, containing get/set methods for the data
• Factory pattern
  • Consists of:
    • 1 or more concrete classes that implement a common interface
    • A common interface
    • A factory class which returns a new instance of one of the concrete classes

 

Generics and Collections

• Generics allow the creation of classes, interfaces and methods where the type is specified as a parameter.
• Generic classes
  • Declared using <> symbols
• Diamond for type inference
  • Means that the new statement to instantiate the object and call the constructor doesn’t require repetition of the type(s) from the reference declaration.
  • The compiler infers the type from the declaration.
  • e.g. ArrayList<string> myList = new ArrayList<>();
• Raw types
  • A raw type is a generic class or interface without a type parameter supplied
  • A parameterized generic type may be assigned to it’s raw type
  • Assigning a raw type to a parameterized type will produce a warning
  • Using a raw type to invoke generic methods will produce a warning
• Collections using raw and generic types
• Wildcards
• Wrapper classes
  • The type argument of a generic cannot be a primative type, they must be object types
  • Always declare generics using object types.
  • Using a primative type will cause a compile error
• Autoboxing
  • Autoboxing will automatically convert a primative variable to the corresponding type wrapper.
  • e.g. an int value can be assigned to ArrayList<Integer> myVals
• Unboxing
  • Unboxing means that a type wrapper object value can be automatically converted to a primative value when assigning a value copied from the type wrapper object.
• Implementations of:
  • List
    • List interface members:
      • add(E e); – appends to list
      • add(int index, E e); – inserts into list at specified index
    • ArrayList
    • LinkedList
  • Set
  • Deque
  • Map
    • Sorted Maps
      • TreeMap
      • NavigableMap
    • HashTable
    • HashMap
• Use java.util.Comparator
• Use java.lang.Comparable
  • Must provide a method int compareTo(T o) which returns -1,  or 1.
• Sort and search arrays and lists
  • To be sorted in collections, the objects contained within must implement the Comparable interface.

String Processing

• Scanner
  • Constructor may take File, InputStream, Path, Readable, ReadableBytChannel and String
  • Can parse text into primitive types and strings using regular expressions
  • useDelimiter method accepts a regular expression as a Pattern or a String to use as a delimiter between fields in the text
  • has and next functions exist for each primitive type and some others
  • nextLine advances to the next text line and returns the current one as a String.
• StringTokenizer
  • Constructors:
    • StringTokenizer(String str) – uses default delimiters – space, tab, newline, carriage return, form feed.
    • StringTokenizer(String str, String delim) – Uses specified delimiter in delim
    • StringTokenizer(String str, String delim, boolean returnDelims) – if returnDelims is true, include the delimiters in the tokens
  • nextElement() returns the next token
  • nextToken() returns the next token or thorws NoSuchElementException
  • countToken() calculates the number of times nextToken can be called before an exception
  • hasMoreTokens() returns true if there is at least one more token after current token
• StringBuilder
  • Constructors:
    • StringBuilder() – creates a blank String Builder
    • StringBuilder(CharSequence seq) – creates a StringBuilder with the same characters as the seq
    • StringBuilder(int capacity) creates a StringBuilder with capacity set
    • StringBuilder(String str) creates a StringBuilder with the contents of the String
  • append(T x) methods exist for all primatives, CharSequence, String and StringBuffer
  • charAt(int index) returns a char from the specified index
  • delete(int start, int end) deletes characters from start to end.
  • insert(int offset, T x) methods exist to insert primatives at an offset
  • substring(int start) returns a String containing all characters from start to the end of the StringBuilder
  • substring(int start, int end) returns a String containing characters from start to end.
  • toString() returns the StringBuiler as a string.
• String
• Formatter
  • Interprets printf style format strings
  • Example: Formatter formatter = new Formatter(sb, Locale.US);
• String processing with regular expressions
  • Used by Pattern and Matcher classes.
  • PatternSyntaxException indicates that a syntax error has occurred in a regular expression pattern.
  • Syntax:
    • ^ – matches beginning of line
    • $ – matches end of line
    • . – matches any single character except newline
    • […] – matches any single character in brackets
    • [^…]- matches any single character not in brackets
    • \A – Beginning of entire string
    • \z – End of entire string
    • \Z – End of entire string except final line terminator
    • a|b – matches either a or b
    • \w – matches word characters
    • \W – matches nonword characters
    • \s – matches whitespace
    • \S – matches non whitespace
    • \d – matches digits, same as [0-9]
    • \D – matches nondigits
    • \n – matches newlines
    • \t – matches tabs
• Format strings
  • See Formatter javadoc
  • %n is used for platform appropriate new line
  • %d is used for a decimal integer
  • %f is use for float
  • a leading 0 after %d indicates use of leading zeroes
  • a number flag indicates characters in width, e.g. 10.3 is ten characters wide with three places after decimal point
  • + indicates use sign, positive or negative
  • , indicates locale specific grouping characters
  • – indicates left justified
  • defaults to right justified
  • To use a % sign in a formatted number for instance for a percentage, use %%.

Exceptions and Assertions

• Each try statement requires at least one catch or a finally block.
• catch statement
  • Once one catch statement executes the otheres are bypassed.
  • Exception subclasses must come before their superclasses. Otherwise an unreachable code error will occur at compile time.
• throw statement
  • thrown object must be of type Thowable or a subclass of it.
  • Execution immediately stops and control is passed to the nearest matching catch statement.
• throws statement
  • Lists the exception types the method may throw.
  • Required for all thrown exceptions except Error or RuntimeException and their subclasses.
• Multiple Exception types in single catch block
  • Called multi-catch
  • Use pipe symbol
  • e.g. catch(FileNotFoundException | IOException e)
• try-with-resources statements
• finally statement
  • Executes regardless of whether or not an exceptions has been thrown.
  • If exception has been thrown executes after the relevant catch block.
• Custom exceptions
• Test invariants by using assertions

Java I/O Fundamentals

• Read and write data from the console
  • Get a Console object with System.console() method.
  • System.console().readPassword() retrieves user input without echoing typed characters.
  • The Java platform supports three standard streams:
    • Standard input – stdin – using System.in
    • Standard output – stdout – using System.out
      • System.setOut may be use to redirect output to a different stream
    • Standard error – stderr – using System.err
  • Can be read using BufferedReader or Scanner
• Using streams to read and write files with:
  • BufferedReader
    • Implements Closeable, AutoCloseable, Readable
    • read() reads a single character – returns -1 if end of stream is reached
    • ready() returns true or false
    • skip(long n) skip n characters
    • Methods throw IOException
  • BufferedWriter
  • File
  • FileReader
  • FileWriter
    • Implements Closeable, Flushable, Appendable, AutoCloseable
    • Defaults to overwrite an existing file use FileWriter(File file, boolean append) constructor with append set to true to append to an existing file.
    • Inherits methods from Writer.
  • RandomAccessFile
    • Implements DataOutput, DataInput, Closeable
    • readUTF reads the file into a String
    • writeUTF writes a String to the file
    • Maintains a cursor also known as a current file pointer marking the current location in the file.
    • seek(long pos) sets the cursor location/file pointer, measured in bytes from beginning of file.
    • read and write methods are provided for all primatives
    • readLine reads bytes from the file, starting from current file pointer and ending at the end of the current line. Line terminating characters are not included.
  • FileInputStream
    • Implements Closeable, AutoCloseable
    • read() reads a single byte or -1 if end of file
    • read(byte[] b]) reads b.length bytes of data into an array of bytes
    • read(byte[] b, int off, int len) reads len bytes starting at offset off into an array of bytes
    • skip(long n) skips n bytes
  • DataInputStream
  • DataOutputStream
  • ObjectOutputStream
  • ObjectInputStream
  • PrintWriter
• Serializable interface
  • Enables a class to be serialized
  • Always include a serialVersionUID field
  • static and transient fields are not serialized
  • Superclass fields are only serialized if the superclass also implements Serializable

Java File I/O (NIO.2)

• Path class
  • Paths.get returns a Path object
  • resolve(Path p) appends a relative path p to this path. If p is an absolute path then p is returned.
  • relativize(Path p) returns a relative path from this path to path p
  • getName(int index) returns the name of the component of the path at that index
  • getRoot() returns root component
• Files class
  • Atomic operations
    • An atomic file operation cannot be interrupted or partially completed.
    • Entire operation is completed or fails.
  • move method supports atomic moves and replacing existing files by means of options provided
• Interfaces:
  • BasicFileAttributes
    • creationTime() returns creation time as a FileTime object
    • fileKey() returns an object that uniquely identifies the file
    • isDirectory() returns true if the file is a director, false otherwise
    • isOther() returns true if the file not a regular file, directory or symbolic link
    • isRegularFile() returns true if the file is a regular file
    • isSymbolicLink() returns true if the file is a symbolic link
    • lastAccessTime() returns the last access time as a FileTime object
    • lastModifiedTime() returns the last modification time as a FileTime object
  • BasicFileAttributeView
    • readAttributes() method returns a BasicFileAttributes object
    • setTimes() allows any or all of a file’s last modified time, last access time and create times to be updated.
  • DosFileAttributes
    • Provides ability to read legacy MS DOS file attributes
    • Provides isArchive(), isHidden(), isReadOnly, isSystem() attributes
  • DosFileAttributeView
    • Provides readAttributes which returns a DosFileAttributes object
    • Provides setArchive(boolean value), setHidden(boolean value), setReadOnly(boolean value), setSystem(boolean value)
  • PosixFileAttributes
    • Provides ability to read file attributes from POSIX operating system file systems. Typically these are Unix, Linux or Mac OS X systems.
  • DirectoryStream
  • FileVisitor
  • PathMatcher
    • Instantiate with for example:
      • PathMatcher matcher = Filesystems.getDefault().getPathMatcher(“glob:*.{txt,doc}”);
    • Uses glob patterns
  • WatchService
    • Watches registered objects for changes and events.
    • May be used to monitor directories or files for changes.
    • If the file system does not support file change notifications the API will poll the file system for changes.

Building Database Applications with JDBC

• JDBC Drivers must include Statement, ResultSet and Connection objects
• JDBC Driver Types:
  • Type 1
    • Not pure Java
    • Require native libraries
    • Use bridging to data source
  • Type 4
    • Are pure Java
    • Use direct connections to data source
    • Can use network protocols
• Core interfaces:
  • Driver
    • acceptsURL(String url) method checks whether the driver thinks it can open a connection to the url.
    • connect(String url) is the most basic way to start a connection
    • connect(String url, Properties info) method tries to start a connection
    • connect(String url, String user, String password) also tries to start a connection
    • url is in format: jdbc:subprotocol:subname
    • getMajorVersion() and getMinorVersion() get the major and minor version numbers respectively
    • SQLException is thrown for most database access errors.
    • See Javadoc for other methods.
  • Connection
    • Connections are AutoCloseable
    • Use DriverManager.getConnection() and DataSource.getConnection() to establish a connection using a JDBC driver
    • Use setAutoCommit(false) to turn off auto commiting of updates
    • Use commit() to commit updates when auto committing is off.
  • Statement
    • Statements default to being commited automatically
  • ResultSet
    • Maintains a pointer/cursor to the current row.
    • next method moves to next row, but returns false if there a no more rows.
    • Current row pointer is initially set before the first row, so will require a next to reach the first row.
    • By default is only cursor only moves forward
    • By default ResultSet is not updateable
    • A ResultSet may be set to be updateable and scrollable.
• Connect to database using DriverManager class
• Submit queries and read results
• Use JDBC transactions
  • Disable auto-commit mode using setAutoCommit(false);
  • Commit changes using commit();
  • Rollback changes rollback() if a SQLException is thrown.
  • setSavepoint() sets a Savepoint within the current transaction. Use rollback(Savepoint s) to rollback to that savepoint.
  • Disable auto-commit only during transaction mode.
  • Savepoint interface
    • Represents a point that can be returned to by the Connection.rollback method.
    • releaseSavepoint(Savepoint s) removes the Savepoint from the current transaction.
• Construct and Use RowSet objects with RowSetProvider class and RowSetFactory interface
  • Get a RowSetFactory using RowSetFactory factory = RowSetProvider.newFactory();
  • Get a JdbcRowSet using JdbcRowSet rs = factory.createJdbcRowSet();
  • A JdbcRowSet can also be obtained by using JdbcRowSet rs = new JdbcRowSetImpl();
• CachedRowSet interfaces
  • These are disconnected RowSets – connected only when reading data and propagating changes to the data source
  • WebRowSet interface
    • Allows a RowSet to be read or written in XML format.
  • FilteredRowSet interface
    • Allows disconnected filtering of a RowSet using a Predicate object
• Statement object
  • Supports only simple SQL statements without parameters
  • Support only a single result set open
• PreparedStatement object
  • Supports only input parameters
• CallableStatement object
  • Supports input and output parameters because they contain stored procedures

Threads

• Create and use Thread class and Runnable interface
  • Implementing Runnable
    • Requires a method call run()
      • e.g. public void run()
    • Must have an object of type Thread and instantiate it
    • Thread constructors:
      • Thread(Runnable threadOb, String threadName)
    • Use start method of the Thread object to start the thread running.
  • Extending Thread
    • Must override run() method
    • Must call start() to begin execution of thread.
    • A thread may only be started once.
    • A thread may not be restarted after completion.
• Manage and control thread lifecycle
  • Threads may be paused in execution using Thead.sleep(long milliseconds) which throws an InterruptedException
  • isAlive() method returns true if the thread is still running
  • join() method waits for a thread to finish execution
• Synchronise thread access to shared data
• Identify code that may not execute correctly in a multi-threaded environment

Concurrency

• Collections from java.util.concurrent package
  • BlockingQueue provides a first-in-first-out queue data structure
  • ConcurrentMap interface has atomic operations to remove or replace a key-value pair if it’s present or add a key-value pair if it is absent. Has implementation ConcurrentHashMap.
    • ConcurrentHashMap is similar to HashMap
    • ConcurrentNavigableMap is sub-interface supporting approximate matches.
      • Standard implementation is ConcurrentSkipListMap which is similar to TreeMap.
  • These collections try to avoid Memory Consistency Errors using a happens-before relationship.
• Use
  • Lock interface
    • lock() acquires the lock
    • unlock() releases the lock
  • ReadWriteLock interface
    • Separate locks for reading and writing
    • Read lock may be held by multiple reader threads at the same time if there are no writers
    • Write lock may only be held by one thread
  • ReentrantLock
    • Mutual exclusion lock – both reading and writing are locked.
    • lock() returns immediately if the lock is acquired, disables thread and waits if lock is not available.
    • Optional constructor Boolean fairness parameter, when set to true locks favour the longest-waiting thread.
  • Executor interface
    • execute(Runnable r) method creates a thread of Runnable object r.
    • May use a worker thread or put Runnable object r in a queue waiting for a worker thread.
  • ExecutorService subinterface of Executor
    • Provides features to manage lifecycle of tasks and Executor
  • Executors
    • Factory and utility methods for Executor, ExecutorService, ScheduledExecutorService, ThreadFactory, Callable.
    • Provides methods for creating a thread pool: newCachedThreadPool, newFixedThreadPool, newScheduledThreadPool, newSingleThreadExecutor, newSinglethreadScheduledExecutor, newWorkStealingPool.
    • Allow separation of thread management and creation from the rest of application.
  • Callable interface
    • Implementations must define a method call with no arguments that returns a result of a generic type. May thrown an exception if unable to produce a result.
  • Future interface
    • Represents the result of an asynchronous computation.
    • Provides method to check for completion, to wait and to retrieve results.
• Use parallel Fork/Join Framework
  • Implementation of ExecutorService interface
  • Uses work-stealing algorithm
  • Extend ForkJoinTask or a more specialised subclass such as RecursiveTask or RecursiveAction
  • RecursiveTask returns a result.
  • RecursiveAction does not return a result.

Localisation

• Read and set locale with Locale object
  • Use Locale constructors
  • Use constants in Locale class
  • Use Locale.forLangaugeTag factory method
  • Locale.Builder class
    • Create a Locale variable with the Locale.Builder class: Locale loc = Locale.Builder().setLangauge(“en”).setScript(“Latn”).setRegion(“AU”).build();
    • build method must be called to create a Locale object withtthe Locale.Builder class.
• Build a resource bundle for each locale
  • Resource bundles stored in properties files using name=value pairs
  • Properties files must be in the CLASSPATH
  • There must be an individual properties file for each language supported
• Call a resource bundle from an application
  • Identified by family name/base name and may have a two character language code.
  • e.g. Sample_en_AU where en is the language code and AU is the country code for Australia
  • Create a local object for the language, country and platform appropriate
  • Load bundle using
    • Locale myLocale = new Locale(“fr”, “CA”, “UNIX”)
    • ResourceBundle myBundle = ResourceBundle.getBundle(“Sample”, myLocale);
    • will search for classes in this order:
      1. Sample_fr_CA_UNIX
      2. Sample_fr_CA
      3. Sample_fr
      4. Sample_en_US
      5. Sample_en
      6. Sample
  • Resource bundle may be stored in classes extending ListResourceBundle, PropertyResourceBundle or a properties file.
• Format dates, numbers and currency values with NumberFormat and DateFormat classes
  • Use DateFormat myDateFormat = DateFormat.getDateInstance(DateFormat.DEFAULT, Locale.US); to format a date to US format.
• Advantages of localising an application
• Define a locale with language and country codes