Open Education – Page 217

Jul 16

SCJP Training: Lesson 2: Flow Control #Java Short Notes #SCJP

By Sayed in FromSitesTree.com, SCJP/OCJP

From: http://sitestree.com/?p=5049
Categories:Java Short Notes, SCJP
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Post Data:2007-05-18 16:05:29

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Jul 16

SCJP Training: Lesson 1: Develop code that declares classes (including abstract and all forms of nested classes), interfaces, and enums, and includes the appropriate use of package and import statements (including static imports). #Java Short Notes #SCJP

By Sayed in FromSitesTree.com, SCJP/OCJP

From: http://sitestree.com/?p=5047
Categories:Java Short Notes, SCJP
Tags:
Post Data:2012-06-10 07:14:48

Jul 16

SCJP Essential Knowledge #Java Short Notes #SCJP

By Sayed in FromSitesTree.com, SCJP/OCJP

int x=5; String y=”3″; System.out.print(x + 1 + y); Output: 63
When there is no live reference to an object, the object becomes eligible for garbage collection.
The concrete interface method implementation must be public.
When a class implements Comparator, it must implement a compare method.
When a class implements Comparable, it must implement a compareTo method.
The Arrays.asList() method creates a fixed-size list
A method like void go(Set a) can be called with objects created as: TreeSet t = new TreeSet(); TreeSet t = new TreeSet();. Remember, TreeSet t = new TreeSet(); TreeSet t = new TreeSet(); will not work where Dog is a sub-class of Animal.
When two enums are equal, both .equals and == always return true.
TreeSet is ordered in their natural order or according to the Comparator provided at creation.
SortedMaps are abstract
Is-a relationships can use inheritance
Tightly encapsulated classes can have a has-a relationship
is-a or has-a relationships does not apply to methods

java command in details:

Usage: java [-options] class [args...]           (to execute a class)   or  java [-options] -jar jarfile [args...]           (to execute a jar file)where options include:    -client	  to select the "client" VM    -server	  to select the "server" VM    -hotspot	  is a synonym for the "client" VM  [deprecated]                  The default VM is client.                      -cp     -classpath                   A ; separated list of directories, JAR archives,                  and ZIP archives to search for class files.    -D=                  set a system property    -verbose[:class|gc|jni]                  enable verbose output    -version      print product version and exit    -version:                  require the specified version to run    -showversion  print product version and continue    -jre-restrict-search | -jre-no-restrict-search                  include/exclude user private JREs in the version search    -? -help      print this help message    -X            print help on non-standard options    -ea[:...|:]    -enableassertions[:...|:]                  enable assertions    -da[:...|:]    -disableassertions[:...|:]                  disable assertions    -esa | -enablesystemassertions                  enable system assertions    -dsa | -disablesystemassertions                  disable system assertions    -agentlib:[=]                  load native agent library , e.g. -agentlib:hprof                    see also, -agentlib:jdwp=help and -agentlib:hprof=help    -agentpath:[=]                  load native agent library by full pathname    -javaagent:[=]                  load Java programming language agent, see java.lang.instrument

From: http://sitestree.com/?p=4893
Categories:Java Short Notes, SCJP
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Post Data:2008-03-02 15:26:47

Jul 16

SCJP Practice Exams #Java Short Notes #SCJP

By Sayed in FromSitesTree.com, SCJP/OCJP

Check these practice exams. Be careful that many java rules may have changed over the time. Check that which jdk version or the exam these practice exams support

From: http://sitestree.com/?p=4892
Categories:Java Short Notes, SCJP
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Post Data:2007-11-16 02:44:22

Jul 16

SCJP: More Rules #Java Short Notes #SCJP

By Sayed in FromSitesTree.com, SCJP/OCJP

java -classpath gFolder/Game.jar civilization.java: In such command, -classpath will override (replace) CLASSPATH environment variable.
java -classpath gFolder/Game.jar civilization.java: if both gFolder and current directory contain Game.jar then the jar file under gFolder will be used.
If you want java compiler to recognize your jar file, either you have to mention the location of the jar file in javac command using -classpath switch or you have to add the jar file in the CLASSPATH environment variable. Mentionable – you need to add the file, adding upto the directory will not work.
Garbage collection: can Java program run out of memory? yes.
Can objects be garbage collected even if it has a valid reference? yes. when no live thread has access to the object.
Can Objects created within inner classes be eligible for garbage collection.? yes.
Do garbage collector deletes objects from the stack? No. As objects reside in heaps not in the stack.
If an object’s finalize method runs to completion will it always be garbage collected? No. The finalize method may create some other references.
Arrays.equals(Object[] a1, Object[] a2) : Returns true if the two specified arrays of Objects are equal to one another
The results of Binary search on an unsorted array are undefined
Alpha go(int i) is a valid method declaration with a generic return type. class Alpha{}

From: http://sitestree.com/?p=4891
Categories:Java Short Notes, SCJP
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Post Data:2010-06-26 02:46:16

Jul 16

SCJP: Short Notes #Java Short Notes #SCJP

By Sayed in FromSitesTree.com, SCJP/OCJP

For the package

package com.sun2;public enum Seasons {SUMMER, FALL, WINTER, SPRING}

Valid import statements are:

import com.sun2.Seasons; // the class import static com.sun2.Seasons.*; //all enum valuesimport static com.sun2.Seasons.FALL; //only one enum value

An interface can extend many interfaces
Interfaces can have variables, overrides and overloads
An enum can have methods and can override those methods for a specific enum constant
var-args can be used to pass arrays, var args can be of 0 or more lengths
A var-args argument must be a method’s last argument
An override cannot change the return type. Override can use covariant as a return type
Overloading does not depend on the return type
From J2se 5.0, a method in a subclass may return an object whose type is a subclass of the type returned by the method with the same signature in the superclass
When a subclass constructor does not explicitly invoke a superclass constructor, the no-argument constructor of the superclass is automatically called from the subclass constructor. If the super class does not have a no-argument constructor, a compile-time error is displayed
When subclasses override superclass methods, these superclass methods can be accesses using the super keyword (from subclass methods)
From java 1.5, enums can be used in a switch
To enable asserts in java program use -ea switch.
assert has two forms: assert Expression1 ;, assert Expression1 : Expression2 ;. In the first form only AssertionErorr is displayed, in the second form you can add additional message(Expression2) to the AssertionError. Assert when finds its arguments to be false, it throws AssertionError
Good places to use assertions: Internal Invariants, Control-Flow Invariants, Preconditions, Postconditions, and Class Invariants
bad use of assertions: argument checking in public methods, to do any work that your application requires for correct operation
Assertions should be free of side effects: evaluating the expression should not affect any state that is visible after the evaluation is complete
An overriding method does not need to throw the overridden method’s exception
Nesting try/catches are ok and normal flow rules apply in case of nesting.
Accessing uninitialized instance/class wrapper variables (Integer, Short) will cause NullPointerException
NumberFormatException extends IllegalArgumentException
JVM when calling a method, to make data types compatible, will widen the parameters before boxing.
java.io.Console class: Only a single instance exists for a given JVM, All of the methods that read user input are synchronized
Among File, FileReader, BufferedReader only BufferedReader has a readLine method
A class’s superclasses don’t have to implement Serializable in order to be serialized
if a superclass doesn’t implement Serializable then it’s constructor will run during deserialization.
A transient variable’s state is lost during serialization
A volatile variable’s state is not lost during serialization
If Dog has-a Collar, then Collar must implement java.io.Serializable in order to serialize an instance of Dog.
If Dog extends Animal and Animal implements java.io.Serializable but Dog does NOT implement java.io.Serializable, you can serialize an instance of Dog.
Serialization rules: # Rule #1: The object to be persisted must implement the Serializable interface or inherit that implementation from its object hierarchy,# Rule #2: The object to be persisted must mark all nonserializable fields transient
To make thread instance unserializable: transient private Thread animator;
java.lang.Object does not implement Serializable. AWT and Swing GUI components, strings, and arrays — are serializable
Serialization Details
NumberFormat, Calendar, and DateFormat are abstract. Use the getInstance method to get instances.
For java.util.Scanner, the default separator is a blank NOT a comma
Multiple threads can be created using the same Runnable object, but a given thread can be started only once
Two invocations of synchronized methods on the same object to interleave – is not possible. When one thread is executing a synchronized method for an object, all other threads that invoke synchronized methods for the same object block (suspend execution) until the first thread is done with the object. When a synchronized method exits, it automatically establishes a happens-before relationship with any subsequent invocation of a synchronized method for the same object. This guarantees that changes to the state of the object are visible to all threads.
In object oriented design, low coupling is more desirable than high coupling. High cohesion is more desirable than low cohesion. Coupling: class interdependence. Cohesion: Class activities should be well focused.
Polymorphism does NOT apply to static methods
Polymorphic calls and co-variant returns can be mixed
no modifier (package private) is weaker than protected. The order is: public, protected, package private, private
Kind of’ translates to extends, ‘contract’ translates to implements, and ‘composed’ translates to a has-a implementation.
TreeMap: headMap() method returns keys that are less than the argument, and tailMap() returns keys that are greater than(>=) the argument.

From: http://sitestree.com/?p=4888
Categories:Java Short Notes, SCJP
Tags:
Post Data:2011-10-20 04:13:02

Jul 16

SCJP: Rules #Java Short Notes #SCJP

By Sayed in FromSitesTree.com, SCJP/OCJP

A class’s superclasses don’t have to implement Serializable in order to be serialized
if a superclass doesn’t implement Serializable then it’s constructor will run during deserialization
A transient variable’s state is lost during serialization, but a volatile variable’s state is not lost
Java:Volatile variable
Transient Variable
NumberFormat, Calendar, DateFormat are abstract classes. Use the getInstance method to get NumberFormat instances.
The + quantifier in a regular expression indicates 1 or more occurrences, * indicates 0 or more, [] just one character in the group, () indicates a whole group match.
The default separator in Scanner class is a blank NOT a comma
Multiple threads can be created using the same Runnable object, but a given thread can be started only once.
Thread.yield(): Causes the currently executing thread object to temporarily pause and allow other threads to execute.
Thread.join(): Waits for this thread to die.
Wait(), Notify(), NotifyAll() methods
Low coupling: classes know the least possible about each other, is preferable over tight coupling.
High cohesion: Each class has well focused responsibilities. Is preferable over low cohesion.
Polymorphism does NOT apply to static methods.
Instance variable cannot be referenced from static method

From: http://sitestree.com/?p=4883
Categories:Java Short Notes, SCJP
Tags:
Post Data:2011-09-04 06:32:26

Jul 16

SCJP: Java Concurrency #Java Short Notes #SCJP

By Sayed in FromSitesTree.com, SCJP/OCJP

Java High Level Concurrency Objects
constructors cannot be synchronized
Liveness of multi-threaded applications
Immutable Objects
An application may have one or more processes. A process may have one or more threads.
Two ways to create threads
Thread.sleep(4000); To pause a thread. The time like 4000 ms is really dependent on the os and not precise
Thread Interrupts
Thread Join
Regular Expression
Regular expression – meta characters: ([{^-$|]})?*+.
Character Classes for Regular Expressions
Pattern Class
Matcher Class
System.getenv(): get environment variables in a hashmap.
System.getProperty(“path.separator”); System.setProperties : read and write system properties.
How to use Properties to keep application properties
Access SecurityManager: SecurityManager appsm = System.getSecurityManager();

From: http://sitestree.com/?p=4879
Categories:Java Short Notes, SCJP
Tags:
Post Data:2009-04-19 03:15:05

Jul 16

SCJP: Basic Java I/O #Java Short Notes #SCJP

By Sayed in FromSitesTree.com, SCJP/OCJP

ByteStream is the basic I/O stream. Handles data as a stream of bytes. Does operation with byte unit and uses 8 bit. FileInputStream, FileOutputStream – can be used to copy files as byte by byte.
Character Streams: FileReader and FileWriter are character streams. They treat file data as 16 bit unicode charater streams.
InputStreamReader, OutputStreamWriter are also character streams. May be used in socket data reading.
Line-Oriented I/O: BufferedReader and PrintWriter
Buffered Streams: BufferedInputStream and BufferedOutputStream create buffered byte streams, while BufferedReader and BufferedWriter create buffered character streams
You can set autoflush() of buffered streams or call manually flush() method.
The scanner API breaks input into individual tokens. The formatting API assembles data into nicely formatted, human-readable form.

Scanner:

Scanner scanner = new Scanner(new BufferedReader(new FileReader("test.txt")));            while (s.hasNext()) {                System.out.println(s.next());            }

To create formatted output streams, instantiate PrintWriter not PrintStream
Check: System.out.format(“The square root of %d is %f.%n”, i, r);
System.out and System.err are PrintStream objects. PrintStream is really a byte stream but uses some mechanism to emulate character stream.
InputStreamReader cin = new InputStreamReader(System.in); System.in is a byte stream. InputStreamReader is used to emulate a character stream.
System.console provides character streams to handle console read/write operations. Provides readPassword method to read password from the console
Data Streams: Support I/O operations of primitive data types. DataInputStream and DataOutputStream.

Check:

out = new DataOutputStream(new            BufferedOutputStream(new FileOutputStream(dataFile)));out.writeDouble(prices);out.writeInt(units);out.writeUTF(descs);

DataStreams detects an end-of-file condition by catching EOFException
Correct type to represent currency values: java.math.BigDecimal – an object type
Object Streams: ObjectInputStream and ObjectOutputStream. Use writeObject and readObject to write and read objects respectively.
A stream can contain only one copy of an object but many references to it when required.
File Objects: File I/O

Random access file:

new RandomAccessFile("test.txt", "r");new RandomAccessFile("test.txt", "rw");RandomAccessFile methods:    * int skipBytes(int)     * void seek(long)      * long getFilePointer()

The new java.nio.* package provides supports to handle file i/o for high performance applications.

From: http://sitestree.com/?p=4878
Categories:Java Short Notes, SCJP
Tags:
Post Data:2012-10-24 02:25:07

Jul 16

Java : SCJP: Important Resources #Java Short Notes #SCJP

By Sayed in FromSitesTree.com, SCJP/OCJP

How to use generics to avoid runtime errors.
More Generics
Class casting in Java: How to avoid runtime exception – ClassCastException: overloading, overriding, variable and method hiding
Java HotSpot virtual machine
What Java Technology can do?
Offers from Java Technology
Java:Common Problems (and Their Solutions)
Benefits of OOP: Modularity, Information-hiding, Code re-use, Pluggability and debugging ease
Object oriented concepts in Java
Variable Naming Conventions in Java
Java Primitive Data Types
Java : Arrays: System.arraycopy method
Java Operators: Carefully look about precedence
continue and break statements in Java
Annotations in Java
Passing Parameters, System.out.printf, Shadowing Variables
The Garbage Collector
You can use interface names as return types. The object returned must implement the specified interface.
when a class name is used as a return type, the returned object must be either a subclass of, or the exact class of, the return type
Controlling Access to Members of a Class
Java initializing variables: initialize at declarations, Static Initialization Blocks, private static method to initialize, Initializer blocks for instance variables, A final method for initialization
Nested Classes
enum in Java: enums implicitly extend java.lang.Enum. An enum cannot extend anything else. Enums can have methods like classes
if a variable is declared to be the type of an interface, its value can reference any object that is instantiated from any class that implements the interface
Interfaces provide an alternative to multiple inheritance
Java Interfaces
Rewriting Interfaces
Covariant return type: An overriding method can return a subtype of the type returned by the overridden method
The access modifier for an overriding method can allow more, but not less, access than the overridden method
Hiding fields makes code difficult to read
java.lang.Object as a super class
final methods can not be overridden, final classes can not be sub-classed, methods called from constructors should generally be declared final (otherwise, a different implementation of the method in subclasses may create unwanted results)
Interfaces can not have static or final methods
System.out.format(…..) and String.format() can be used as alternatives to System.out.println()
Using Number class vs. primitive data types
String, StringBuilder, StringBuffer: StringBuilder has capacity(), reverse() methods that String does not have. StringBuffer = StringBuilder but StringBuffer is thread-safe i.e. synchronized. String, and StringBuilder can be converted to each other
Three kinds of exceptions
Chained Exceptions: Stack Trace: Logging

From: http://sitestree.com/?p=4876
Categories:Java Short Notes, SCJP
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Post Data:2008-03-19 18:31:53

ABC Learning Design: A Practical Guide to the Arena Blended Connected (ABC) Toolkit

SMTP: Purpose, Installation, Steps, Data Format

Which ETF performed similarly to Bitcoin or closely? What performed the closet?

If I were putting CAD $5,000 into either SPSK or SPRE — which I’d choose, based on risk/reward and probabilities as of Today (Nov 28, 2025)

is SPRE a good purchase today

SCJP Training: Lesson 2: Flow Control #Java Short Notes #SCJP

SCJP Training: Lesson 1: Develop code that declares classes (including abstract and all forms of nested classes), interfaces, and enums, and includes the appropriate use of package and import statements (including static imports). #Java Short Notes #SCJP

SCJP Essential Knowledge #Java Short Notes #SCJP

SCJP Practice Exams #Java Short Notes #SCJP

SCJP: More Rules #Java Short Notes #SCJP

SCJP: Short Notes #Java Short Notes #SCJP

SCJP: Rules #Java Short Notes #SCJP

SCJP: Java Concurrency #Java Short Notes #SCJP

SCJP: Basic Java I/O #Java Short Notes #SCJP

Java : SCJP: Important Resources #Java Short Notes #SCJP

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Which ETF performed similarly to Bitcoin or closely? What performed the closet?

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is SPRE a good purchase today

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