python help pages

python list : http://effbot.org/zone/python-list.htm

rollback deployment: http://benosullivan.co.uk/internet/google-app-engine/google-app-engine-fix-another-transaction-by-user-acct-is-already-in-progress/

sample rollback:
"C:\Python27\python" "C:\Program Files (x86)\Google\google_appengine\appcfg.py" -verbose --no_cookies -

-email=XXX@gmail.com --passin rollback "C:\\Documents and Settings\\kli\\My Documents\\Python\\helloworld"

some java interview questions(1)

http://www.interview-questions-java.com/java-questions/interview-questions-on-java

What if the main method is declared as private?

The program compiles properly but at runtime it will give “Main method not public.” message.

What are the differences between == and .equals() ?

The == operator compares two objects to determine if they are the same object in memory i.e. present in the same memory location. It is possible for two String objects to have the same value, but located in different areas of memory.

== compares references while .equals compares contents. The method public boolean equals(Object obj) is provided by the Object class and can be overridden. The default implementation returns true only if the object is compared with itself, which is equivalent to the equality operator == being used to compare aliases to the object. String, BitSet, Date, and File override the equals() method. For two String objects, value equality means that they contain the same character sequence. For the Wrapper classes, value equality means that the primitive values are equal.

What is the difference between final, finally and finalize? What do you understand by the java final keyword?

o final - declare constant
o finally - handles exception
o finalize - helps in garbage collection

Variables defined in an interface are implicitly final. A final class can’t be extended i.e., final class may not be subclassed. This is done for security reasons with basic classes like String and Integer. It also allows the compiler to make some optimizations, and makes thread safety a little easier to achieve. A final method can’t be overridden when its class is inherited. You can’t change value of a final variable (is a constant). finalize() method is used just before an object is destroyed and garbage collected. finally, a key word used in exception handling and will be executed whether or not an exception is thrown. For example, closing of open connections is done in the finally method.

What is the Java API?

The Java API is a large collection of ready-made software components that provide many useful capabilities, such as graphical user interface (GUI) widgets.

What is the GregorianCalendar class?

The GregorianCalendar provides support for traditional Western calendars.

What is the ResourceBundle class?

The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program’s appearance to the particular locale in which it is being run.

When is static variable loaded? Is it at compile time or runtime? When exactly a static block is loaded in Java?

Static variable are loaded when classloader brings the class to the JVM. It is not necessary that an object has to be created. Static variables will be allocated memory space when they have been loaded. The code in a static block is loaded/executed only once i.e. when the class is first initialized. A class can have any number of static blocks. Static block is not member of a class, they do not have a return statement and they cannot be called directly. Cannot contain this or super. They are primarily used to initialize static fields.

What is reflection API? How are they implemented?

Reflection is the process of introspecting the features and state of a class at runtime and dynamically manipulate at run time. This is supported using Reflection API with built-in classes like Class, Method, Fields, Constructors etc. Example: Using Java Reflection API we can get the class name, by using the getName method.

Does JVM maintain a cache by itself? Does the JVM allocate objects in heap? Is this the OS heap or the heap maintained by the JVM? Why

Yes, the JVM maintains a cache by itself. It creates the Objects on the HEAP, but references to those objects are on the STACK.

What is phantom memory?

Phantom memory is false memory. Memory that does not exist in reality.

Can a method be static and synchronized?

A static method can be synchronized. If you do so, the JVM will obtain a lock on the java.lang.
Class instance associated with the object. It is similar to saying:

synchronized(XYZ.class) {

}

Garbage collection

Garbage collection is one of the most important features of Java. The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused.

A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used. Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists.

In Java, it is good idea to explicitly assign null into a variable when no more in use. In Java on calling System.gc() and Runtime.gc(), JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected. Garbage collection is an automatic process and can’t be forced. There is no guarantee that Garbage collection will start immediately upon request of System.gc().

Garbage collector is a daemon thread

The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup, before the object gets garbage collected. For example, closing an opened database Connection.

java options for Garbage collection

JDK 1.4.1 adds some new options to control garbage collection.

The options for server applications should be:

-server -Xms(size) -Xmx(size) -XX:+UseParallelGC -XX:UseAdaptiveSizePolicy

where (size) is the amount of memory your application will use (128m is 128 megabytes, 128 is 128 bytes). These options select the parallel scavenge collector for young generation garbage, and the mark-sweep-compact collector for old generation garbage. Old generation collection is still single threaded but the mark-sweep-compact collector is very efficient.

The options for applications where pauses are undesirable should be:

-server -Xms(size) -Xmx(size) -XX:+UseParNewGC -XX:+UseConcMarkSweepGC -XX:SurvivorRatio=2 -XX:NewRatio=8

where (size) is the amount of memory your application will use (128m is 128 megabytes, 128 is 128 bytes). These options select the parallel copying collector for the new generation, and the concurrent mark-sweep collector for the old generation. The new generation collection uses one thread for each processor on the machine. 1/8 of the memory is used for the new generation, and 7/8 for the old generation. The new generation memory is broken into three pieces, 1/16 of the memory is the eden where new objects are created, and the rest of the new generation is stored in two semi spaces.