Garbage Collection

• Java itself does memory management. You do not need to allocate memory at the time of object creation; also you do not need to free memory explicitly
• Object is created either on the heap or on a stack
• Memory heap: Objects created with new keyword are placed in heaps. This memory remains allocated throughout the life cycle of the object. When the object is no more referred, the memory becomes eligible for garbage collection
• Stack: During method calls, objects are created for method arguments and method variables. These objects are created on stack. Such objects are eligible for garbage-collection when they go out of scope.
• Garbage Collection is a low-priority thread in java
• Garbage Collection cannot be forced explicitly. JVM may do garbage collection if it is running short of memory.
• The call System.gc() does NOT force the garbage collection but only suggests that the JVM may make an effort to do garbage collection.
• Garbage Collection is hardwired in Java runtime system. Java runtime system keeps the track of memory allocated. Therefore, it is able to determine if memory is still usable by any live thread. If not, then garbage collection thread will eventually release the memory back to the heap.
• Garbage Collection usually adopts an algorithm, which gives a fair balance between responsiveness (how quickly garbage-collection thread yields?) and speed of memory recovery (important for memory-intensive operations). Responsiveness is especially important in real time systems.
• An object is eligible for garbage collection when no object refers to it.
• An object also becomes eligible when its reference is set to null. (Actually all references to the object should be null for it to be eligible.)
• The objects referred by method variables or local variables are eligible for garbage collection when the method or their container block exits

From: http://sitestree.com/?p=4863
Categories:Java Short Notes, SCJP
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Post Data:2009-03-10 13:59:45

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Class declaration and java source file.

• Only “one” top-level public class is allowed per java source file.
• The name of the java source file and the name of the top-level public class MUST be the same.
• If no public class is there in a file, after compiling separate class files will be created for all classes in the file.
• Package statement, import statements and class definition MUST appear in the order given.

Keywords and Identifiers

• Keywords are always in a lower case.
• Some keywords: const, goto, strictfp, and volatile
• Identifiers must start with either letter, $ or _ (underscore) and can have letter, $, _ or digits in it.
• no keyword is allowed as identifiers

Default Values, Local Variables

• Each primitive data type has a default value specified. Variable of primitive data type may be initialized
• Only class member variables are automatically initialized. Method variables need explicit initialization
• Local variables (also known as automatic variables) are declared in methods and in code blocks
• Automatic variables are not automatically initialized
• local variables should be explicitly initialized before first use. These are automatically destroyed when they go out of scope

Arrays

• Fixed-sized ordered collection of homogeneous data elements
  int[] ints; // array declaration
  ints = new ints[25]; // array construction at runtime.
• Array declared, constructed and initialized at the same time.
  int[] ints = {1,2,3,4,5}; // array declaration,
• An array of primitive data type created using the new keyword is automatically initialized. Each array element is initialized to its default value.
  For example,
  char[] arrayOfChars = new char[10];
• Array of object references: An array of object references created using the new keyword is also initialized. Each array element is initialized to its default value, i.e. null.
  String[] names = new String[10];
• length is a property of array (and not a method)
• java.lang.Object is the superclass of an array

Argument passing during method calls

• Always a copy of argument value is passed to calling method
• Arguments of primitive data types: first, a copy of the passing variable is made and then it is passed. The original copy remains unaffected
• Object reference as an argument: A copy of object reference is passed for method calls. It still points to the same object. The original copy is affected.

From: http://sitestree.com/?p=4862
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Post Data:2008-07-28 12:18:41

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Exams like SCJP test your understanding of Java operators and how to use them like:
assignment operators: =, +=, -=
arithmetic operators: +, -, *, /, %, ++, —
relational operators: < , , >=, ==, !=
logical operators: &, |, ^, !, &&, ||
conditional operators: ? :
Also operators to check the equality of two objects or two primitives

In the following table operators and their precedences are shown. Upper rows operators have higher precedence. Same row operators have equal precedence. For equal precedence operators, All binary operators except the assignment operators work from left to right while assignment operators work from right to left.

The following quick reference summarizes the operators supported by the Java programming language.

Simple Assignment Operator

=	Simple assignment operator

Arithmetic Operators

+ 	Additive operator (also used for String concatenation)- 	Subtraction operator*	Multiplication operator/ 	Division operator%	Remainder operator

Unary Operators

+ 	Unary plus operator; indicates positive value (numbers are positive without this, however)- 	Unary minus operator; negates an expression++  	Increment operator; increments a value by 1--    	Decrement operator; decrements a value by 1!     	Logical compliment operator; inverts the value of a boolean

Equality and Relational Operators

==	Equal to!=	Not equal to>	Greater than>=	Greater than or equal to< Less than<=	Less than or equal to

Conditional Operators

&& 	Conditional-AND|| 	Conditional-OR?:      Ternary (shorthand for if-then-else statement)

Type Comparison Operator

instanceof	Compares an object to a specified type 

Bitwise and Bit Shift Operators

~	Unary bitwise complement< >	Signed right shift>>>	Unsigned right shift&	Bitwise AND^	Bitwise exclusive OR|	Bitwise inclusive OR

Details about java operators

From: http://sitestree.com/?p=4848
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Post Data:2012-12-29 02:01:30

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Java certification exams like SCJP test your knowledge about java classpath. Check here for an excellent resource on the topic .

System classpath

We can specify classpath in the command line or make use of a `system’ class path. The IDEs have their own way of maintaining class paths. System class paths will be used by both the Java compiler and the JVM in the absence of specific instructions.

Set Linux System Classpath:
CLASSPATH=/myclasses/myclasses.jar;export CLASSPATH

Windows:
set CLASSPATH=c:myclassesmyclasses.jar

For permanent result, in Linux, put the commands in the file .bashrc in your home directory. On Windows 2000/NT, set it from `Control Panel’, System, Environment Variables.

You can also specify .jar files full of your required classes in your classpath.

CLASSPATH=/usr/j2ee/j2ee.jar:.;export CLASSPATH

where the `.’ indicates `current directory’.

From command line, if you want to add more paths to the classpath use:

Linux:javac -classpath $CLASSPATH:dir1:dir2 …

Windows:javac -classpath %CLASSPATH%;dir1:dir2 …

if you have spaces in your path use double quote to concatenate words.

Also check this resource

From: http://sitestree.com/?p=4849
Categories:Java Short Notes, SCJP, Blog
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Post Data:2013-06-05 14:27:56

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Assertions in Java: http://java.sun.com/j2se/1.4.2/docs/guide/lang/assert.html
Enum in Java:http://www.java2s.com/Code/Java/Language-Basics/Enum.htm
StringBuffer and StringBuilder Classes have similar methods where StringBuffer is synchronized: http://java.sun.com/j2se/1.5.0/docs/api/java/lang/StringBuilder.html
Serialize and Deserialize: http://java.sun.com/j2se/1.4.2/docs/api/java/io/Serializable.html
java.util package is very important: http://java.sun.com/j2se/1.4.2/docs/api/java/util/package-summary.html

From: http://sitestree.com/?p=4845
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Post Data:2013-06-11 18:38:25

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