SCJP: Language Fundamentals #Java Short Notes #SCJP

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
Categories:Java Short Notes, SCJP
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Post Data:2008-07-28 12:18:41

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