uCertify Articles » 310-035 http://www.ucertify.com/article Just another WordPress weblog Wed, 04 Nov 2009 00:00:00 +0000 http://wordpress.org/?v=2.8.4 en hourly 1 What is an assertion? http://www.ucertify.com/article/what-is-an-assertion.html http://www.ucertify.com/article/what-is-an-assertion.html#comments Tue, 12 Aug 2008 00:00:00 +0000 uCertify 4101075

An assertion is a statement that is assumed to be true during the execution of a program. It returns a boolean result. If the result is true, the code executes normally and no other action takes place. This confirms that the assumed statement is true. If the result is false, an AssertionError is thrown. The assertion condition is tested using the assert keyword. The assert keyword has the following two forms:

  • assert assertCondition;
    Here, assertCondition is an expression that evaluates to a boolean expression.
  • assert assertCondition : exp;
    Here, an expression exp is passed to the AssertionError in case an error is thrown at runtime.

An assertion can be enabled or disabled for a class or a package during runtime as follows:

  • -da ClassName or -da PackageName: This will disable the assertion for a class or a package respectively.

  • -ea ClassName or -ea PackageName: This will enable the assertion for a class or a package respectively.
Share/Bookmark]]> http://www.ucertify.com/article/what-is-an-assertion.html/feed 0 What is the Vector class? http://www.ucertify.com/article/what-is-the-vector-class.html http://www.ucertify.com/article/what-is-the-vector-class.html#comments Sat, 22 Sep 2007 00:00:00 +0000 uCertify 2600070

The Vector class is used to create a generic dynamic array known as vector. It is contained in the java.util package. It implements the List interface. A vector can be created with or without explicitly specifying its size. Vector is synchronized. It can be expanded to hold more elements when needed. As all classes in Java inherit from the Object class, a vector can hold objects of any class. However, it cannot directly store primitive data types.

Share/Bookmark]]> http://www.ucertify.com/article/what-is-the-vector-class.html/feed 0 What is the toHexString() method of the Integer class? http://www.ucertify.com/article/what-is-the-tohexstring-method-of-the-integer-class.html http://www.ucertify.com/article/what-is-the-tohexstring-method-of-the-integer-class.html#comments Sat, 22 Sep 2007 00:00:00 +0000 uCertify 2600082

The toHexString() method converts an integer value into a hexadecimal string. The Integer class defines this method with the following signature:

public static String toHexString(int i)

The given signature of the method suggests that it takes an int argument and returns a String object. The String object returned by the method is a string that represents an unsigned integer in base 16.

The toHexString() method works as follows:

  1. If the argument to the method is a positive value, then the method converts this value to a string of ASCII digits in hexadecimal (base 16) with no extra leading zeros.
  2. If the argument to the method is a negative value, the method first adds the value 232 to the value. The value so obtained is then converted to a string of ASCII digits in hexadecimal (i.e., base 16) format with no extra leading zeros.
Share/Bookmark]]> http://www.ucertify.com/article/what-is-the-tohexstring-method-of-the-integer-class.html/feed 0 What is the default value of the elements of an array? http://www.ucertify.com/article/what-is-the-default-value-of-the-elements-of-an-array.html http://www.ucertify.com/article/what-is-the-default-value-of-the-elements-of-an-array.html#comments Fri, 21 Sep 2007 00:00:00 +0000 uCertify 2600004

As soon as an array is created using a new operator, its elements are automatically initialized to their default values. This automatic initialization of the elements depends on the data type of the array. The following table shows the initial value of the elements of an array:

Element Type Initial Value
boolean false
byte 0
short 0
int 0
long 0L
char ‘u0000′
float null
double 0.0D
Object reference null
Share/Bookmark]]> http://www.ucertify.com/article/what-is-the-default-value-of-the-elements-of-an-array.html/feed 0 What are keywords? http://www.ucertify.com/article/what-are-keywords_2600013.html http://www.ucertify.com/article/what-are-keywords_2600013.html#comments Fri, 21 Sep 2007 00:00:00 +0000 uCertify 2600013

Every computer language uses some reserved words known as keywords. They are an essential part of the language. In Java, keywords are defined in lower case and cannot be used as identifiers. The table below shows the list of keywords used in the Java language:

abstract assert boolean break byte case catch
char class const continue default do double
else extends final finally float for goto
if implements import instanceof int interface long
native new package private protected public return
short static strictfp super switch synchronized this
throw throws transient try void volatile while
enum

The keywords ‘const’ and ‘goto’ have not yet been implemented in Java. Java has reserved them for use in future expansion of the language.

Note: In addition to the keywords shown in the table, Java reserves the boolean values (true and false) and the reference value (null) for its own use.

Share/Bookmark]]> http://www.ucertify.com/article/what-are-keywords_2600013.html/feed 0 What is the min() method? http://www.ucertify.com/article/what-is-the-min-method.html http://www.ucertify.com/article/what-is-the-min-method.html#comments Fri, 21 Sep 2007 00:00:00 +0000 uCertify 2600026

The min() method is a static method of the java.lang.Math class. It takes two values as its arguments and returns the smaller of the two values. The min() method is overloaded with separate versions for the int, long, float, and double arguments. The signatures of the different versions of the min() method are given below:

public static int min(int p, int q)
public static long min(long p, long q)
public static float min(float p, float q)
public static double min(double p, double q)

The first version takes two int type values as its arguments and returns the smaller of the two values. The second version takes two long type values as its arguments and returns the smaller of the two values, and so on.

Share/Bookmark]]> http://www.ucertify.com/article/what-is-the-min-method.html/feed 0 What are character literals? http://www.ucertify.com/article/what-are-character-literals.html http://www.ucertify.com/article/what-are-character-literals.html#comments Fri, 21 Sep 2007 00:00:00 +0000 uCertify 2600042

Character literals are 16 bit Unicode characters that are used to represent a single character. They are generally used to initialize variables declared as char. They can also be converted into integer types and manipulated with integer operators.

Character literals are formed by enclosing a single printable character or the escape sequence for a character within a pair of single quotes. A character literal can be formed using any of the four options given below:

  1. Printable characters available on the keyboard: A character literal can be formed using a printable character available on the keyboard except the backslash and single quote characters. For example: ‘a’,’ A’, ‘#’, etc.
  2. Special escape sequences: Some special characters are defined using escape sequences. The escape sequences for these characters are shown below:

    Escape sequence Description
    ‘ Single quote
    \” Double quote
    Backslash
    r Carriage return
    n New line
    f Form feed
    t Tab
    \ Backspace

    3. A character literal can be formed using one of the escape sequences given in the table.

  3. Unicode escape sequences: A Unicode escape sequence consists of backslash-u (u) characters followed by the hexadecimal representation of the Unicode character. The syntax used to represent the Unicode values for characters is uxxxx where, each x represents a hexadecimal digit. The range of the hexadecimal escape sequence is from u0000 to uffff.

    There are few Unicode sequences that cannot be used to form a character literal. These are shown below:

    u0027    (this represents a single quote)
    u000a    (this represents a line feed)
    u000d    (this represents a carriage return)
    u005c    (this represents a backslash)

  4. Octal escape sequences: Any Unicode character whose value ranges from 0 to 255 can be represented using octal escape sequences. An octal escape sequence consists of a backslash character followed by three octal digits. The range of the octal sequence is from 00 to 377.
Share/Bookmark]]> http://www.ucertify.com/article/what-are-character-literals.html/feed 0 What is an interface? http://www.ucertify.com/article/what-is-an-interface.html http://www.ucertify.com/article/what-is-an-interface.html#comments Thu, 13 Sep 2007 00:00:00 +0000 uCertify 1501145

An interface is a reference type that defines a contract. An interface body consists of method declarations and constants. All methods and constants in an interface are public. Interfaces are left completely unimplemented, i.e., no method in the interface is implemented. All methods of an interface are abstract and the method body is absent. A class may access one or more interfaces simultaneously. An interface is defined using the syntax below:

access_modifier interface interface_name extends Super_interfaces{
return_type method1(parameter_list);
.
.
.
return_type methodn(parameter_list);
datatype variable1 = value;
.
.
.
datatype variablen = value;
}

The syntax defined above has two components: the interface declaration and the body of the interface. They are discussed in the following sections:

An interface declaration

An interface declaration consists of the following components:

  • access_modifier: An interface can be declared by using the public access modifier or without any access modifier. When an interface is declared as public, it can be accessed by any class in any package. If no access modifier is used in the declaration, the interface is accessible only within the package in which it is declared.
  • interface interface_name: The keyword interface in the declaration tells the compiler that it is an interface declaration, and name of the interface is the one that follows the interface keyword. The name of an interface must be a valid identifier.
  • extends Super_interfaces: An interface can extend other interfaces in the same way as a class extends another class. The only difference between these two is that whereas a class is allowed to extend only one class, an interface can extend any number of interfaces. The extends keyword in an interface declaration tells the compiler that the interface extends the list of interfaces that follows the extends keyword.

The interface body

The interface body contains method declarations for all the methods included in the interface. A method declared within an interface does not have a body, i.e., the method declaration ends with a semicolon. This is because an interface does not provide implementations for the methods declared within it. Apart from method declarations, an interface contains constant declarations.

All the constants defined in an interface are by default static and final. Therefore, there is no need to explicitly use these keywords to declare variables inside the interface body. All methods and variables in an interface are implicitly public if the interface is declared as public. It is also not necessary to declare the methods as abstract as well. The following example demonstrates how to define an interface:

public interface MyInterface{
void method1();
int method2(int k);
int x = 1;
int y = 2;
}

Share/Bookmark]]> http://www.ucertify.com/article/what-is-an-interface.html/feed 0 What is an abstract class? http://www.ucertify.com/article/what-is-an-abstract-class_1501058.html http://www.ucertify.com/article/what-is-an-abstract-class_1501058.html#comments Tue, 11 Sep 2007 00:00:00 +0000 uCertify 1501058

An abstract class is a class that is partially implemented. It provides design convenience. An abstract class is made up of one or more abstract methods that are declared but left unimplemented. An abstract class may also contain non-abstract methods. Abstract classes cannot be instantiated, but they can be subclassed. When an abstract class is subclassed, the subclass usually provides implementations for all of the abstract methods in its parent class. However, if it does not, the subclass must also be declared abstract.

Share/Bookmark]]> http://www.ucertify.com/article/what-is-an-abstract-class_1501058.html/feed 0 What is a constructor? http://www.ucertify.com/article/what-is-a-constructor.html http://www.ucertify.com/article/what-is-a-constructor.html#comments Wed, 11 May 2005 00:00:00 +0000 uCertify 2238

A constructor is a special type of method. It initializes an object immediately upon creation. It has the same name as the class in which it resides. A constructor is automatically called immediately after an object is created, before the new operator completes. A constructor can be marked as public, protected, or private. It has no explicit return type, not even void.

Share/Bookmark]]> http://www.ucertify.com/article/what-is-a-constructor.html/feed 0