Fundamental Classes in the java.lang Package
In this section you should be able to:
1. Write code using the following methods or the java.lang.Math class: abs, ceil, floor, max, min, random, round, sin, cos, tan, sqrt.
2. Describe the significance of the immutability of
String objects.
3. Make appropriate selections in the wrapper classes to
suit specified behaviour requirements.
4. State the result of executing a fragment of code that
includes an instance of one of the wrapper classes.
5. Write code using the following methods of the wrapper
classes (e.g. Integer, Double, etc.): doublevalue, floatValue, intValue,
longValue, parseXxx, getXxx, toString, toHexString.
Fundamental
Classes in the java.lang Package
The
Object class
All classes in Java™ are directly or
indirectly derived from the object class. Some of the classes are –
Boolean, Number, Void, Math, String, StringBuffer etc.
Some of the important methods defined in the
Object class are given below. These methods are available to all Java™
classes.
·
boolean equals(Object obj)
– The equals method in the Object class returns true if two references
point to the same object. Some classes like String and Boolean overload this
method. The difference between the equals function and the equality operator
is covered here.
·
String
toString() – This function is used
to convert objects to String. If a subclass does not override this method, the
method returns a textual representation of the object, which has the following
format: <name_of_the_class>@<hash_code_value_of_the_object>”.
·
The following
methods related to threads are also defined in the Object class –
void
notify()
void
notifyall()
void
wait(long millisec) throws InterruptedException
void
wait(long millisec, int nanosec) throws InterruptedException
void
wait() throws InterruptedException
Wrapper
classes
Corresponding to all the primitive types,
Java™ defines wrapper classes.Some examples of these wrapper classes are –
Character, Boolean, Integer, Double.
Important
methods in the Math class
Some of the methods defined in the Math
class are used frequently. These are explained below. Besides the
functionality, it is important to understand the arguments and return types of
these functions.
static double ceil(double val) – The method ceil returns the smallest double
value equal to a mathematical integer, that is not less than the argument. For
example:
ceil(3.4) returns 4.0
ceil(-2.3) returns -2.0
ceil(3.0) returns 3.0
Static double floor(double val) – The method floor returns the largest double value
equal to a mathematical integer, that is not greater than the argument. For
example:
floor(3.4) returns 3.0
floor(-2.3) returns -3.0
floor(3.0) returns 3.0
static int round(float val) and
static long round(double val) – The
method round returns the integer value closest to the argument. For example:
round(3.7) returns 4
round(3.2) returns 3
round(3.0) returns 3
round(-3.1) returns 3
String
class
The String class is used to implement
immutable character strings. This means that the character string cannot be
changed once it has been created. Some of the important methods are explained
below.
int length() – The number of characters in the String class
are returned by the length() method.
String substring(int
startIndex)
String substring(int
startIndex, int endIndex)
The method substring extracts a substring
from the string. The method extracts a string from the startIndex to the index
endIndex. Note: if the endIndex is not specified then string till the end of
the input string is returned. The example below illustrates this:
String str = “I
am a string”;
int len = str.length();
String str2 = str.substring(2,5);
If I run the above code in the test class
public class test
{
public static void main(String[] args)
{
String str = “I am a string”;
int len = str.length();
String str2 = str.substring(2, 5);
System.out.println(“\nlen = “
+ len + “\nsubstring = \”” + str2 + “\””);
}//end of main method
}//end of test class
I get the following result
After the above statements have run, the
string str2 contains the string “am “. The string str
still contains the same value “I am a string”. The variable len
has the value 13.
StringBuffer
class
The StringBuffer class implements mutable
strings. This means that the characters stored in the string and the capacity of
the string can be changed.