8 java lang package

8/8/2019 8 Java Lang Package

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Language Package (lang)

The java.lang package contains classes that are

central to the operation of the java language

and environment. For example: very little can

be done without String class and the Objectclass is completely indispensable.

Due to this importance, java compiler

automatically import all the classes in the package into every source file.



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Object Class

Object class is the super class of all javaclasses. If a class does not extends any class,

the compiler build a class that directly inherit

from Object.All the methods of Object class inherited by

all the classes. Two important methods are:

public boolean equals(Object o)

public String toString( )



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equals(Object o) compares the reference

of the calling object with the o object. If they refer to the same object the method

return true otherwise returns false.

This method is overridden by String,Date and Wrapper Classes. In those

classes it compare the contents of the

objects rather than their memoryreference.



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toString( ) method provides a string

representation of an object¶s state. The

method provided by Object class returnsclass name followed by hash code. The

classes should override this method to

display class specific information.When we print any object the compiler

calls toString( ) method to display the

string representation of the object.



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Math Class

Math class is final class so you can notextends it. Constructors are private so you can

not create object of it.

All the methods in this class are static and can be accessed with class name.

The methods in Math class cover a broad

range of mathematical functions including

trigonometry, logarithms and exponentiationand rounding.



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Wrapper Classes

Each java primitive data type has acorresponding wrapper class. A wrapper class

is simply a class that contain a single,

immutable value.All the wrapper classes can be constructed by

passing the value to be wrapped into the

constructor.

The wrapper classes are useful whenever itwould be convenient to treat a piece of

primitive data type as if it were an object.



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Wrapped values inside wrapper classes cannot

be modified.

Following is the list of wrapper classes:

Byte byte

Character char

Shortshort

Boolean boolean

Integer int

Wrapper ClassPrimitive Data Type



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Longlong

Floatfloat

Doubledouble

The values wrapped inside two

wrappers of the same type can be

checked for equality by using equals( )

method, which are overridden by all

wrapper classes.



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Some examples of wrapper classes:

int a = 5;

Integer a1 = new Integer(a);

short s = 43;

Short s1 = new Short(s);

char c = µA¶;Character c1 = new Character(c);



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boolean b = true;

Boolean b = new Boolean(b);

double d = 54.43;

Double d1 = new Double(d);

Integer i = new Integer(4);

Double d = new Double(33.33);

Character c = new Character(µA¶);



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String Class

The String class contain an immutable string.Once an instance of string class is created, the

string it contains cannot be changed.

There are ten constructors in the string class

which allow you to create string out of many

forms like character arrays or another string

etc.

Note that java support 16 bit Unicodecharacter set.



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The most common constructor is:

String s = new String(³Hello´);

can be abbreviated as:

String s = ³Hello´;

Both create a object of String class



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We can compare two string object contents

using equals( ) method.

String s1 = ³Hello´;

String s2 = ³Hell0´;

boolean b = s1.equals(s2);

System.out.println(b);

In this case true is printed on the screen.



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String class contain several methods to

perform transformations on Strings.

Some methods are:

toUpperCase( );

toLowerCase( );length( );

substring( );

indexOf( );

charAt( );

etc.



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String class overload + operator to perform

concatenation of two strings and other objects.

String s1 = ³Hello´;

String s2 = ³World´;

Int i = 43;

String s = s1 + s2 + i;

Now s contains : HelloWorld43;



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StringBuffer Class

The instance of StringBuffer class represents astring that can be dynamically modified.

The most commonly used constructor takes a

String instance.

StringBuffer sb = new StringBuffer(³Hello´);

StringBuffer has several methods which areused to dynamically change string.



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append( String s) method append the s to the

end of the calling StringBuffer object.


String s = ³World´;

sb.append(s);

Now sb represents:

HelloWorld



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insert( int index, String s) method insert the s

at the position of index number specified of the

calling StringBuffer object.


String s = ³World´;

sb.insert(2, s);

Now sb represents:

HeWorldllo



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reverse( ) method return the string in reverse

order.


sb.reverse( );

Now sb represents:

olleH



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