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8/9/2019 Java notes By Pradeep Goud

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Arrays

• Arrays are objects in Java that store multiplevariables of the same type.

• Arrays can hold either primitives or object references

• For this arrays, you need to know three things:

How to make an array reference variable declare!

How to make an array object construct!How to populate the array with elements initiali"e!



#eclaring an Array

• Arrays are declared by stating the type of element the array will

hold followed by s$uare brackets to the left or right of the

identifier.

Example:

int[] key; // brackets before name (recommended)

int key []; // brackets after name (legal but less readable)

// spaces between the name and [] legal, but bad

• %t is never legal to include the si"e of the array in your declaration.

int[5] scores;

• &emember, the J'( doesn)t allocate space until you actually

instantiate the array object.



Constructing an Array

•*onstructing an array means creating the array object on theheap.

• +o create an array object, Java must know how much space to

allocate on the heap, so you must specify the si"e of the array

at creation time.• +he si"e of the array is the number of elements the array will

hold.

• +he most straightforward way to construct an array is to use

the keyword new followed by the array type, with a bracketspecifying how many elements of that type the array will hold.

int[] testScores; // eclares the array of ints

testScores ! new int["];



int[] testScores;

testScores = new int[4]; ----values-------

The Heap

0 1 23

0 0 0 0



• ou can also declare and construct an array in onestatement as follows:

int[] testScores ! new int["];

int[] car#ist ! new int[];// $ompilation %ails



Initializing an Array

•

%nitiali"ing an array means putting elements into it.

&ote

• if an array has three elements, trying to access the ['] element

will raise an rraynde*+ut+founds-*ception, because inan array of three elements, the legal inde* .alues are , 0, and

12

int[] * ! new int[5];

*["] ! 1; // +3, the last element is at inde* " *[5] ! ';



int[] 4 ! new int[1];

int y ! ';

4[y] ! ";

Declaring, Constructing, and Initializing on One Line

int[] dots ! 67,8,9:;



*onstructing and %nitiali"ing an Anonymous Array

• +he second shortcut is called -anonymous array creation- and

can be used to construct and initiali"e an array, and then assignthe array to a previously declared array reference variable:

int[] testScores;

testScores ! new int[] 6",,1:;

• <emember that you do not specify a si4e when using

anonymous array creation synta*2



Multidimensional Arrays

• %n Java, multidimensional arrays are actually arrays of arrays2

• +o declare a multidimensional array variable, specify each

additional inde using another set of s$uare brackets.

• For eample, the following declares a two dimensional array

variable called twoD.

int two[][] ! new int["][5];



Multidimensional Arrays



/trings



• 0roup of characters are called 1strings2

• %n java, a string is an object of /tring class

• %n Java, each character in a string is a 345bit 6nicode

character

• /tring class is declared as final, which means that /tring class

not subclassed



*reation of /trings3!7e can create a string just by assigning a group of characters to

a string type variableString s;// declare a string type .ariable

S!=>ello?;//assign a group of characters to it

&ote@ String s!=>ello?;

8! 7e can create an object to /tring class by allocating memoryusing new operator

String s!new String(=>ello?);

9!*reating string by converting character array into strings

char[] arr!6A>B,BeB,BlB,BlB,BoB:;

String s!new String(arr);

String s!new String(arr,0,'); //ell



• /tring s;abc<=

• /tring s3new /tring;abc<!=



trings Are Immuta!le O!"ects

• >nce a /tring object is created, it can never be changed it)s

immutable

• +he good news is that while the /tring object is immutable, itsreference variable is not.

String s!=abcdef?; // create a new String obCect, with .alue DabcdefD,

refer s to it

String s1 ! s; // create a 1nd reference .ariable referring to the sameString

s ! s2concat(D more stuffD); //create a new String obCect, with .alue

Dabcdef more stuffD, refer s to it2



/tring objects and their reference variables

• /tring s;abc<=

abc

s



• /tring s8s=

abc

s

s2



• s = s.concat !"e#!$;

abc

abc"e#

s2

s

s



String * ! DEa.aD;

*2concat(D <ulesFD);

System2out2println(D* ! D G *);

String * ! DEa.aD;

* ! *2concat(D <ulesFD);

System2out2println(D* ! D G *);



String s0 ! Dspring D;

String s1 ! s0 G Dsummer D;

s02concat(Dfall D);

s12concat(s0);

s0 G! Dwinter D;

System2out2println(s0 G D D G s1);

Answer: spring winter spring summer.• +here are two reference variables, s3 and s8. +here were

a total of eight /tring objects created as follows: -spring-,

-summer - lost!, -spring summer-, -fall- lost!, -spring

fall- lost!, -spring summer spring- lost!, -winter- lost!,-spring winter- at this point -spring- is lost!. >nly two

of the eight /tring objects are not lost in this process.



/tring class methodsCharacter Extraction

charAt!

get*hars!

get?ytes!

to*harArray!

chart()

+o etract a single character from a /ting

public char chart(int inde*)

Example:

String * ! DairplaneD;

System2out2println( *2chart(1) ); // output is HrA



get$hars()

+o etract more than one character at a time

public .oid get$hars(int start, int end,char target[], int targetStart)

Example:

String s!=Ihis is a demo of the get$hars method?;

int start!0;

int end!0";

char[] buf!new char[endstart];

s2get$hars(start,end,buf,);

System2out2println(buf); //demo



getytes()

public byte[] getytes()

to$harrray()

*onvert all the characters in a string object into a

character array

public char[] to$harrray()



String Comparison

e$uals!

e$uals%gnore*ase!

region(atches!

starts7ith!

ends7ith!

compare+o!



eJuals()

3! +o compare two strings for e$uality, use e$uals!

8! %t returns true if the strings contains same characters in the

same order, and false otherwise9! +he comparison is case5sensitive

public boolean eJuals(+bCect str)

String s0!=e*it?;

String s1!=e*it?;

Strrong s'!=-*it?;

System2out2println(s02eJuals(s1));

System2out2println(s02eJuals(s'));



regionLatches()

+his method compares a specific region inside a string with

another specific region in other string

boolean regionLatches(int startinde*, String s1, int s1Startinde*,int num$hars)

String s0!DhelloworldD;

String s1!DworldD;

System2out2println(s02regionLatches(5,s1,,'));

boolean regionLatches(boolean ignore$ase,int startinde*, String

s1, int s1Startinde*, int num$hars)



startsMith()

+his method determines whether a given string begins with a

specified string

public boolean startsMith(String str) String s0!=hello world?;

System2out2println(s02startsMith(=hello?);

public boolean startsMith(String str, int startinde*)

System2out2println(s02startsMith(DworldD,7));



endsMith()

+his method determines the string ends with a specified string

public boolean endsMith(String str)

String s0!Dhello worldD;

System2out2println(s02endsMith(DrldD));



compareIo()

+his method is useful to compare two strings and to know which

string is bigger or smaller

+his should be used as s3.compare+os8! %f s3 and s8 strings are e$ual, then this method gives @

%f s3 is greater than s8, then this method a ve number

%f s3 is less than s8, then it returns a Bve number

7hen we arrange the strings in dictionary order, which ever

string comes first is lesser than the string that comes net.

Cample: ;?o< is lesser than ;?oy<

+his method case sensitive. %t means ;?>D< and ;bo< are not

same for this method

public int compareIo(String s)

public int compareIognore$ase(String s)



Modifying a Strings

3! substring!

8! concat!

9! replace!

E! trim!



substring()

+his method is useful to etract sub string from a main string

public String substring(int *)

%t return a new string consisting all characters starting from the

position 12 until the end of the string

-*ample@

String * ! D01'"5798D;

System2out2println( *2substring(5) ); // 5798

public String substring(int stratinde*, int endinde*)

+he string returned contains all the characters from the beginning

inde, up to, but not including the ending inde

System2out2println( *2substring(5, 9));



String concat()

+his method concatenates or joins two strings s3 and s8! and

return a third string s9! as result.

public String concat(String s)

-*ample@

String * ! Dta*iD;

System2out2println( *2concat(D cabD) );



String replace(char old, char new)

public String replace(char old, char new)

+his method replaces all the occurrences of character by a new

character =String * ! Do*o*o*o*=

System2out2println( *2replace(H*H, HKH) ); // output is

DoKoKoKoK=



String trim()

public String trim()

+his method removes spaces from the beginning and ending of a

string

String * ! = <ish.ad Lohan D;

System2out2println( *2trim() );



Searching String

inde*+f()

• +his method is called in the form s3.inde>fs8!, and it

returns an integer value.

• %f s3 contains s8 as sub string, then the first occurrence of s8 in

the string s3 will be returned by this method

public int inde*+f(String s)

String s0!=Ihis is a book?

String s1!=is?

int n!s02inde*+f(s1);



lastnde*+f(String s)

+his method returns the last occurrence of the sub string 1s2 in the

main string. %f 1s2 is not found, then it returns negative value

public int lastnde*+f(String s)

String s0!=Ihis is a book?

String s1!=is?

int n!s02lastnde*+f(s1);

l h()



length()

public int length()

+his method returns the length of the /tring used to invoke the

methodString * ! D01'"57D;

System2out2println( *2length() ); // returns D9D



public String to#ower$ase()

+his method returns a /tring whose value is the /tring used to

invoke the method, but with any uppercase characters

converted to lowercaseString * ! D &ew LoonD;

System2out2println( *2to#ower$ase() );

public String toNpper$ase()

+his method returns a /tring whose value is the /tring used to

invoke the method, but with any lowercase characters

converted to uppercase

String * ! D &ew LoonD;

System2out2println( *2toNpper$ase() );



/tring?uffer

• /tring?uffer is a class which represents strings in

such a way that their data can be modified

• %t means /tring?uffer class objects are mutable



*reating /tring?uffer class objects

Stringuffer sb!new Stringuffer(=>ello?);

Stringuffer sb!new Stringuffer();

Here, we are creating a /tring?uffer object as an empty object.

%n this case, a /tring?uffer object will be created a default capacity of 34

characters

Stringuffer sb!new Stringuffer(5);

%n this case, a /tring?uffer object will be created a default capacity of @

charactersCven if we declare the capacity as @, it is possible to store more than @

characters into this /tring?uffer.

+he reason is /tring?uffer is mutable and can epand dynamically in

memory



(ethods of /tring?uffer class

public Stringuffer append(*)

12 may be boolean, byte, int, long, float, double, char, character

array,/tring or other /tring?uffer

%t will be append to the /tring?uffer object

Cample:

Stringuffer sb!new Stringuffer(=uni?);

sb2append(=.ersityB);

System2out2println(sb);



public Stringuffer insert(int i , *)

%t will be inserted into /tring?uffer at the position represented by

i

Cample:Stringuffer sb!new Stringuffer(=intelligent person?);

sb2insert(00,?young?);


bli St i ff d l t (i t i t C)



public Stringuffer delete(int , int C)

+his method removes characters from ith position till j53th

position in the /tring?uffer

sb2delete(,');

public Stringuffer re.erse()

+his method reverse the characters in the /tring?uffer

Stringuffer sb!new Stringuffer(=abc?);

sb2re.erse();




public int length()

public int inde*+f(String str)

public int lastnde*+f(String str)

public Stringuffer replace(int , int C,String str)

+his replace characters from % to j53, by the string 1str2 in the

/tring?uffer object

Stringuffer sb!new Stringuffer(=>igh $ost?);

sb2replace(,",?low?);
