The String Class (1)

8/8/2019 The String Class (1)

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The String Class


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Objectives:

Learn about literal strings

Learn about String constructors Learn about commonly used

methods

Understand immutability ofstrings Learn toformat numbers into

strings


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String class facts

In java a string is a sequence of

characters.But unlike many other

languages that implement strings as

character arrays, java implements

strings as objects of type String.

When you create a String object,youare creating a string that cannot be

changed.


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String class facts An object ofthe String class represents a

string ofcharacters.

The String class belongs to the java.langpackage, which does not require an importstatement.

Like other classes, String has constructorsand methods.

Unlike other classes, String has twooperators, + and += (used forconcatenation).


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Literal Strings are anonymous objects ofthe String class

are defined by enclosing text in double

quotes. This is a literal String

dont have tobe constructed.

can be assigned to String variables.

can be passed tomethods and constructorsas parameters.

have methods you can call.


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Literal String examples//assign a literal to a String variable

String name = Robert;

//calling a method on a literal String

char firstInitial = Robert.charAt(0);

//calling a method on a String variable

char firstInitial = name.charAt(0);


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Immutability Once created, a string cannot be changed:

none of its methods changes the string.

Such objects are called immutable. To say that the strings within objects oftype

String are unchangeable means that thecontents ofthe String instance cannot be

changed after it has been created. However, a variable declared as a String

reference can be changed topoint at someother String object any time.


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Empty Strings

An empty String has no characters. Its

length is 0.

Not the same as an uninitialized String.

String word1 = "";String word2 = new String();

private String errorMsg; errorMsg

is null

Empty strings


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No Argument Constructors

No-argument constructor creates an

empty String. Rarely used.

A more common approach is to

reassign the variable to an empty

literal String. (Often done to reinitialize a variableused to store input.)

String empty = ;//nothing between quotes

String empty = new String();


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Constructors String(char chars[])

char chars[]={a,b,c};

String s=new String(chars);This constructor initializes s with the stringabc

String(char chars[], int startIndex,int

numchars)

char chars[]={a,b,c,d,e};

String s=new String(chars,2,3);

This initializes s with the characters cde.


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Constructors

] String(String strobj)strobj is a String object

class Makestring{

public static void main(String args[])

{

char c[]={j,a,v,a};

String s1=new String(c);

String s2=new String(s2);

System.out.println(s1);


}

}


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Constructors String(byte asciichars[])

String(byte asciichars[],int

startIndex,int numchars)Class Substring{public static void main(String args[])

{ byte ascii[]={65,66,67,68,69,70};

String s1=new String (ascii);System.out.println(s1);

String s2= new String(ascii,2,3);


}}


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The contents of the array are copies

whenever you create a String object

from an array.

Ifyoumodify the contents of the array

after you have created the string, the

String will be unchanged.


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Copy Constructors Copy constructor creates a copy ofan existing

String. Also rarely used.

Not the same as an assignment.

String word = new String(Java);

String word2 = new String(word);

word

word2

Java"

Java"

Copy Constructor: Each variable points to a different copy of the String.

String word = Java;

String word2 = word;

wordJava"

word2

Assignment: Both variables point to the same String.


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Other Constructors

Most other constructors take an array as

a parameter to create a String.

char[] letters = {J, a, v, a};

String word = new String(letters);//Java


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Methods length, charAtint length();

char charAt(i);

Returns the numberofcharacters in

the string

Returns the char at position i.

7

n'

Problem".length();

Window".charAt (2);

Returns:

Characterpositions in strings are numbered

starting from 0 just like arrays.


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char chars[]={J,a,v,a};

String s = new String(chars);

System.out.println(s.length));

String literal

char chars[]={J,a,v,a};

String s = new String(chars);

String s2=abc;

System.out.println(abc.length());


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CharacterE

xtraction charcharAt(int where)

Where is the index of the character that u want.

The value ofwhere must be nonnegative andspecify a location within the string

charAt() returns the character at the specified

location

char ch;ch=abc.charAt(1);


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CharacterE

xtraction void getChars( int sourcestart,int sourceend,char

target[], int targetstart)

Sourcestart specifies the index of the beginning ofthe substring

Sourceend specifies an index that is one past the

end ofthe desired substring.

The array that will receive the characters is

specified by target

The index within target at which the ststring will be

copied is passed in targetstart


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Class getchardemo{ public static void main(String args[])

{

String s=demoofthe getcharmethod;

int start=10;

int end=14;

Charbuf[]=new char[end-start];

s.getChars(start, end,buf,0);

System.out.println(buf);

}

}


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Changing the case

String toLowerCase()

String toUpperCase()

String upper= s.toUpperCase();

String l=s.toUpperCase();


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Modifying a String

String substring(int startindex)

String substring(int startindex, int

endindex)


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Methods substring

lev"

mutable"

"" (empty string)

television".substring (2,5);

immutable".substring (2);

bob".substring (9);

Returns:

television

i k

television

i

String subs = word.substring (i, k);

returns the substring ofchars in

positions fromi tok-1

String subs = word.substring (i);

returns the substring from the i-th

char to the end

Returns a new String by copying characters from an existing String.


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Concatenation

String concat(String str)

String s1=one;

String s2= s1.concat(andtwo);

It can alsobe written as

String s1=one;String s2=s1+ two;


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Methods Concatenation

String word1 = re, word2 = think; word3 = ing;

int num = 2;

String result = word1 + word2;//concatenates word1 and word2 rethink

String result = word1.concat(word2);//the same as word1 + word2 rethink

result += word3;//concatenates word3 to result rethinking


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Replace

To replace all occurrences ofone

character in the invoking string with

another character.

String replace(charoriginal, char

replacement);

String s= Hello.replace(l,w);


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Trim

Trimmethod returns a copy of the

invoking string from which any leading

and trailing white space has beenremoved.

String trim()

String s= Hello world .trim();


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Searching Strings

String class provides twomethods that

allow you to search a string for a

specified characteror substring.

indexOf() ---searches forfirst

occurrence ofa characteror substring

lastIndexof()---- searches for lastoccurrence ofa characteror substring


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Methods Find (indexOf)

String name =President George Washington";

name.indexOf (P'); 0name.indexOf (e'); 2

name.indexOf (George"); 10

name.indexOf (e', 3); 6

name.indexOf (rock"); -1

name.lastIndexOf (e'); 15

Returns:

(not found)

(starts searching

at position 3)

0 2 6 10 15


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String comparison

Equals and EqualsIgnoreCase

String s1=Hello;

String s1=Hello;

String s3=HELLO;

s1.equals(s2);s1.equalsIgnoreCase(s3);


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Methods Equality

boolean b = word1.equals(word2);

returns true if the string word1 is equal toword2

boolean b = word1.equa

lsIgnoreCa

se(word2);returns true if the string word1matches word2,

case-blind

b = Raiders.equals(Raiders);//true

b = Raiders.equals(raiders);//false

b = Raiders.equalsIgnoreCase(raiders);//true

if(team.equalsIgnoreCase(raiders))

System.out.println(Go You + team);


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Function comparTo

To know which is less than, or greater

than the next.

A string is less than another if it comes

before the other in dictionary order

A string is greater than another if it

comes after the other in dictionaryorder.


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int compareTo(String str)

Value= Zeromeans the two strings are

same

Less than zero then the invoking string

is less than str

Greater than zeromeans the invoking

string is greater than str


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Methods Comparisons

int diff= word1.compareTo(word2);returns the difference word1 - word2

int diff= word1.compareToIgnoreCase(word2);returns the difference word1

-word2,

case-blind

Usually programmers dont care what the numerical difference of

word1 - word2 is, just whether the difference is negative (word1

comes before word2), zero (word1 and word2 are equal) orpositive(word1 comes after word2). Often used in conditional statements.

if(word1.compareTo(word2) > 0){

//word1 comes after word2

}


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Equals versus ==

equals() method compares the

characters inside a String object.

The == operator compares twoobject

references to see whether they refer to

the same instance.


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class Equals{

public static void main(String args[]){

String s1=Hello;

String s2= new String(s1);

System.out.println(s1.equals(s2));

System.out.println(s1.equals(s2));System.out.println(s1==s2);}}

Output

True

False


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Comparison Examples

//negative differences

diff = apple.compareTo(berry);//a before b

diff = Zebra.compareTo(apple);//Z before a

diff = dig.compareTo(dug);//i before u

diff = dig.compareTo(digs);//dig is shorter

//zero differences

diff = apple.compareTo(apple);//equal

diff = dig.compareToIgnoreCase(DIG);//equal

//positive differences

diff = berry.compareTo(apple);//b after a

diff = apple.compareTo(Apple);//a after A

diff = BIT.compareTo(BIG);//T after G

diff = huge.compareTo(hug);//huge is longer


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Methods trim

String word2 = word1.trim ();

returns a new string formed fromword1 by

removing white space at both ends

does not affect whites space in the middle

String word1 = Hi Bob ;

String word2 = word1.trim();

//word2 is Hi Bob no spaces on either end

//word1 is still Hi Bob with spaces


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Methods replace

String word2 = word1.replace(oldCh, newCh);


replacing all occurrences ofoldCh with newCh

String word1 = rare;

String word2 = rare.replace(r, d);

//word2 is dade, but word1 is still rare


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Methods Changing Case

String word2 = word1.toUpperCase();

String word3 = word1.toLowerCase();


converting its characters toupper (lower) case

String word1 = HeLLo;

String word2 = word1.toUpperCase();//HELLO

String word3 = word1.toLowerCase();//hello

//word1 is still HeLLo


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Replacements

Example: to convert word1 toupper case, replace

the reference with a new reference.

A common bug:

word1 = word1.toUpperCase();

word1.toUpperCase();word1

remains

unchanged


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Numbers to Strings

Three ways to convert a number into a string:

1. String s = "" + num;

2. String s = Integer.toString (i);

String s = Double.toString (d);

3. String s = String.valueOf (num);

Integerand Double

are wrapper classes

fromjava.lang that

represent numbers as

objects. They also

provide useful static

methods.

s = String.valueOf(123);//123

s = + 123;//123

s = Integer.toString(123);//123

s = Double.toString(3.14); //3.14


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Review Questions:

1. The String class is part ofwhatpackage?

2. What does the String class have thatother classes do not have?

3. Text enclosed in quotes is called ?

4. What is the returned value forRumplestiltskin.length()?

5. Define immutable objects.


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Review (contd):

6. How does immutability ofStrings

make Java more efficient?

7. How does immutability ofStringsmake Java less efficient?

8. How do you declare an empty string?

9. Why are String constructors not usedvery often?

10. Bob + + Smith is called ____ ?


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Review (contd):

11. String city = "Bloomington;

What is returned by city.charAt (2)?

12. By city.substring(2, 4)?

13. By city.lastIndexOf(o)?

14. By city.indexOf(3)?

15. What does the trimmethod do?


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Review (contd):

16. sam.equals(Sam) returns ?

17. What kind ofvalue does

sam.compareTo(Sam) return?18. What will be stored in s?

s = mint.replace(t, e);

19. What does s.toUpperCase() do to s?20. Name a simple way to convert a

number into a string.

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The String Class (1)