string class in java

String Class in Java

• java.lang Class String

• java.lang.Object java.lang.String

• We do not have to import the String class since it comes from java.lang.

• An object of the String class represents a string of characters. “abcde”;

Two ways to create a String object

• Create a String literal:

String str = "abc";

• With the key word new since it is an object

String str = new String(“abc”);

10-3

Empty Strings• An empty string has no characters;

• Its contents are “null”.

String s1 = “"; String s2 = new String();

Empty strings

String indexes

• String is a sequence of characters.

Each letter in the string has its own index location and can be accessed by it.

index location

10-4

0 1 2 3 4 5 6 7

c o m p u t e r

The length of the String is how many letters it contains: 8The last index of the String is length – 1; 7

10-5

Index locations Index locations are from 0 to length-1

String s = “strawberry”;

strawberry // 10 letters in the word

// index from 0 to 9

int len = s.length() // 10

int lastIndex = s.length()-1 //9

S is the String object I created. Objects call methods with a dot operator.

10-6

String Methods: • Page 78

• There are many ways to manipulate Strings.

• Look on page 78 those tested on AP

• You always use the period to separate the object from the method.

• int len = s1.length();

10-7

Methods — length()

int length (); returns an int

• Returns the number of characters in the string

6

4

int lenF = f.length();

int lenW = w.length();

Returns:

String f = “Flower”;

String w = “Wind”;

10-8

Methods — substring

String s2 = s.substring (i, k);

returns the substring of chars in positions from i to k-1

String s3 = s.substring (i);

returns the substring from i char to the end

String s = “strawberry”;

rawrawberry

String s2 = s.substring (2,5); start at 2 end at 4 String s3 = s.substring (2); start at 2 thru end

Returns:

strawberry

i k

strawberry

i

Strings have index locations from 0 to length-1

String n = “computer";

String one = n.substring(0,7);

String two = n.substring(1,6);

String three = n.substring(2,5);

String four = n.substring(4);

String five = n.substring(3);

String six = n.substring(1,n.length()-2);

String seven = six.substring(0, n.length()/2);

10-9

0 1 2 3 4 5 6 7

c o m p u t e r

s.substring(i, k);returns the substring of chars in positions from i to k-1

s.substring(i);returns the substring from i char to the end

10-10

Methods — Concatenation

String s1 = “obi”;String s2 = “wan”;

String result = s1 + s2;obiwan

String result = s1.concat (s2);the same as s1 + s2 obiwan

10-11

Methods — Find (indexOf)

String date ="July 5, 2012 1:28:19 PM";

date.indexOf ('J'); 0

date.indexOf ('2'); 8

date.indexOf ("2012"); 8

date.indexOf ('2', 9); 11

date.indexOf ("2020"); -1

date.lastIndexOf ('2'); 15

Returns:

(not found)

(starts searching at position 9)

0 8 11 15

Index of return the index location of the first occurrence of the character requested.