 Strings
 String Constructors
 String Methods
 An object of the String class represents a string of characters.
 The String class belongs to the java.lang package, which does not require an
import statement.
 Like other classes, String has constructors and methods.
 Unlike other classes, String has two operators, + and += (used for concatenation).
Example:
//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);
 No argument Constructor: No-argument constructor creates an empty String.
Rarely used.
String empty = new String();
A more common approach is to reassign the variable to an empty literal String.
(Often done to reinitialize a variable used to store input.)
String empty = “”;//nothing between quotes
 Copy Constructors: Copy constructor creates a copy of an existing String.
 Not the same as an assignment.
Copy Constructor: Each variable points to a different copy of the String.
String word = new String(“Java”);
String word2 = new String(word);
word
word2
“Java"
“Java"
Assignment: Both variables point to the same String.
String word = “Java”;
String word2 = word;
word
word2
“Java"
 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”
 For Example:
char[ ] charArray = {‘b’, ‘i’, ‘r’, ‘t’, ‘h’, ‘ ‘, ‘d’, ‘a’, ‘y’};
String s = new String(“Hello”);
String s1 = new String();
String s2 = new String(s);
Empty
Hello
String s3 = new String (charArray);
String s4 = new String(charArray, 3, 6);
birth day
day
 The String class defines a number of methods that allow us to accomplish a
variety of string manipulation tasks. Some of the most commonly used string methods
are discussed.
length, charAt:
int length();

Returns the number of characters in the
string.
char charAt(i);

Returns the char at position i.
 For Example:
”Problem".length();
Character positions in
strings are numbered
starting from 0 – just like
arrays.
”Window".charAt (2);
7
‘n’
substring: Returns a new String by copying characters from an existing String.
String subs = word.substring(i)

Returns the substring from the ith
character
to
the
end.
String subs = word.substring(i, k)

Returns the substring of characters in
position from i to k-1.
 For Example:
”television".substring (2, 5);
“lev”
“immutable".substring (2);
“mutable”
“bob".substring (4);
error
concat: String concatenation is the operation of joining two character strings end to
end For example: the string “snow” and “ball” may be concatenated to give
“snowball”.
 For Example:
String word1 = “re”;
String word2 = “think;
String word3 = “ing”;
int num = 2;
String result = word1 + word2;
String result1 = result.concat(word3);
result1 += num;
rethink
rethinking
rethinking2
Find: Often it’s useful to search a string for a characters. indexOf and lastIndexOf
are two methods that search for a specified characters or substring in a string.
 For Example:
String letters = “abcdefghijklmabcdefghijklm”;
letters.indexOf(‘c’);
2
letters.indexOf(‘a’, 1);
letters.indexOf(‘$’);
13
-1
letters.lastIndexOf(‘c’);
15
letters.lastIndexOf(‘a’, 25);
13
letters.lastindexOf(‘$’);
letters.indexOf(”def”);
-1
3
letters.indexOf(“def”, 7);
letters.indexOf(“hello”);
16
-1
letters.lastIndexOf(“def”);
letters.lastIndexOf(“def”, 25);
letters.lastindexOf(“hello”);
16
16
-1
Changing Case: converts the string either to lower or upper, the methods are
toLowerCase and toUpperCase.
 For Example:
String word = “heLLo”;
String word1 = word.toUpperCase();
HELLO
String word2 = word.toLowerCase();
hello
Trim: removes the white spaces at the beginning and end of the string.
 For Example:
String word = “ heLLo
JAVA ”;
String word1 = word.trim();
heLLo
Replace: replaces all the appearances of character x with y.
 For Example:
String word = “rare”
String word1 = word.replace(‘r’, ‘d’);
dade
JAVA
Equals: Compare two strings if equal returns true if not false. The methods are
equals() and equalsIgnoreCase(). It returns Boolean type.
 For Example:
boolean b = "Raiders".equals("Raiders");
true
boolean b1 = "Raiders".equals("raiders");
false
boolean b2 = "Raiders".equalsIgnoreCase("raiders");
true
compareTo: Compares this instance with a specified String object and indicates
whether this instance precedes, follows, or appears in the same position in the sort
order as the specified String.
int diff = word1.compareTo(word2);
returns the “difference” word1 - word2
int diff = word1.compareToIgnoreCase(word2);
returns the “difference” word1 - word2, case-blind
Usually programmers don’t 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) or positive (word1 comes after word2). Often used in
conditional statements.
 For Example:
int diff = “apple".compareTo(“berry");
<0
int diff1 = “Zebra".compareTo(“apple");
<0
int diff2 = “dig".compareTo(“dug");
<0
int diff3 = “dig”.compateTo(“digs”);
<0
int diff4 = “apple”.compateTo(“apple”);
0
int diff5 = “dig”.compateToIgnoreCase(“dIG”);
0
int diff3 = “berry”.compateTo(“apple”);
int diff3 = “BIT”.compateTo(“BIG”);
int diff3 = “apple”.compateTo(“Apple”);
int diff3 = “huge”.compateTo(“hug”);
>0
>0
>0
>0
Numbers to Strings: converts the parameter value to string representation. Three
ways to convert a number into a string
1. String s = "" + num;
s = “” + 123;//”123”
2. String s = Integer.toString (i);
String s = Double.toString (d);
s = Integer.toString(123);//”123”
s = Double.toString(3.14); //”3.14”
3. String s = String.valueOf (num);
s = String.valueOf(123);//”123”
1. What is the returned value for “Rumplestiltskin”.length()
2. String city = "Bloomington“;
3. city.charAt (2)?
4. city.substring(2, 4)?
5. city.lastIndexOf(‘o’)?
6. city.indexOf(3)?
7. "sam".equals("Sam") returns ?
8. What kind of value does "sam".compareTo("Sam") return?
9. What will be stored in s? s = "mint".replace(‘t’, ‘e’);
10. What does s.toUpperCase() do to s?