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Java Strings
Presented By:
Rabia Saeed 301

Strings
Intr0duction
The String class
The Character class
The StringBuffer class

Introduction
• A string is sequence (series) of characters.
• A string is NOT an array of characters.
E.g. in C/C++:

char s[20];

• In Java, A String is an object.

• Java has 3 String classes:
• String
• StringBuffer
• StringTokenizer

The String Class(1)
• String is in java.lang package.
• Since java.lang.* is always imported automatically, we don’t
need to import the String class.
• Declaration:
String s1;

• Initialization:
s1=“Information Technology”;

• Or, short-cut:
String s1=“Information Technology”;

String Class(2)
• Because String is a class, then s1 is an object.
• So there should be constructors, methods, or properties.
• String constructors:
• String()
• String(String value)
• String(char[] value)
Ex: String s1 = new String(“IT”);

• You just say:
String s1 = “IT”;

The String Class
• Constructing a String:
• String message = "Welcome to Java“;
• String message = new String("Welcome to Java“);

• String s = new String();

• Obtaining String length and Retrieving Individual Characters
in a string

C0nstructing Strings
String newString = new String(stringLiteral);
String message = new String("Welcome to Java");
Since strings are used frequently, Java provides a shorthand
initializer for creating a string:

String message = "Welcome to Java";

The String Class
• How do I get those methods to use?
• A. You just declare a variable as String.
String s1=“”;
Then, you call a method, say length().
System.out.println(s1.length());

• Let’s try this out:
Public class TestString {
public static void main(String[] args){
String s1=“npic”;

Strings Are Immutable
A String object is immutable; its contents cannot be changed.
• has no setter method).
• The String class is final so we cannot inherit from it. Does the
following code change the contents of the string?
String s = "Java";
s = "HTML";

Trace C0de
String s=“Java”;
s = "HTML";

After executing s = "HTML";

After executing String s = "Java";
s

: String
String object for "Java"

Contents cannot be changed

s

: String

: String
String object for "HTML"

This string object is
now unreferenced

Trace C0de
String s = "Java“;

s= “HTML”;

After executing s = "HTML";

After executing String s = "Java";
s

: String

Contents cannot be changed

s

: String

: String
String object for "HTML"

This string object is
now unreferenced

Interned Strings
• Since strings are immutable and are frequently used, to improve
efficiency and save memory, the JVM uses a unique instance for
string literals with the same character sequence. Such an instance
is called interned. You can also use a String object’s intern method
to return an interned string. For example, the following
statements:

Examples
String s = "Welcome to Java";

s
s2

String s1 = new String("Welcome to Java");

• String
d

s3

: String
Interned string object for
"Welcome to Java"

s2 = s1.intern();

String s3 = "Welcome to Java";
System.out.println("s1 == s is " + (s1 == s));
System.out.println("s2 == s is " + (s2 == s));
System.out.println("s == s3 is " + (s == s3));

display
s1 == s is false
s2 == s is true
s == s3 is true

s1

: String
A string object for
"Welcome to Java"

A new object is created if you use the new
operator.
If you use the string initializer, no new object
is created if the interned object is already
created.

Trace C0de

•t
String s2 = s1.intern();

s

: String
"Welcome to Java"

Trace C0de

s

•d

: String
"Welcome to Java"


s1

: String
A string object for
"Welcome to Java"

Trace C0de
•String s = "Welcome to Java";
d

s
s2

: String
"Welcome to Java"


s1

: String
A string object for
"Welcome to Java"

Trace C0de

s

•f

s2
s3

: String
"Welcome to Java"


s1

: String
A string object for
"Welcome to Java"

Finiding String Length
Finding string length using the length() method:
message = "Welcome";
message.length() (returns 7)

Retrieving Individual Characters in a String
• Do not use message[0]
• Use message.charAt(index)
• Index starts from 0
Indices

0

1

2

3

4

5

6

message

W

e

l

c

o

m

e

message.charAt(0)

7

8

9

t

o

message.length() is 15

10 11 12 13 14
J

a

v

a

message.charAt(14)

String C0ncatinati0n
String s3 = s1.concat(s2);
String s3 = s1 + s2;
s1 + s2 + s3 + s4 + s5 same as
(((s1.concat(s2)).concat(s3)).concat(s4)).concat(s5);

Extracting Substring
You can extract a single character from a string
using the charAt method. You can also extract a
substring from a string using the substring method in
the String class.
String s2 = s1.substring(0, 11) + "HTML";

Indices

0

1

2

3

4

5

6

message

W

e

l

c

o

m

e

7

8

9

t

o

message.substring(0, 11)

10 11 12 13 14
J

a

v

a

message.substring(11)

String Comparisons
• equals
String s1 = new String("Welcome“);
String s2 = "welcome";
if (s1.equals(s2)){
// s1 and s2 have the same contents

}
if (s1 == s2) {
// s1 and s2 have the same reference

}

String Comparisons, cont.
• compareTo(Object object)
String s1 = new String("Welcome“);
String s2 = "welcome";
if (s1.compareTo(s2) > 0) {
// s1 is greater than s2
}
else if (s1.compareTo(s2) == 0) {
// s1 and s2 have the same contents
}
else
// s1 is less than s2

String C0nversi0ns
The contents of a string cannot be changed once the string is
created. But you can convert a string to a new string using the
following methods:
•
•
•
•

toLowerCase
toUpperCase
trim
replace(oldChar, newChar)

Finding a Character or a Substring in a String
"Welcome
"Welcome
"Welcome
"Welcome
"Welcome
"Welcome
"Welcome

to
to
to
to
to
to
to

Java".indexOf('W') returns 0.
Java".indexOf('x') returns -1.
Java".indexOf('o', 5) returns 9.
Java".indexOf("come") returns 3.
Java".indexOf("Java", 5) returns 11.
Java".indexOf("java", 5) returns -1.
Java".lastIndexOf('a') returns 14.

The StringBuffer Class
• The StringBuffer class is an alternative to the String class. In
general, a string buffer can be used wherever a string is used.
StringBuffer is more flexible than String. You can add,
insert, or append new contents
into a string buffer. However, the value of
a String object is fixed once the string is created

Example
• Count the number of words in a given String
Example:
Input: National Polytechnic Institute of Cambodia
Output: Word Count: 5 words

java.lang.StringBuffer
+StringBuffer()

Constructs an empty string buffer with capacity 16

+StringBuffer(capacity: int)

Constructs a string buffer with the specified capacity

+StringBuffer(str: String)

Constructs a string buffer with the specified string

+append(data: char[]): StringBuffer

Appends a char array into this string buffer

+append(data: char[], offset: int, len: int): StringBuffer

Appends a subarray in data into this string buffer

+append(v: aPrimitiveType): StringBuffer

Appends a primitive type value as string to this buffer

+append(str: String): StringBuffer

Appends a string to this string buffer

+capacity(): int

Returns the capacity of this string buffer

+charAt(index: int): char

Returns the character at the specified index

+delete(startIndex: int, endIndex: int): StringBuffer

Deletes characters from startIndex to endIndex

+deleteCharAt(int index): StringBuffer

Deletes a character at the specified index

+insert(index: int, data: char[], offset: int, len: int):
StringBuffer

Inserts a subarray of the data in the array to the buffer at
the specified index

+insert(offset: int, data: char[]): StringBuffer

Inserts data to this buffer at the position offset

+insert(offset: int, b: aPrimitiveType): StringBuffer

Inserts a value converted to string into this buffer

+insert(offset: int, str: String): StringBuffer

Inserts a string into this buffer at the position offset

+length(): int

Returns the number of characters in this buffer

+replace(int startIndex, int endIndex, String str):
StringBuffer

Replaces the characters in this buffer from startIndex to
endIndex with the specified string

+reverse(): StringBuffer

Reveres the characters in the buffer

+setCharAt(index: int, ch: char): void

Sets a new character at the specified index in this buffer

+setLength(newLength: int): void

Sets a new length in this buffer

+substring(startIndex: int): String

Returns a substring starting at startIndex

+substring(startIndex: int, endIndex: int): String

Returns a substring from startIndex to endIndex

StringBuffer Constructors
• public StringBuffer()
No characters, initial capacity 16 characters.
• public StringBuffer(int length)
No characters, initial capacity specified by the length argument.
• public StringBuffer(String str)
Represents the same sequence of characters
as the string argument. Initial capacity 16
plus the length of the string argument.

Appending New Contents
into a String Buffer
StringBuffer strBuf = new StringBuffer();
strBuf.append("Welcome");
strBuf.append(' ');
strBuf.append("to");
strBuf.append(' ');
strBuf.append("Java");

The StringTokenizer Class
java.util.StringTokenizer
+StringTokenizer(s: String)

Constructs a string tokenizer for the string.

+StringTokenizer(s: String, delimiters: Constructs a string tokenizer for the string
with the specified delimiters.
String)
+StringTokenizer(s: String, delimiters: Constructs a string tokenizer for the string
with the delimiters and returnDelims.
String, returnDelimiters: boolean)
+countTokens(): int

Returns the number of remaining tokens.

+hasMoreTokens(): boolean

Returns true if there are more tokens left.

+nextToken(): String

Returns the next token.

+nextToken(delimiters: String): String Returns the next token using new delimiters.

Example
String s = "Java is cool.";
StringTokenizer tokenizer = new StringTokenizer(s);
System.out.println("The total number of tokens is " +
tokenizer.countTokens());
The total number of tokens is
while (tokenizer.hasMoreTokens())
3
System.out.println(tokenizer.nextToken());
Java
The code displays
is
cool.

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