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Unit 6 String in Java


1. Introduction to String

In Java, a String is an object that represents a sequence of characters.
Strings are written using double quotation marks (" ").

Example:

String s = "Java";

A character array can also be used to create a String.

Example:

char[] ch = {'j','a','v','a'};

String s = new String(ch);

This is the same as:

String s = "java";

 Java StringBuffer Class

The StringBuffer class is used to create mutable (modifiable) string objects.

Unlike the String class, StringBuffer objects can be modified after creation.

·         String → Immutable (cannot change)

·         StringBuffer → Mutable (can change)

Features of StringBuffer

·         Characters can be inserted, replaced, or deleted

·         Automatically grows when more characters are added

·         Stores data in heap memory

·         Thread-safe (synchronized)

Important Constructors of StringBuffer

Constructor

Description

StringBuffer()

Creates an empty StringBuffer with initial capacity 16

StringBuffer(String str)

Creates a StringBuffer with the given string

StringBuffer(int capacity)

Creates an empty StringBuffer with specified capacity

 Java StringBuilder Class

The StringBuilder class is also used to create mutable strings.

It is similar to StringBuffer, but it is non-synchronized, meaning it is faster but not thread-safe.

It was introduced in JDK 1.5.

Important Constructors of StringBuilder

Constructor

Description

StringBuilder()

Creates an empty StringBuilder with capacity 16

StringBuilder(String str)

Creates a StringBuilder with the specified string

StringBuilder(int capacity)

Creates an empty StringBuilder with given capacity

 StringBuffer Methods

1. append() Method

The append() method adds a string at the end of the existing string.

Example:

class StringBufferExample {

    public static void main(String args[]) {

        StringBuffer sb = new StringBuffer("Hello ");

        sb.append("Java");   // original string changes

        System.out.println(sb);

    }

}

Output

Hello Java

 insert() Method

The insert() method inserts a string at a specified position.

Example:

class StringBufferExample2 {

    public static void main(String args[]) {

        StringBuffer sb = new StringBuffer("Hello ");

        sb.insert(1, "Java");

        System.out.println(sb);

    }

}

Output

HJavaello

 

3. replace() Method

The replace() method replaces characters between the specified index positions.

Example:

class StringBufferExample3 {

    public static void main(String args[]) {

        StringBuffer sb = new StringBuffer("Hello");

        sb.replace(1, 3, "Java");

        System.out.println(sb);

    }

}

Output

HJavalo

4. delete() Method

The delete() method removes characters between specified indexes.

Example:

class StringBufferExample4 {

    public static void main(String args[]) {

        StringBuffer sb = new StringBuffer("Hello");

        sb.delete(1, 3);

        System.out.println(sb);

    }

}

Output

Hlo

5. reverse() Method

The reverse() method reverses the string.

Example:

class StringBufferExample5 {

    public static void main(String args[]) {

        StringBuffer sb = new StringBuffer("Hello");

        sb.reverse();

        System.out.println(sb);

    }

}

Output

olleH

 Difference Between String and StringBuffer

No

String

StringBuffer

1

Immutable (cannot be changed)

Mutable (can be changed)

2

Creates new object when modified

Modifies existing object

3

Slower for concatenation

Faster for concatenation

4

Overrides equals() method

Does not override equals()

 

 Java String length() Method

The length() method returns the number of characters in a string.

Syntax

public int length()

Example:

class CalcLength {

    public static void main(String args[]) {

        String name = "Santosh";

        int length = name.length();



        System.out.println("The length of the String \"" + name + "\" is: " + length);

    }

}

Output

The length of the String "Santosh" is: 7

7. Java String concat() Method

The concat() method joins two or more strings together.

Syntax

string1.concat(string2)

Example:

public class ConcatExample {

    public static void main(String[] args) {



        String str1 = "Hello ";

        String str2 = "Ramesh ";

        String str3 = "Sharma";



        String str4 = str1.concat(str2);

        System.out.println(str4);



        String str5 = str1.concat(str2).concat(str3);

        System.out.println(str5);

    }

}

Output

Hello Ramesh

Hello Ramesh Sharma

 

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