Lesson Overview

Text data is used almost everywhere in software applications.

For example:

• User name
• Email address
• Course title
• Product description
• Error message
• Search query
• Notification

In Java, we use String to represent text.

String courseName = "Java and OOP Foundation";

String is not a primitive data type. It is a class from the Java standard library.

In this lesson, we will learn:

• What String is
• String literals
• Empty, blank, and null
• String concatenation
• String length and character access
• Common String methods
• String comparison
• String immutability
• Escape sequences
• Text blocks
• The difference between String and char

Learning Objectives

After completing this lesson, you will be able to:

• Create String variables
• Distinguish between empty, blank, and null
• Concatenate Strings
• Access String length and characters
• Use common String methods
• Compare String content using .equals()
• Understand what String immutability means
• Use escape sequences
• Use text blocks for multi-line text

What Is String?

String represents text.

String studentName = "Sakib";
String courseName = "Java Foundation";
String email = "[email protected]";

Here:

String courseName = "Java Foundation";

• String — type
• courseName — variable name
• "Java Foundation" — String value

String Is Not Primitive

Java's primitive types are:

byte
short
int
long
float
double
char
boolean

String is not part of this list.

String is a class, and a String variable is a reference-type variable.

However, Java provides convenient syntax for String literals:

"Java Foundation"

String Literal

Text written inside double quotes is a String literal.

"Java"

"Welcome to LiveKlass"

""

The last value is an empty String.

String Variable

To declare a variable:

String name;

To initialize it:

String name = "Sakib";

or:

String name;

name = "Sakib";

Empty String

An empty String contains no characters.

String value = "";

Length:

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

Output:

0

Blank String

A blank String can be empty or contain only whitespace.

Examples:

""

" "

"   "

You can check this using Java's isBlank() method:

String value = "   ";

System.out.println(value.isBlank());

Output:

true

Difference Between Empty and Blank

String emptyValue = "";
String blankValue = "   ";

System.out.println(emptyValue.isEmpty());
System.out.println(blankValue.isEmpty());

Output:

true
false

But:

System.out.println(emptyValue.isBlank());
System.out.println(blankValue.isBlank());

Output:

true
true

Difference:

• isEmpty() → checks whether the length is 0
• isBlank() → checks whether the String is empty or contains only whitespace

null

null means that the variable currently does not reference a String object.

String courseName = null;

This is not an empty String.

String emptyValue = "";
String missingValue = null;

Difference:

""   → A String exists, but it contains no characters
null → There is no String object reference

Calling a Method on null

The following code will fail at runtime:

String courseName = null;

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

because courseName does not reference a String object.

This can cause a NullPointerException.

This is why understanding the difference between null and an empty String is important.

String Concatenation

Combining multiple Strings is called concatenation.

In Java, the + operator can be used.

String firstName = "Sakib";
String lastName = "Sami";

String fullName = firstName + " " + lastName;

System.out.println(fullName);

Output:

Sakib Sami

Concatenating Text and Other Values

String courseName = "Java Foundation";
int durationInWeeks = 8;

System.out.println(
        "Course: " + courseName
        + ", Duration: " + durationInWeeks + " weeks"
);

Output:

Course: Java Foundation, Duration: 8 weeks

Concatenation and Calculation

Look at this code:

System.out.println("Total: " + 10 + 20);

Output:

Total: 1020

because evaluation happens from left to right:

"Total: " + 10
→ "Total: 10"

"Total: 10" + 20
→ "Total: 1020"

Use parentheses if the calculation should happen first:

System.out.println("Total: " + (10 + 20));

Output:

Total: 30

length()

To get the length of a String:

String language = "Java";

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

Output:

4

For simple Latin text, you can think of this as the character count. Technically, Java's String.length() counts UTF-16 code units.

String Index

String positions start from 0.

String: Java
Index:  0123

Character	Index
J	0
a	1
v	2
a	3

charAt()

To get the character at a specific position:

String language = "Java";

char firstCharacter = language.charAt(0);

System.out.println(firstCharacter);

Output:

J

Last Character

The last index is:

length - 1

Example:

String language = "Java";

char lastCharacter =
        language.charAt(language.length() - 1);

System.out.println(lastCharacter);

Output:

a

Invalid Index

String language = "Java";

language.charAt(4);

This will fail at runtime.

The valid indexes are:

0
1
2
3

substring()

You can use substring() to extract part of a String.

String value = "Java Foundation";

String language = value.substring(0, 4);

System.out.println(language);

Output:

Java

Here:

• Start index 0 is included
• End index 4 is excluded

From an Index to the End

String value = "Java Foundation";

String secondPart = value.substring(5);

System.out.println(secondPart);

Output:

Foundation

toUpperCase() and toLowerCase()

String language = "Java";

System.out.println(language.toUpperCase());
System.out.println(language.toLowerCase());

Output:

JAVA
java

strip()

To remove whitespace from the beginning and end of a String:

String name = "   Sakib   ";

String cleanName = name.strip();

System.out.println(cleanName);

Output:

Sakib

Whitespace in the middle is not removed.

String name = "Sakib Sami";

System.out.println(name.strip());

Output:

Sakib Sami

contains()

Checks whether specific text exists inside a String.

String courseName = "Java and OOP Foundation";

System.out.println(courseName.contains("OOP"));

Output:

true

It is case-sensitive.

System.out.println(courseName.contains("oop"));

Output:

false

startsWith() and endsWith()

String courseName = "Java Foundation";

System.out.println(
        courseName.startsWith("Java")
);

Output:

true

File name example:

String fileName = "Main.java";

System.out.println(
        fileName.endsWith(".java")
);

Output:

true

replace()

To replace text in a String:

String message = "I am learning Python";

String updatedMessage =
        message.replace("Python", "Java");

System.out.println(updatedMessage);

Output:

I am learning Java

Common String Methods

Method	Purpose
length()	Returns the length
charAt(index)	Returns the character at a specific position
substring()	Returns part of a String
toUpperCase()	Returns an uppercase version
toLowerCase()	Returns a lowercase version
strip()	Removes surrounding whitespace
contains()	Checks whether text exists
startsWith()	Checks a prefix
endsWith()	Checks a suffix
replace()	Replaces text
isEmpty()	Checks whether the length is 0
isBlank()	Checks whether the String is empty or whitespace-only

Comparing Strings

Use .equals() to compare the content of two Strings.

String firstLanguage = "Java";
String secondLanguage = "Java";

System.out.println(
        firstLanguage.equals(secondLanguage)
);

Output:

true

Case Matters

String firstLanguage = "Java";
String secondLanguage = "java";

System.out.println(
        firstLanguage.equals(secondLanguage)
);

Output:

false

Compare While Ignoring Case

System.out.println(
        firstLanguage.equalsIgnoreCase(secondLanguage)
);

Output:

true

Do Not Use == to Compare String Content

Look at the following example:

String firstValue = new String("Java");
String secondValue = new String("Java");

System.out.println(firstValue == secondValue);
System.out.println(firstValue.equals(secondValue));

Output:

false
true

because:

• == compares whether the references point to the same object
• .equals() compares String content

Therefore, for String content use:

firstValue.equals(secondValue)

String Is Immutable

Java String is immutable.

This means:

After a String object is created, its existing content cannot be changed.

Look at this code:

String language = "Java";

language.toUpperCase();

System.out.println(language);

Output:

Java

Why?

toUpperCase() does not modify the original String. It returns a new String.

Store the Returned String

String language = "Java";

language = language.toUpperCase();

System.out.println(language);

Output:

JAVA

Remember the same behavior for methods such as strip(), replace(), and toLowerCase().

Escape Sequences

Escape sequences are used to represent special characters inside String literals.

Common escape sequences:

Escape	Meaning
\n	New line
\t	Tab
\"	Double quote
\\	Backslash

New Line

System.out.println("Java\nFoundation");

Output:

Java
Foundation

Tab

System.out.println("Name\tScore");
System.out.println("Sakib\t90");

Double Quote

System.out.println(
        "He said, \"Learn Java.\""
);

Output:

He said, "Learn Java."

Backslash

Windows path:

C:\Java\Projects

Java String:

String path = "C:\\Java\\Projects";

Text Blocks

Java supports text blocks for writing readable multi-line text.

String message = """
        Java and OOP Foundation
        Language: Bangla
        Level: Beginner
        """;

Print:

System.out.println(message);

Output:

Java and OOP Foundation
Language: Bangla
Level: Beginner

Text blocks can be useful for:

• Multi-line messages
• JSON
• SQL
• HTML
• Test data

String and char

String and char are different types.

char grade = 'A';
String courseName = "Java";

char	String
Primitive type	Reference type
Single UTF-16 code unit	Text sequence
Single quotes	Double quotes
'A'	"A"

There Is No Empty char

Invalid:

char value = '';

But an empty String is valid:

String value = "";

Common Errors

Using Single Quotes for a String

Wrong:

String language = 'Java';

Correct:

String language = "Java";

Calling a Method on null

Wrong:

String value = null;

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

This can cause a NullPointerException at runtime.

Comparing Content With ==

Avoid:

firstValue == secondValue

To compare content:

firstValue.equals(secondValue)

Ignoring the Returned String

Wrong expectation:

String value = "java";

value.toUpperCase();

System.out.println(value);

Output:

java

Correct:

value = value.toUpperCase();

Invalid Index

String value = "Java";

value.charAt(4);

This will fail at runtime.

Valid last index:

3

Concatenation Instead of Addition

System.out.println("Result: " + 10 + 20);

Output:

Result: 1020

Correct:

System.out.println(
        "Result: " + (10 + 20)
);

Output:

Result: 30

Complete Example

public class Main {

    public static void main(String[] args) {
        String rawCourseName =
                "   Java and OOP Foundation   ";

        String courseName = rawCourseName.strip();

        System.out.println(
                "Course: " + courseName
        );

        System.out.println(
                "Length: " + courseName.length()
        );

        System.out.println(
                "Starts with Java: "
                + courseName.startsWith("Java")
        );

        System.out.println(
                "Contains OOP: "
                + courseName.contains("OOP")
        );

        System.out.println(
                "Uppercase: "
                + courseName.toUpperCase()
        );
    }
}

Possible output:

Course: Java and OOP Foundation
Length: 23
Starts with Java: true
Contains OOP: true
Uppercase: JAVA AND OOP FOUNDATION

Important Terms

String

A Java class used to represent a sequence of text.

String Literal

Text written inside double quotes.

"Java"

Empty String

A zero-length String.

""

Blank String

An empty or whitespace-only String.

null

The absence of an object reference.

Concatenation

Combining Strings and other values.

Index

A position inside a String, starting from 0.

Immutability

The property that an existing String object's content cannot be changed.

Escape Sequence

Syntax used to represent special characters inside a String.

Text Block

Syntax used to write multi-line Strings.

Practice Exercise 1: Create Strings

Create String variables for the following information:

1. Student name
2. Course name
3. Email address
4. Country
5. Error message

Practice Exercise 2: Empty, Blank, or null?

Classify each value:

String firstValue = "";
String secondValue = "   ";
String thirdValue = null;
String fourthValue = "Java";

Practice Exercise 3: Length and Index

String language = "Java";

Answer:

1. What is the length?
2. What is the index of the first character?
3. What is the index of the last character?
4. What does charAt(2) return?

Practice Exercise 4: Concatenation

String firstName = "Sakib";
String lastName = "Sami";

Expected output:

Full name: Sakib Sami

Practice Exercise 5: Predict the Output

System.out.println("Result: " + 10 + 20);
System.out.println("Result: " + (10 + 20));

Practice Exercise 6: Normalize Text

Input:

String rawName = "   md sakib   ";

Use strip() and toUpperCase() to produce:

MD SAKIB

Practice Exercise 7: Compare Strings

String firstValue = "Java";
String secondValue = "java";

Print:

• The .equals() result
• The .equalsIgnoreCase() result

Practice Exercise 8: Fix the Immutability Issue

String language = "java";

language.toUpperCase();

System.out.println(language);

Expected output:

JAVA

Practice Exercise 9: Escape Sequence

Use a String to produce:

Course: "Java Foundation"
Path: C:\Java\Projects

Practice Exercise 10: Text Block

Use a text block to produce:

Course: Java and OOP Foundation
Language: Bangla
Level: Beginner

Knowledge Check

Question 1

What is String?

Question 2

Is String a primitive type?

Question 3

Which quotation marks are used for String literals?

Question 4

What is the difference between an empty String and null?

Question 5

What is the difference between isEmpty() and isBlank()?

Question 6

Which operator can be used for String concatenation?

Question 7

Where does a String index start?

Question 8

What does charAt() do?

Question 9

In substring(0, 4), is index 4 included?

Question 10

Which method should be used to compare String content?

Question 11

What is the difference between == and .equals()?

Question 12

What does it mean that String is immutable?

Question 13

Does toUpperCase() modify the original String?

Question 14

What does strip() do?

Question 15

What are text blocks used for?

Question 16

What is the main difference between String and char?

Knowledge Check Answers

Answer 1

String is a Java class used to represent a sequence of text.

Answer 2

No. String is a reference type.

Answer 3

Double quotes:

"Java"

Answer 4

An empty String is a valid zero-length String object. null means that the variable does not reference a String object.

Answer 5

isEmpty() checks whether the length is 0. isBlank() checks whether the String is empty or contains only whitespace.

Answer 6

+

Answer 7

The index starts from 0.

Answer 8

It returns the char value at a specific index.

Answer 9

No. The end index is excluded.

Answer 10

equals()

Answer 11

== compares reference identity. .equals() compares String content.

Answer 12

The content of an existing String object cannot be changed.

Answer 13

No. It returns a new String.

Answer 14

It removes whitespace from the beginning and end of a String and returns a new String.

Answer 15

Text blocks are used to write readable multi-line Strings.

Answer 16

char is a primitive type that represents a single UTF-16 code unit. String is a reference type that represents a sequence of text.

Lesson Summary

In this lesson, we learned:

• String represents text
• String is not a primitive type
• String literals are written inside double quotes
• Empty String, blank String, and null are different
• Calling a method on null can cause a NullPointerException
• + can be used to concatenate Strings
• Parentheses help control the order of calculation and concatenation
• length() returns the length of a String
• String indexes start from 0
• charAt() returns the character at a specific position
• substring() returns part of a String
• strip() removes surrounding whitespace
• contains(), startsWith(), and endsWith() help inspect Strings
• replace() returns a new modified String
• .equals() should be used to compare String content
• .equalsIgnoreCase() can be used for case-insensitive comparison
• == should not be used for String content comparison
• String is immutable
• String methods generally return new Strings instead of modifying the original
• Escape sequences represent special characters
• Text blocks help write readable multi-line text
• String and char are different types