Strings in Java

What is a string in Java?

A string in Java is a sequence of characters. It is used to store and process text data.

Example:

String name = "Groot Academy";

Here is a string variable that stores the text value "Groot Academy".

A string can contain letters, numbers, symbols, and spaces.

Example:

String message = "Welcome to Core Java Course";
String email = "info@grootsoftware.com";
String password = "Java@123";

How Strings Work in Java

In Java, String String is not a primitive data type. It is a class available in the java.lang package.

When we create a string, Java creates an object of the String class.

Example:

String course = "Core Java";

Java stores string values in a special memory area called the String Constant Pool. This helps Java save memory when the same string value is used multiple times.

Example:

String a = "Java";
String b = "Java";

Here, both a can refer to the same value in the String Constant Pool.

Ways to Create a String in Java

There are two common ways to create a string in Java.

1. Using String Literal

String name = "Java Programming";

This is the most common and recommended way to create a string.

2. Using New Keyword

String name = new String("Java Programming");

This creates a new string object in memory.

Simple String Example

public class StringExample {
    public static void main(String[] args) {
        String course = "Core Java";

        System.out.println(course);
    }
}

Output:

Core Java

Common String Methods in Java

Java provides many built-in methods to work with strings. These methods help us perform different operations on text data.

1. length()

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

String name = "Java";
System.out.println(name.length());

Output:

4

2. charAt()

The charAt() method returns a character at a specific index.

String name = "Java";
System.out.println(name.charAt(0));

Output:

J

Java string indexing starts from 0.

3. toUpperCase()

The toUpperCase() method converts a string into uppercase letters.

String name = "java";
System.out.println(name.toUpperCase());

Output:

JAVA

4. toLowerCase()

The toLowerCase() method converts a string into lowercase letters.

String name = "JAVA";
System.out.println(name.toLowerCase());

Output:

java

5. concat()

The concat() method joins two strings.

String first = "Core";
String second = " Java";

System.out.println(first.concat(second));

Output:

Core Java

6. equals()

The equals() method compares the actual value of two strings.

String a = "Java";
String b = "Java";

System.out.println(a.equals(b));

Output:

true

7. contains()

The contains() method checks whether a string contains a specific word or character sequence.

String course = "Core Java Programming";

System.out.println(course.contains("Java"));

Output:

true

8. substring()

The substring() method returns a part of a string.

String course = "Java Programming";

System.out.println(course.substring(0, 4));

Output:

Java

String Comparison in Java

String comparison is used to compare two string values.

There are two common ways to compare strings:

1. Using ==
2. Using equals()

Using == Operator

The == operator compares memory references, not actual string values.

String a = "Java";
String b = "Java";

System.out.println(a == b);

Output:

true

But when we create a string using the new keyword

String a = new String("Java");
String b = new String("Java");

System.out.println(a == b);

Output:

false

This happens because both objects are created at different memory locations.

Using equals() Method

The equals() method compares the actual value of two strings.

String a = new String("Java");
String b = new String("Java");

System.out.println(a.equals(b));

Output:

true

That is why it equals() is mostly used for string comparison in Java.

String Immutability in Java

Strings in Java are immutable. “Immutable” means once a string object is created, its value cannot be changed.

Example:

String name = "Java";
name.concat(" Programming");

System.out.println(name);

Output:

Java

The value did not change because strings are immutable.

To store the changed value, we need to assign it again:

String name = "Java";
name = name.concat(" Programming");

System.out.println(name);

Output:

Java Programming

Why are strings immutable in Java?

Strings are immutable in Java for better memory management, security, and performance.

String immutability helps in:

• Saving memory using the string constant pool
• Improving security
• Making strings safe in multi-threaded programs
• Improving performance in Java applications

Difference Between String, StringBuffer, and StringBuilder

StringBuffer in Java

StringBuffer is used when we need to modify string data multiple times.

Example:

public class StringBufferExample {
    public static void main(String[] args) {
        StringBuffer sb = new StringBuffer("Java");
        sb.append(" Programming");

        System.out.println(sb);
    }
}

Output:

Java Programming

Unlike String changes, the same object is created again and again.

StringBuilder in Java

StringBuilder is also used for mutable strings. It is faster StringBuffer because it is not synchronized.

Example:

public class StringBuilderExample {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("Core");
        sb.append(" Java");

        System.out.println(sb);
    }
}

Output:

Core Java

StringBuilder is commonly used when we need better performance in single-threaded programs.

Real-Life Example of String

Suppose we want to store student information such as name, email, and course.

public class StudentInfo {
    public static void main(String[] args) {
        String name = "Rahul Sharma";
        String email = "rahul@example.com";
        String course = "Core Java";

        System.out.println("Student Name: " + name);
        System.out.println("Email: " + email);
        System.out.println("Course: " + course);
    }
}

Output:

Student Name: Rahul Sharma
Email: rahul@example.com
Course: Core Java

This shows how strings are used in real-world programs to store text data.

Important Points About Strings in Java

• A string is a sequence of characters.
• String is a class in Java, not a primitive data type.
• Strings are used to store text data.
• String indexing starts from 0.
• Strings are immutable in Java.
• The equals() method is used to compare string values.
• String methods are used to perform operations on text.
• StringBuffer and StringBuilder are used for mutable strings.

Advantages of Strings in Java

Strings are useful because they make text handling easy. Java provides many built-in string methods, which help in searching, comparing, joining, converting, and modifying text data.

Strings are commonly used in forms, login systems, student records, email validation, search features, and many real-world applications.

Practice Programs for Students

Students should practice the following programs after learning strings:

1. Print a string in Java
2. Find the length of a string
3. Convert string to uppercase
4. Convert string to lowercase
5. Compare two strings
6. Reverse a string
7. Check palindrome string
8. Count vowels in a string
9. Count words in a string
10. Find duplicate characters in a string

Conclusion

Strings in Java are used to store and process text data. They are one of the most important topics in Core Java because almost every real-world application uses strings in some form.

In this lesson, students learned what strings are, how to create strings, how strings work in memory, common string methods, string comparison, string immutability, StringBuffer, StringBuilder, and practical uses of strings.

After completing this topic, students can move to the next lesson: String Methods in Java, where they will learn important string functions in detail with examples.