Chapter 3: Strings in Java

Introduction to Strings

A string is a sequence of characters.

String is a class but can be used like a data type.

Strings are immutable and cannot be changed.

String Creation

Method 1: Using new keyword

StringCreation1.java

String name;
name = new String("Harry");

Method 2: Direct assignment (Recommended)

StringCreation2.java

String name = "Harry";

name

→

Reference

"Harry"

→

Object

Different Ways to Print in Java

1. System.out.print()

No newline at the end

Print.java

System.out.print("Hello ");
System.out.print("World");
// Output: Hello World

2. System.out.println()

Prints a newline at the end

Println.java

System.out.println("Hello");
System.out.println("World");
// Output: 
// Hello
// World

3. System.out.printf()

Formatted printing

Printf.java

System.out.printf("Name: %s, Age: %d", "John", 25);
// Output: Name: John, Age: 25

Format Specifiers:

%d for int

%f for float

%c for char

%s for string

4. System.out.format()

Same as printf()

String Methods

String methods operate on Java Strings. They can be used to find length, convert to lowercase, etc.

StringExample.java

String name = "Harry";
//           01234 (index positions)

1. length()

Returns length of String

Length.java

name.length() // Returns 5

2. toLowerCase()

Returns new String with lowercase characters

ToLowerCase.java

name.toLowerCase() // Returns "harry"

3. toUpperCase()

Returns new String with uppercase characters

ToUpperCase.java

name.toUpperCase() // Returns "HARRY"

4. trim()

Removes leading and trailing spaces

Trim.java

String str = "  Harry  ";
str.trim() // Returns "Harry"

5. substring(int start)

Returns substring from start to end

Substring1.java

name.substring(3) // Returns "ry"

6. substring(int start, int end)

Returns substring from start to end (exclusive)

Substring2.java

name.substring(1, 4) // Returns "arr"

7. replace(char old, char new)

Returns new string after replacing characters

Replace.java

name.replace('r', 'p') // Returns "Happy"

8. startsWith(String prefix)

Returns true if string starts with prefix

StartsWith.java

name.startsWith("Ha") // Returns true

9. endsWith(String suffix)

Returns true if string ends with suffix

EndsWith.java

name.endsWith("ry") // Returns true

10. charAt(int index)

Returns character at given index

CharAt.java

name.charAt(2) // Returns 'r'

11. indexOf(String str)

Returns index of first occurrence

IndexOf.java

name.indexOf("ar") // Returns 1
name.indexOf("z") // Returns -1

12. equals(String str)

Returns true if strings are equal (case sensitive)

Equals.java

name.equals("Harry") // Returns true
name.equals("harry") // Returns false

Escape Sequence Characters

Sequence of characters after backslash '\' = Escape sequence characters.

Escape sequence characters consist of more than one character but represent one character when used within strings.

Escape Sequence	Description	Example
`\n`	Newline	`"Hello\nWorld"`
`\t`	Tab	`"Hello\tWorld"`
`\'`	Single quote	`"It\'s working"`
`\"`	Double quote	`"He said \"Hello\""`
`\\`	Backslash	`"C:\\Users\\Name"`

Practice Set

1. Convert string to lowercase

StringLowercase.java

public class StringLowercase {
    public static void main(String[] args) {
        String str = "HELLO WORLD";
        System.out.println(str.toLowerCase());
    }
}

2. Replace spaces with underscores

ReplaceSpaces.java

public class ReplaceSpaces {
    public static void main(String[] args) {
        String str = "Hello World Java";
        String result = str.replace(' ', '_');
        System.out.println(result); // Hello_World_Java
    }
}

3. Fill letter template

LetterTemplate.java

public class LetterTemplate {
    public static void main(String[] args) {
        String letter = "Dear <|name|>, Thanks a lot";
        String name = "Harry";
        String result = letter.replace("<|name|>", name);
        System.out.println(result);
    }
}

Chapter 3 - Practice Set

1. String to Lowercase

Write a Java program to convert a string to lowercase.

2. Replace Spaces

Write a Java program to replace spaces with underscores.

3. Letter Template

Write a Java program to fill in a letter template which looks like: letter = "Dear <|name|>, Thanks a lot". Replace <|name|> with a string (some name).

4. Detect Multiple Spaces

Write a Java program to detect double and triple spaces in a string.

5. Format with Escape Sequences

Write a program to format the following letter using escape sequence characters: letter = "Dear Harry, This Java Course is nice. Thanks"

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