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In real programs, strings are not static — they constantly get combined, searched, cleaned up, and transformed. This is called string manipulation: applying operations to change, inspect, or extract information from text.

String Concatenation

The simplest operation is concatenation — joining strings together using the + operator:

Main.java


              1234567891011
            
package com.example;

public class Main {
   public static void main(String[] args) {

       String customerName = "Ann";
       String welcomeMessage = "Welcome " + customerName;

       System.out.println(welcomeMessage); // Welcome Ann
   }
}

All other string operations are based on methods — built-in actions Java provides. Instead of writing logic from scratch, we call these methods and let Java handle the operation. Think of them as ready-made tools applied directly to a string.

length()

If you need to know how many characters a string contains — including spaces — use the .length() method. It returns an int.

Main.java


              12345678910111213
            
package com.example;

public class Main {
   public static void main(String[] args) {

       String customerName = "Ann";
       String welcomeMessage = "Welcome " + customerName;

       int messageLength = welcomeMessage.length();

       System.out.println(messageLength);
   }
}

"Welcome Ann" has 11 characters including the space, so messageLength stores 11 and that's what gets printed.

toUpperCase()

If you need to convert all characters in a string to uppercase — for example on receipts or labels — use .toUpperCase(). It returns a new String.

Main.java


              123456789101112
            
package com.example;

public class Main {
   public static void main(String[] args) {

       String customerName = "anna";

       String upperCaseName = customerName.toUpperCase();

       System.out.println(upperCaseName);
   }
}

.toUpperCase() doesn't modify the original customerName — it creates a new String with all letters capitalized, so "anna" becomes "ANNA".

toLowerCase()

If you need all characters in lowercase — useful for consistency or comparison logic — use .toLowerCase(). It returns a new String.

Main.java


              123456789101112
            
package com.example;

public class Main {
   public static void main(String[] args) {

       String order = "Large Latte with oat milk";

       String lowerCaseOrder = order.toLowerCase();

       System.out.println(lowerCaseOrder);
   }
}

A new String is created with every character converted to lowercase, so the output is "large latte with oat milk".

contains()

If you need to check whether a string includes a specific word or piece of text, use .contains(). It returns a boolean.

Main.java


              123456789101112
            
package com.example;

public class Main {
   public static void main(String[] args) {

       String order = "Large Latte with oat milk";

       boolean containsLatte = order.contains("Latte");

       System.out.println(containsLatte);
   }
}

.contains() scans the entire string and checks if "Latte" appears anywhere inside it. Since it does, the result is true.

isEmpty()

If you need to check whether a string has any content at all, use .isEmpty(). It returns true if the string contains no characters, and false otherwise.

Main.java


              123456789101112
            
package com.example;

public class Main {
   public static void main(String[] args) {

       String customerNote = "";

       boolean emptyNote = customerNote.isEmpty();

       System.out.println(emptyNote);
   }
}

customerNote is an empty string with no characters, so .isEmpty() returns true.

replace()

If you need to swap out part of a string without rewriting it completely, use .replace(). It finds a specific piece of text and replaces it with something new. It returns a new String.

Main.java


              123456789101112
            
package com.example;

public class Main {
   public static void main(String[] args) {

       String order = "Large Latte with oat milk";

       String updatedMessage = order.replace("Latte", "coffee with milk");

       System.out.println(updatedMessage);
   }
}

.replace() finds every occurrence of "Latte" inside order and swaps it with "coffee with milk", producing "Large coffee with milk with oat milk".

charAt() and Indexing

If you need to access a single character at a specific position in a string, use .charAt(). It returns a char.

Main.java


              123456789101112
            
package com.example;

public class Main {
   public static void main(String[] args) {

       String customerName = "Daniel";

       char firstLetter = customerName.charAt(0);

       System.out.println(firstLetter);
   }
}

.charAt(0) accesses position 0, which is the first character 'D', and stores it in the char variable firstLetter.

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Thanks for your feedback!

Section 4. Chapter 2

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String Manipulation Operations in Java

String Concatenation

The simplest operation is concatenation — joining strings together using the + operator:

Main.java


              1234567891011
            
package com.example;

public class Main {
   public static void main(String[] args) {

       String customerName = "Ann";
       String welcomeMessage = "Welcome " + customerName;

       System.out.println(welcomeMessage); // Welcome Ann
   }
}

length()

If you need to know how many characters a string contains — including spaces — use the .length() method. It returns an int.

Main.java


              12345678910111213
            
package com.example;

public class Main {
   public static void main(String[] args) {

       String customerName = "Ann";
       String welcomeMessage = "Welcome " + customerName;

       int messageLength = welcomeMessage.length();

       System.out.println(messageLength);
   }
}

"Welcome Ann" has 11 characters including the space, so messageLength stores 11 and that's what gets printed.

toUpperCase()

If you need to convert all characters in a string to uppercase — for example on receipts or labels — use .toUpperCase(). It returns a new String.

Main.java


              123456789101112
            
package com.example;

public class Main {
   public static void main(String[] args) {

       String customerName = "anna";

       String upperCaseName = customerName.toUpperCase();

       System.out.println(upperCaseName);
   }
}

.toUpperCase() doesn't modify the original customerName — it creates a new String with all letters capitalized, so "anna" becomes "ANNA".

toLowerCase()

If you need all characters in lowercase — useful for consistency or comparison logic — use .toLowerCase(). It returns a new String.

Main.java


              123456789101112
            
package com.example;

public class Main {
   public static void main(String[] args) {

       String order = "Large Latte with oat milk";

       String lowerCaseOrder = order.toLowerCase();

       System.out.println(lowerCaseOrder);
   }
}

A new String is created with every character converted to lowercase, so the output is "large latte with oat milk".

contains()

If you need to check whether a string includes a specific word or piece of text, use .contains(). It returns a boolean.

Main.java


              123456789101112
            
package com.example;

public class Main {
   public static void main(String[] args) {

       String order = "Large Latte with oat milk";

       boolean containsLatte = order.contains("Latte");

       System.out.println(containsLatte);
   }
}

.contains() scans the entire string and checks if "Latte" appears anywhere inside it. Since it does, the result is true.

isEmpty()

If you need to check whether a string has any content at all, use .isEmpty(). It returns true if the string contains no characters, and false otherwise.

Main.java


              123456789101112
            
package com.example;

public class Main {
   public static void main(String[] args) {

       String customerNote = "";

       boolean emptyNote = customerNote.isEmpty();

       System.out.println(emptyNote);
   }
}

customerNote is an empty string with no characters, so .isEmpty() returns true.

replace()

If you need to swap out part of a string without rewriting it completely, use .replace(). It finds a specific piece of text and replaces it with something new. It returns a new String.

Main.java


              123456789101112
            
package com.example;

public class Main {
   public static void main(String[] args) {

       String order = "Large Latte with oat milk";

       String updatedMessage = order.replace("Latte", "coffee with milk");

       System.out.println(updatedMessage);
   }
}

.replace() finds every occurrence of "Latte" inside order and swaps it with "coffee with milk", producing "Large coffee with milk with oat milk".

charAt() and Indexing

If you need to access a single character at a specific position in a string, use .charAt(). It returns a char.

Main.java


              123456789101112
            
package com.example;

public class Main {
   public static void main(String[] args) {

       String customerName = "Daniel";

       char firstLetter = customerName.charAt(0);

       System.out.println(firstLetter);
   }
}

.charAt(0) accesses position 0, which is the first character 'D', and stores it in the char variable firstLetter.

Everything was clear?

Thanks for your feedback!

Section 4. Chapter 2