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Introduction to Java

Extracting Information from Strings in Java

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When working with text in real applications, you rarely need the entire string — most of the time you only need a small piece of it. A name inside an email, a product inside an order message, or a specific keyword hidden in a sentence. Java gives us simple but powerful tools to pull out exactly what we need.

substring()

If you need to take a "slice" of a string based on character positions, use .substring(). It relies on zero-based indexing — counting starts from 0. It returns a new String.

Main.java

Main.java


              
123456789101112

            
package com.example;

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

       String order = "Large Latte with oat milk";

       String drinkName = order.substring(6, 11);

       System.out.println(drinkName);
   }
}

substring(6, 11) tells Java to start at index 6 (included) and stop right before index 11 (excluded) — extracting characters at positions 6 through 10, which gives us "Latte".

If you use substring(start) with only one argument, Java extracts everything from that position to the end of the string:

order.substring(6); // "Latte with oat milk"

indexOf()

If you need to find where a specific piece of text is located inside a string, use .indexOf(). Instead of cutting text, it locates it. It returns an int representing the starting index of the match.

Main.java

Main.java


              
123456789101112

            
package com.example;

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

       String order = "Large Latte with oat milk";

       int lattePosition = order.indexOf("Latte");

       System.out.println(lattePosition);
   }
}

Java scans the string from left to right until it finds "Latte" and returns the position of its first character — 6. If the text is not found anywhere in the string, Java returns -1.

Combining Both — Real Example

This real example combines indexOf() and substring() to extract a username from an email address.

Main.java

Main.java


              
1234567891011121314

            
package com.example;

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

       String customerEmail = "anna.coffee@gmail.com";

       int atPosition = customerEmail.indexOf("@");

       String username = customerEmail.substring(0, atPosition);

       System.out.println(username);
   }
}

indexOf("@") finds the position of the @ symbol, then substring(0, atPosition) extracts everything from the start of the string up to — but not including — the @. The result is "anna.coffee". Instead of hardcoding positions, we let Java locate the boundary and extract exactly what we need.

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Section 4. Chapter 7

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Section 4. Chapter 7
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