Strings and ArrayList

Why this matters

Almost every project stores a list of objects — students, books, products — and ArrayList is the simplest way to do it.

Example

import java.util.ArrayList;

public class Demo {
    public static void main(String[] args) {
        String name = "  Ananya Rao  ";
        System.out.println(name.trim().toUpperCase());
        System.out.println("Length: " + name.trim().length());
        System.out.println(name.contains("Rao"));

        ArrayList<String> subjects = new ArrayList<>();
        subjects.add("Java");
        subjects.add("DBMS");
        subjects.add("Maths");
        subjects.remove("Maths");

        for (String s : subjects) {
            System.out.println("Subject: " + s);
        }
        System.out.println("Count: " + subjects.size());
    }
}

Step by step

1. 1`trim()` removes spaces at both ends; `toUpperCase()` returns a new uppercase String.
2. 2String is immutable: these methods return new Strings, the original never changes.
3. 3`length()` is a method for Strings (arrays use the field `length`).
4. 4`ArrayList<String>` is a growable list of Strings.
5. 5add(), remove() and size() are the three methods you will use most.
6. 6The enhanced for loop reads each item in turn.

Key points

• Compare Strings with .equals(), never ==
• String is immutable; StringBuilder is mutable
• ArrayList grows automatically, arrays do not
• Use generics: ArrayList<Student> instead of a raw ArrayList

Common mistakes

• Expecting name.toUpperCase() to change name itself
• Comparing Strings with ==
• Forgetting to import java.util.ArrayList

Try it yourself

• Store five subject names in an ArrayList and print them sorted.
• Count how many vowels a String contains.