Module 3: Operators, Expressions, and Strings

Useful String Methods

Use a few high-value string methods to inspect and transform text without overwhelming the lesson with too many APIs.

Author

Java Learner Editorial Team

Reviewer

Technical review by Java Learner

Last reviewed

2026-04-16

Java version

Java 25 LTS

How this lesson was prepared: AI-assisted draft, edited by hand, and checked against current Java 25 documentation and runnable examples.

Learning goals

Read string length
Convert case with `toUpperCase()` or `toLowerCase()`
Check whether text contains a smaller piece of text

Start small: You do not need dozens of string APIs on day one. A few useful methods go a long way.

Common methods: length() tells you how many characters are in the string. toUpperCase() and toLowerCase() change letter case. contains(...) checks whether one string appears inside another.

Method calls return new results: Many string methods return a value. That means you can print the result directly or store it in another variable.

Why this matters: String methods show how real Java APIs feel without throwing you into advanced topics too early.

Runnable examples

Length and case

public class Main {
    public static void main(String[] args) {
        String word = "Java";
        System.out.println(word.length());
        System.out.println(word.toUpperCase());
    }
}

Expected output

4
JAVA

Check if text contains something

public class Main {
    public static void main(String[] args) {
        String sentence = "Java learner";
        System.out.println(sentence.contains("learn"));
    }
}

Expected output

true

Common mistakes

Writing `length` without parentheses

Use `length()` because it is a method call.

Mini exercise

Create a string and print its length and uppercase version.

Summary

Methods are actions you can call on a string.
`length()` and case conversion are very common.

Next step

Finish the module by learning how Java evaluates longer expressions.

Sources used

Useful String Methods

Length and case

Check if text contains something

What does `"Java".length()` return?