Java course for Peru, from zero and in English

If you are deciding between Java and Python as a first language, either is a good choice; what matters more is a course that makes you think. Our Spanish page curso de Java en inglés compares the two for Peruvian learners, and the curso de Python en inglés covers the other path. Whichever you choose, the classes are taught in English.

Learners join a batch of people at a similar stage, or learn one to one. Each class is live: the mentor explains, the learner types, and when something does not compile, they read the error message together, in English, and fix it. Those error messages are some of the best English reading practice a programmer gets.

1. Setting up and a first program. Installing a current JDK, writing main, compiling and running. Learners meet the compiler's error messages on day one and learn to read them calmly.
2. Types, variables and operators. The table above, in practice: choosing int, long, double or boolean, and the overflow lesson on this page.
3. Decisions and loops. if, switch, for and while, with problems checked by hand first.
4. Methods. Splitting a program into named pieces with clear inputs and outputs, and testing each one.
5. Classes and objects. The heart of Java: designing classes, constructors, encapsulation, then inheritance and interfaces once the basics are steady.
6. Collections and generics. Lists, sets and maps from the standard library, and choosing between them.
7. Exceptions, files and testing. Handling failure on purpose, reading and writing data, and writing tests that catch bugs before users do.
8. A real project. For teenagers, often a game or a school tool; for adults, a back-end service with a database. Every learner explains their project, in English, to their mentor.

public class Milliseconds {
    public static void main(String[] args) {
        int days = 30;
        int ms = days * 24 * 60 * 60 * 1000;
        long stillWrong = days * 24 * 60 * 60 * 1000;
        long right = days * 24L * 60 * 60 * 1000;
        System.out.println("int:         " + ms);
        System.out.println("long, late:  " + stillWrong);
        System.out.println("long, early: " + right);
        System.out.println("int maximum: " + Integer.MAX_VALUE);
        try {
            System.out.println(Math.multiplyExact(days * 24 * 60 * 60, 1000));
        } catch (ArithmeticException e) {
            System.out.println("multiplyExact: " + e.getMessage());
        }
    }
}

The right answer comes from making the calculation a long from the start: writing 24L turns the second number into a long, so every multiplication after it happens in 64 bits. The last line shows the other professional tool. Math.multiplyExact, part of Java's standard library, throws an exception if the result overflows an int, so a silent wrong number becomes a loud error you can see and fix.

Bugs like this are not exotic. They appear when programs count time, money in cents, views on a video or rows in a database, and they often hide for months because the small test cases work. The habit we teach is to ask, for every calculation, "how big can this get?", and to choose the type before writing the code. The lesson is ours, written for our classes.

1. Forecast. Before you press run, note your forecast for every printed line. Then run it and mark which forecasts held.
2. Find the edge. Change days to 24, then 25. At which number of days does the int version first go wrong? Work it out with a calculator before you test it.
3. Move the L. Put the L on 1000 instead of 24. Does the answer change? Explain why in two English sentences.
4. Go loud. Rewrite the whole calculation with Math.multiplyExact on long values, and test it with 30 days and with 300 million days.

Based on the IANA tz database entries America/Lima and Asia/Kolkata, neither of which moves its clocks during the year. We did the conversions.

Mentors teach only in English and match each learner's pace: short sentences, technical words typed into the chat, and each new idea shown on screen before it is used. Learners in the capital who want more about learning in English can read our page on English-medium coding classes in Lima.