Method Overloading

public Decimal calculateArea(Decimal side) {
    return side * side; // square
}

public Decimal calculateArea(Decimal length, Decimal width) {
    return length * width; // rectangle
}

1. public Decimal calculateArea(Decimal side) { — one version, taking a single Decimal.
2. return side * side; // square — squares it.
3. } — closes the first version.
4. (blank line)
5. public Decimal calculateArea(Decimal length, Decimal width) { — a second version, same name, but two parameters.
6. return length * width; // rectangle — multiplies them.
7. } — closes the second version.

Both methods are named calculateArea, but one takes a single Decimal and the other takes two. Apex tells them apart by their parameter list — this is called overloading.

Decimal squareArea = calculateArea(5);        // calls the one-parameter version → 25
Decimal rectArea = calculateArea(4, 6);       // calls the two-parameter version → 24

1. Decimal squareArea = calculateArea(5); — one argument, so Apex calls the one-parameter version — 25.
2. Decimal rectArea = calculateArea(4, 6); — two arguments, so Apex calls the two-parameter version — 24.

You don't have to name them calculateSquareArea and calculateRectangleArea — Apex matches each call to the overload whose parameter list fits, based purely on how many arguments (and of what type) you pass.

public Decimal calculatePrice(Decimal monthlyRate) {
    return monthlyRate;
}

public Decimal calculatePrice(Decimal monthlyRate, Integer months) {
    return monthlyRate * months;
}

Decimal oneMonth = calculatePrice(299);        // 299
Decimal sixMonths = calculatePrice(299, 6);    // 1794

1. public Decimal calculatePrice(Decimal monthlyRate) { — one version, just the monthly rate.
2. return monthlyRate; — one month costs exactly the monthly rate.
3. } — closes the first version.
4. (blank line)
5. public Decimal calculatePrice(Decimal monthlyRate, Integer months) { — a second version, adding a number of months.
6. return monthlyRate * months; — multiplies rate by months.
7. } — closes the second version.
8. (blank line)
9. Decimal oneMonth = calculatePrice(299); — one argument, calls the first version — 299.
10. Decimal sixMonths = calculatePrice(299, 6); — two arguments, calls the second version — 1794.

A billing system naturally has both a "price this month" and a "price for N months" calculation — overloading lets both live under one intuitive name instead of two awkwardly different ones.

Changing only the return type while keeping the same parameter list is not a valid overload — Apex (like most languages) can't tell two methods apart based on return type alone, since the call site doesn't always make the return type obvious.