Java constructors and encapsulation

A constructor initializes a new instance. Private fields keep normal external code from modifying state directly; methods define allowed operations.

Java

class Counter {
    private int value;
    Counter(int start) {
        if (start < 0) throw new IllegalArgumentException("negative start");
        value = start;
    }
    void increment() { value++; }
    int getValue() { return value; }
}
class Main {
    public static void main(String[] args) {
        Counter first = new Counter(2);
        Counter second = new Counter(10);
        first.increment();
        System.out.println(first.getValue());
        System.out.println(second.getValue());
    }
}

Read the full lesson