Constructors and references

A constructor initializes a new object. Rules the exam tests:

• Same name as the class, no return type (not even void).
• If you write no constructor, Java silently provides a no-argument default (all fields zero/false/null) — but only then. Write any constructor and the default disappears.

public class Counter {
    private int count;

    public Counter() {           // no-arg: start at 0
        count = 0;
    }

    public Counter(int start) {  // overloaded: start elsewhere
        count = start;
    }
}

Two constructors with different parameter lists = overloading. The call new Counter(5) picks the matching one.

Multiple references, one object — aliasing:

Counter a = new Counter();
Counter b = a;          // b refers to the SAME object; no copy

Now a and b are two names for one counter. Compare with primitives, where int b = a really copies. And the null reference: an object variable can hold null ("no object") — calling a method on it crashes with a NullPointerException.