Consider this class:

class Point {
  int x;
  int y;
}

The problem is subtle. Java generates a no-argument constructor by default, so this compiles without complaint:

Point p = new Point();
p.x = 10;

The object is incomplete. x is 10, y is 0 — but only because Java zero-initializes fields. There is no way to distinguish this from a Point where y was intentionally set to zero:

Point p = new Point();
p.x = 10;
p.y = 0;

The two are indistinguishable, which is the root of the problem.

The fix is to define an explicit constructor:

class Point {
  int x;
  int y;

  Point(int x, int y) {
    this.x = x;
    this.y = y;
  }
}

Once any constructor is defined, Java stops generating the default one. This means:

Point p = new Point();

no longer compiles. The only valid construction is:

Point p = new Point(3, 5);

A Point now cannot exist in a partially initialized state — the type itself enforces completeness.