Parents first, and final fields¶

☕ Java 17 · The Order Java Builds Your Objects In · 3/4

Objects are built in layers: the parent part first, then the child part. Here is how constructors make that happen.

Parents are built before children¶

class Content {
    Content() { System.out.print("Content-"); }
}

class Movie extends Content {
    Movie()      { System.out.print("Movie1-"); }
    Movie(int y) { System.out.print("Movie2-"); }
}

class Trailer extends Movie {
    Trailer() {
        super(2026);
        System.out.print("Trailer-");
    }
}

// new Trailer();

Output:

Content-Movie2-Trailer-

Always start from the new call:

Trailer() calls super(2026), so Movie(int) runs.
Movie(int) has no explicit first line: the compiler adds super();, so Content() runs.
The bodies then finish top-down: Content, then Movie2, then Trailer.

Constructors are called bottom-up, but their bodies run top-down. Only constructors on the real path run: Movie() is never called, so Movie1- never prints.

Since every constructor on the path runs, each one has a responsibility toward final fields.

Final fields: set once, never twice¶

A final instance field must be assigned exactly once — never zero times, never twice.

This version does not compile:

class Episode {
    private final String title;
    private final int season = 1;

    Episode() { season = 2; }   // ❌ season assigned twice, title never
}

season already gets 1 on its declaration, so season = 2 is a second assignment.
This constructor never assigns title.

This version compiles:

class Episode {
    private final String title;
    private final int season = 1;

    Episode(String t) { title = t; }
    Episode() { this(null); }    // ✅ delegates to the first constructor
}

A final field can be assigned in three places: on its declaration, in an instance block, or in a constructor. The compiler checks each constructor separately: every path must leave every final field assigned once. null is a valid value, as long as you assign it.

You now have the whole picture. Time to see it run.

💻 Code: Java17_Part_6_Object_Oriented_Basics on GitHub

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