Writing and overloading constructors¶

☕ Java 17 · Building Objects the Right Way with Constructors · 1/3

Anatomy of a constructor¶

public class Movie {
    private String title;

    public Movie(String t) {
        title = t;
    }
}

// somewhere else
Movie m = new Movie("Dune");

Its name is exactly the class name, capital letter included.
It has no return type, not even void.
new reserves memory, then calls the constructor whose parameters match the arguments.
Parameters can be of any type, except var.

Lookalikes that aren't constructors¶

public class Movie {
    public movie() { }          // ❌ wrong case: a method with no return type → doesn't compile
    public void Movie() { }     // ⚠️ a valid method, never called by new
    public Movie(var t) { }     // ❌ var isn't allowed for parameters
}

A return type, even void, turns a "constructor" into an ordinary method.

Several ways to build a movie¶

A class can declare several constructors, each with a different list of parameter types. This is constructor overloading:

public Movie() { title = "Untitled"; }
public Movie(String title) { this.title = title; }
public Movie(String title, int minutes) { /* … */ }
public Movie(String title, String... genres) { /* … */ }

new Movie();              // runs the first one
new Movie("Dune", 155);   // runs the third one

As with methods, each signature must be unique: two constructors can't have the same list of parameter types. The arguments of new decide which one runs.

The constructor Java writes for you¶

When a class declares no constructor at all, the compiler adds a default constructor: no parameters, empty body.

public class Clip { }                               // gets a default constructor
public class Teaser { public Teaser() { } }         // declares its own: no default added
public class Poster { public Poster(boolean hd) { } }
public class Banner { private Banner() { } }

Class	`new X()` from another class
`Clip`	✅ the default constructor
`Teaser`	✅ its own no-argument constructor
`Poster`	❌ it has a constructor, so no default one
`Banner`	❌ its only constructor is `private`

A private constructor prevents other classes from creating instances at all.

With several constructors, the same assignments tend to repeat. The next page shows how one constructor reuses another, and how a child builds its parent first.

💻 Code: Java17_Part_6_Object_Oriented_Basics on GitHub

Next: chaining with this() and super() →