Java constructor of "immutable class" with many fields with default values?

Question

I have a class with lots of fields. They should basically be set at the constructor phase and never change. Semantically the class then is an immutable one.

public class A{
    final int a;
    final short b;
    final double e;
    final String f;
    final String g;
    //and more
}

The problem is that normally these fields have default values and therefore I do not want to always burden the user with a constructor with all of them. Most time, they just need to set a couple of them. There are a couple of ways to solve this:

1. I would need lots of constructor with different signature.
2. Create a bunch of set method of these field and only set those non-default value. But this somehow indicate a different semantics other than immutable nature.
3. Create a new parameter class that is mutable and use that class as constructor.

None of that is totally satisfactory. Is there any other approach? Thanks. One way

Answer 1

I would use a combination of a parameter class and a fluent builder API for creating the parameter:

public class A {
    private final int a;
    private final short b;
    private final double e;
    private final String g;

    public static class Aparam {
        private int a = 1;
        private short b = 2;
        private double e = 3.141593;
        private String g = "NONE";

        public Aparam a(int a) {
            this.a = a;
            return this;
        }

        public Aparam b(short b) {
            this.b = b;
            return this;
        }

        public Aparam e(double e) {
            this.e = e;
            return this;
        }

        public Aparam g(String g) {
            this.g = g;
            return this;
        }

        public A build() {
            return new A(this);
        }
    }

    public static Aparam a(int a) {
        return new Aparam().a(a);
    }

    public static Aparam b(short b) {
        return new Aparam().b(b);
    }

    public static Aparam e(double e) {
        return new Aparam().e(e);
    }

    public static Aparam g(String g) {
        return new Aparam().g(g);
    }

    public static A build() {
        return new Aparam().build();
    }

    private A(Aparam p) {
        this.a = p.a;
        this.b = p.b;
        this.e = p.e;
        this.g = p.g;
    }

    @Override public String toString() {
        return "{a=" + a + ",b=" + b + ",e=" + e + ",g=" + g + "}";
    }
}

Then create instances of A like this:

A a1 = A.build();
A a2 = A.a(7).e(17.5).build();
A a3 = A.b((short)42).e(2.218282).g("fluent").build();

Class A is immutable, the parameters are optional, and the interface is fluent.