Named Parameter idiom in Java

Question

How to implement Named Parameter idiom in Java? (especially for constructors)

I am looking for an Objective-C like syntax and not like the one used in JavaBeans.

A small code example would be fine.

Thanks.

Answer 1

The best Java idiom I've seem for simulating keyword arguments in constructors is the Builder pattern, described in Effective Java 2nd Edition.

The basic idea is to have a Builder class that has setters (but usually not getters) for the different constructor parameters. There's also a build() method. The Builder class is often a (static) nested class of the class that it's used to build. The outer class's constructor is often private.

The end result looks something like:

public class Foo {
  public static class Builder {
    public Foo build() {
      return new Foo(this);
    }

    public Builder setSize(int size) {
      this.size = size;
      return this;
    }

    public Builder setColor(Color color) {
      this.color = color;
      return this;
    }

    public Builder setName(String name) {
      this.name = name;
      return this;
    }

    // you can set defaults for these here
    private int size;
    private Color color;
    private String name;
  }

  public static Builder builder() {
      return new Builder();
  }

  private Foo(Builder builder) {
    size = builder.size;
    color = builder.color;
    name = builder.name;
  }

  private final int size;
  private final Color color;
  private final String name;

  // The rest of Foo goes here...
}

To create an instance of Foo you then write something like:

Foo foo = Foo.builder()
    .setColor(red)
    .setName("Fred")
    .setSize(42)
    .build();

The main caveats are:

1. Setting up the pattern is pretty verbose (as you can see). Probably not worth it except for classes you plan on instantiating in many places.
2. There's no compile-time checking that all of the parameters have been specified exactly once. You can add runtime checks, or you can use this only for optional parameters and make required parameters normal parameters to either Foo or the Builder's constructor. (People generally don't worry about the case where the same parameter is being set multiple times.)

You may also want to check out this blog post (not by me).

Answer 2

Java does not support Objective-C-like named parameters for constructors or method arguments. Furthermore, this is really not the Java way of doing things. In java, the typical pattern is verbosely named classes and members. Classes and variables should be nouns and method named should be verbs. I suppose you could get creative and deviate from the Java naming conventions and emulate the Objective-C paradigm in a hacky way but this wouldn't be particularly appreciated by the average Java developer charged with maintaining your code. When working in any language, it behooves you to stick to the conventions of the language and community, especially when working on a team.

Answer 3

this is worth of mentioning

Foo foo = new Foo(){{ color=red;  name="Fred"; size=42; }};

the so called "double brace initializer". actually an anonymous class with instance initializer.

Answer 4

You could use a usual constructor and static methods that give the arguments a name:

public class Something {

    String name;
    int size; 
    float weight;

    public Something(String name, int size, float weight) {
        this.name = name;
        this.size = size;
        this.weight = weight;
    }

    public static String name(String name) { 
        return name; 
    }

    public static int size(int size) {
        return size;
    }

    public float weight(float weight) {
        return weight;
    }

}

Usage:

import static Something.*;

Something s = new Something(name("pen"), size(20), weight(8.2));

Limitations compared to real named parameters:

• argument order is relevant
• variable argument lists are not possible with a single constructor
• you need a method for every argument
• not really better than a comment (new Something(/*name*/ "pen", /*size*/ 20, /*weight*/ 8.2))

If you have the choice look at Scala 2.8. http://www.scala-lang.org/node/2075

Answer 5

Here's a little variation of the technique given in Joshua Bloch's Effective Java. Here I have made an attempt to make the client code more readable (or perhaps more DSLish).

/**
 * Actual class for which we want to implement a 
 * named-parameter pseudo-constructor
 */
class Window{
    protected int x, y, width, height;
    protected boolean isResizable;
    protected String title;

    public void show(){
        // Show the window
        System.out.printf("Window \"%s\" set visible.%n",title);
    }

    /**
     * This class is only used to set the parameter values
     */
    static class HavingProperties extends Window{

        public HavingProperties x(int value){
            this.x=value;
            return this;
        }

        public HavingProperties y(int value){
            this.y=value;
            return this;
        }

        public HavingProperties width(int value){
            this.width=value;
            return this;
        }

        public HavingProperties height(int value){
            this.height=value;
            return this;
        }

        public HavingProperties resizable(boolean value){
            this.isResizable=value;
            return this;
        }

        public HavingProperties title(String value){
            this.title=value;
            return this;
        }
    }
}

public class NamedParameterIdiomInAction {
    public static void main(String... args){
        Window window=new Window.HavingProperties().x(10).y(10).width(100).
                height(100).resizable(true).title("My App");
        window.show();
    }
}

Please note that with this variation, you can also give meaningful names to your pseudo-constructors.