Question

Im having trouble with some Java, How do I give in default parameter values in java?.
for example I have this in c++

DVD(int i, string t, int y, string d="Unknown"): Items(i,t,y),director(d){}
and in Java I tried

public Dvd(int i, String t,int y, String d="Unknown"){
        super(i,t,y);
        director = d;
    }

which fails to build. So how do I go about giving in default values?

also In my main testing class I tried giving in 3 arguments insead of 4 but this fails also. How do I get around this problem?.

Answer 1

Java does not support default argument construct like that, unfortunately. The traditional way to implement it, for better or worse, is to use what is called “telescoping” methods.

Here’s a quote from Effective Java 2nd Edition, Item 2: Consider a builder pattern when faced with many constructor parameters:

Traditionally, programmers have used the telescoping constructor pattern, in which you provide a constructor with only the required parameters, another with a single optional parameters, a third with two optional parameters, and so on…

The telescoping constructor pattern is essentially something like this:

public class Telescope {
    final String name;
    final int levels;
    final boolean isAdjustable;

    public Telescope(String name) {
        this(name, 5);
    }
    public Telescope(String name, int levels) {
        this(name, levels, false);
    }
    public Telescope(String name, int levels, boolean isAdjustable) {       
        this.name = name;
        this.levels = levels;
        this.isAdjustable = isAdjustable;
    }
}

And now you can do any of the following:

new Telescope("X/1999");
new Telescope("X/1999", 13);
new Telescope("X/1999", 13, true);

You can’t, however, currently set only the name and isAdjustable, and leaving levels at default. You can provide more constructor overloads, but obviously the number would explode as the number of parameters grow, and you may even have multiple boolean and int arguments, which would really make a mess out of things.

As you can see, this isn’t a pleasant pattern to write, and even less pleasant to use (What does “true” mean here? What’s 13?).

Bloch recommends using a builder pattern, which would allow you to write something like this instead:

Telescope telly = new Telescope.Builder("X/1999").setAdjustable(true).build();

Note that now the parameters are named, and you can set them in any order you want, and you can skip the ones that you want to keep at default values. This is certainly much better than telescoping constructors, especially when there’s a huge number of parameters that belong to many of the same types.

So Java does not have default argument mechanism, but the builder pattern is a much better idiom anyway.

See also

• Wikipedia/Builder pattern
• Effective Java 2nd Edition, Item 2: Consider a builder pattern when faced with many constructor parameters (excerpt online)

Related questions

• When would you use the Builder Pattern?
• Is this a well known design pattern? What is its name?