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Constructor

These pages make up the course notes for my Java programming course (run at Dallam, Milnthorpe, Cumbria). Hopefully they also make a useful self-learning tutorial.

Sometimes when we create an object from a class we want somethings to be set up, or set to default values. Java provides a constructor method to do this.

A constructor method has a slightly different method signature froma normal method. A normal method has a type (e.g. void, String, etc) whereas a construtor has no type, it also has the same name as the class (including the upper/lower case).

Here is an example:

package tuorial.files;

public class Bookmarks {

        public String url;
        public String title;
        public String file;

        /**
        * constructor, set up a default file name if none supplied
        *
        * @param file - the name of the bookmark file or null to default
        */
        public Bookmarks(String file) {
                if (file == null) {
                        this.file = "Bookmarks.html";
                }
                else {
                        this.file = file;
                }


        }

        ...
}

Notice that this contructor takes a parameter (file). This means that we must supply a file name when we make objects from this class - notice also that it handles the situation of the file name being null. If the file is null a default value is used.

Constructors may have any number of parameters, or they may have none.

This work is Copyright Chris Hunter 2007, you may use it for non-commercial purposes
under the Creative Commons license Creative Commons Attribution-Noncommercial-Share Alike.