Guaranteed initialization with the constructor

Bruce Eckel's Thinking in Java Contents | Prev | Next

with the constructor

You can imagine creating a method called initialize( ) for every class you write. The name is a hint that it should be called before using the object. Unfortunately, this means the user must remember to call the method. In Java, the class designer can guarantee initialization of every object by providing a special method called a constructor. If a class has a constructor, Java automatically calls that constructor when an object is created, before users can even get their hands on it. So initialization is guaranteed.

Here’s a simple class with a constructor: (See page 97 if you have trouble executing this program.)

//: SimpleConstructor.java
// Demonstration of a simple constructor
package c04;
 
class Rock {
  Rock() { // This is the constructor
    System.out.println("Creating Rock");
  }
}
 
public class SimpleConstructor {
  public static void main(String[] args) {
    for(int i = 0; i < 10; i++)
      new Rock();
  }
} ///:~ 

Now, when an object is created:

class Rock {
  Rock(int i) {
    System.out.println(
      "Creating Rock number " + i);
  }
}
 
public class SimpleConstructor {
  public static void main(String[] args) {
    for(int i = 0; i < 10; i++)
      new Rock(i);
  }
}

Constructor arguments provide you with a way to provide parameters for the initialization of an object. For example, if the class Tree has a constructor that takes a single integer argument denoting the height of the tree, you would create a Tree object like this:

Tree t = new Tree(12); // 12-foot tree

If Tree(int) is your only constructor, then the compiler won’t let you create a Tree object any other way.

Constructors eliminate a large class of problems and make the code easier to read. In the preceding code fragment, for example, you don’t see an explicit call to some initialize( ) method that is conceptually separate from definition. In Java, definition and initialization are unified concepts – you can’t have one without the other.



Comments

  • There are no comments yet. Be the first to comment!

Leave a Comment
  • Your email address will not be published. All fields are required.

Top White Papers and Webcasts

  • Live Event Date: December 11, 2014 @ 1:00 p.m. ET / 10:00 a.m. PT Market pressures to move more quickly and develop innovative applications are forcing organizations to rethink how they develop and release applications. The combination of public clouds and physical back-end infrastructures are a means to get applications out faster. However, these hybrid solutions complicate DevOps adoption, with application delivery pipelines that span across complex hybrid cloud and non-cloud environments. Check out this …

  • VMware vCloud® Government Service provided by Carpathia® is an enterprise-class hybrid cloud service that delivers the tried and tested VMware capabilities widely used by government organizations today, with the added security and compliance assurance of FedRAMP authorization. The hybrid cloud is becoming more and more prevalent – in fact, nearly three-fourths of large enterprises expect to have hybrid deployments by 2015, according to a recent Gartner analyst report. Learn about the benefits of …

Most Popular Programming Stories

More for Developers

RSS Feeds