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Language: Java
Expertise: Intermediate

Sep 30, 1999

There are techniques for creation of objects (read allocation of memory) widely known as lazy Instantiation and Eager Instantiation.
Lazy instantiation is a memory conservation technique, by which, a program delays the creation of objects until those objects are needed. In Java, there are two categories of lazy instantiation:

1. Lazy class loading
The Java runtime loads classes into memory only when they're first referenced. This may happen due to a first call to a static method of a class as in:

 
	aClass.someStaticMethod();

or a first call to the "new" operator to create an instatnce of a class as in:

 
            AClass aClass = new AClass();

This is a very important feature of the Java runtime. Memory usage can be significantly reduced. For example, if a part of a program is never run, classes that are only referenced by that part of the program will never be loaded.

2. Lazy object creation
This is tightly coupled to lazy class loading. It refers to when you delay the creation of an instance of a class until you really need it in the code.

Eager Instantiation, on the other hand, refers to technique that instances of objects are created before any part of the code actually asks for them. This may considerably improve runtime performance since live objects are sitting in runtime memory waiting to be called. An example of eager instantiation is creation of resource pools, e.g. a pool of database connections waiting for client calls.

Suppose you have to load images on request based corresponding image file names. You can write code like:

 
public class AnImageFile
{
   private String m_filename;
   private Image m_image;

   public AnImageFile(String filename)
   {
       m_filename=filename;
       //Now load the image in constructor
   }
   public String getName(){ m_return filename;}
   public Image getImage(){return m_image;}
}

This is an eager approach that guarantees the availability of an image right after creation of an instance of AnImageFile class. This will boost performance when a call to getImage(...) is made. But if you have many images, and create AnImageFile instances of them, you may use up a lot of memory unnecessarily. You can trade performance benefits to memory uasage via lazy instantiation, as in:

 
public class ImageFile
{
   private String m_filename;
   private Image m_image=null;

   public ImageFile(String filename)
   {
       //Here we just store the filename
       m_filename=filename;
   }
   public String getName(){ return m_filename;}
   public Image getImage()
   {
       if(m_image==null)
       {
           //load the image
       }
       return m_image;
    }
}

Here, the actual image is loaded only on the first call to getImage().

So, you can reduce memory usage and startup time by using lazy instantiation and paying the price for performance hit when you create an object when it is actually need. Or, you can increase the startup time, and use up more memory upfront to eagerly instantiate objects in order to boost performance.