Loading classes, static data…

"Hello, Amigo! I heard Rishi explained something new and exciting to you?!"

"That's right, Kim."

"My topic will be no less interesting. I want to tell you about how classes are loaded into memory."

Classes in Java are files on the disk that contain bytecode, which is compiled Java code.

"Yes, I remember."

The Java machine doesn't load them if it doesn't need to. As soon as there's a call to a class somewhere in the code, the Java machine checks to see if it is loaded. And if not, then it loads and initializes it.

Initializing a class involves assigning values to all of its static variables and calling all static initialization blocks.

"That seems similar to calling a constructor on an object. But what's a static initialization block?"

"If you need to execute complex code (for example, loading something from a file) to initialize objects, we can do it in a constructor. However, static variables don't have this opportunity. But since the need still remains, you can add a static initialization block or blocks to classes. They are basically equivalent to static constructors."

This is how it looks:

class Cat
{
public static int catCount = 0 ;
public static String namePrefix;

static
{
Properties p = new Properties();
p.loadFromFile("cat.properties");
namePrefix = p.get("name-prefix");
}

public static int maxCatCount = 50;

static
{
Properties p = new Properties();
p.loadFromFile("max.properties");
if (p.get("cat-max") != null)
maxCatCount = p.getInt("cat-max");
}

}

class Cat
{
public static int catCount;
public static String namePrefix;
public static int maxCatCount;

//Static constructors aren't allowed in Java,
//but if they were, everything
//would look like this
public static Cat()
{
catCount = 0;

Properties p = new Properties();
p.loadFromFile("cat.properties");
namePrefix = p.get("name-prefix");

maxCatCount = 50;

Properties p2 = new Properties();
p2.loadFromFile("max.properties");
if (p2.get("cat-max")!=null)
maxCatCount = p2.getInt("cat-max");
}
}

It's a lot like what happens when a constructor is called. I've even written it as a (nonexistent) static constructor.

"Yes, I get it."

"Very good."