Thursday, 8 December 2016

What is Instance Initializer in Java?

In this post, I will illustrate what are instance variable initializer, instance initializer, static initializer, and how the instance initializer works in Java.

1. Execution Order

Look at the following class, do you know which one gets executed first?

public class Foo {

//instance variable initializer
String s = "abc";

//constructor
public Foo() {
System.out.println("constructor called");
}

//static initializer
static {
System.out.println("static initializer called");
}

//instance initializer
{
System.out.println("instance initializer called");
}

public static void main(String[] args) {
new Foo();
new Foo();
}
}

Output:

static initializer called
instance initializer called
constructor called
instance initializer called
constructor called

2. How does Java instance initializer work?

The instance initializer above contains a println statement. To understand how it works, we can treat it as a variable assignment statement, e.g., b = 0. This can make it more obvious to understand.

Instead of

int b = 0

, we can write it as

int b;
b = 0;

Therefore, instance initializers and instance variable initializers are pretty much the same.

3. When are instance initializers useful?

The use of instance initializers are rare, but still it can be a useful alternative to instance variable initializers if:

(1) initializer code must handle exceptions
(2) perform calculations that can't be expressed with an instance variable initializer.

Of course, such code could be written in constructors. But if a class had multiple constructors, you would have to repeat the code in each constructor.

With an instance initializer, you can just write the code once, and it will be executed no matter what constructor is used to create the object. (I guess this is just a concept, and it is not used often.)

Another case in which instance initializers are useful is anonymous inner classes, which can't declare any constructors at all.