Java Tutorial 24: More on Classes-Initializing Fields

Before you start reading:

A variable declared inside a class but not inside any particular method is a field.

Another name for a field is an instance variable.

As you have seen, you can often provide an initial value for a field in its declaration:

public class BedAndBreakfast {

    // initialize to 10
    public static int capacity = 10;

    // initialize to false
    private boolean full = false;
}

This works well when the initialization value is available and the initialization can be put on one line. However, this form of initialization has limitations because of its simplicity. If initialization requires some logic (for example, error handling or a for loop to fill a complex array), simple assignment is inadequate. Instance variables can be initialized in constructors, where error handling or other logic can be used. To provide the same capability for class variables, the Java programming language includes static initialization blocks.

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Note: It is not necessary to declare fields at the beginning of the class definition, although this is the most common practice. It is only necessary that they be declared and initialized before they are used.

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Static Initialization Blocks

A static initialization block is a normal block of code enclosed in braces, { }, and preceded by the static keyword. Here is an example:

static {
    // whatever code is needed for initialization goes here
}

A class can have any number of static initialization blocks, and they can appear anywhere in the class body. The runtime system guarantees that static initialization blocks are called in the order that they appear in the source code.
There is an alternative to static blocks — you can write a private static method:

class Whatever {
    public static varType myVar = initializeClassVariable();
        
    private static varType initializeClassVariable() {

        // initialization code goes here
    }
}

The advantage of private static methods is that they can be reused later if you need to reinitialize the class variable.

Initializing Instance Members

Normally, you would put code to initialize an instance variable in a constructor. There are two alternatives to using a constructor to initialize instance variables: initializer blocks and final methods.
Initializer blocks for instance variables look just like static initializer blocks, but without the static keyword:

{
    // whatever code is needed for initialization goes here
}

The Java compiler copies initializer blocks into every constructor. Therefore, this approach can be used to share a block of code between multiple constructors.
A final method cannot be overridden in a subclass. This is discussed in the lesson on interfaces and inheritance. Here is an example of using a final method for initializing an instance variable:

class Whatever {
    private varType myVar = initializeInstanceVariable();
        
    protected final varType initializeInstanceVariable() {

        // initialization code goes here
    }
}

This is especially useful if subclasses might want to reuse the initialization method. The method is final because calling non-final methods during instance initialization can cause problems.

Summary of Creating and Using Classes and Objects

A class declaration names the class and encloses the class body between braces. The class name can be preceded by modifiers. The class body contains fields, methods, and constructors for the class. A class uses fields to contain state information and uses methods to implement behavior. Constructors that initialize a new instance of a class use the name of the class and look like methods without a return type.
You control access to classes and members in the same way: by using an access modifier such as public in their declaration.
You specify a class variable or a class method by using the static keyword in the member's declaration. A member that is not declared as static is implicitly an instance member. Class variables are shared by all instances of a class and can be accessed through the class name as well as an instance reference. Instances of a class get their own copy of each instance variable, which must be accessed through an instance reference.
You create an object from a class by using the new operator and a constructor. The new operator returns a reference to the object that was created. You can assign the reference to a variable or use it directly.
Instance variables and methods that are accessible to code outside of the class that they are declared in can be referred to by using a qualified name. The qualified name of an instance variable looks like this:

objectReference.variableName

The qualified name of a method looks like this:

objectReference.methodName(argumentList)

or:

objectReference.methodName()

The garbage collector automatically cleans up unused objects. An object is unused if the program holds no more references to it. You can explicitly drop a reference by setting the variable holding the reference to null.

Source:oracle