Initialization

Goals:

• Present the default initial values for instance fields.
• Describe how to use variable initializers.
• Show how to write and invoke constructors.

Overview

Here we discuss the various options available to initialize the instance fields of a class.

Default values

Instance fields of a class are set to default values when an object is created. The default value depends on the type of the field.

Numeric fields such as int, float, double, etc. are set to zero.

Fields of type bool are set to false.

Characters fields are set to the null character (often written '\x0000').

References are set to null. We will discuss references in the module on reference types.

Variable initializer

Instance fields can be initialized inline, that is, at the point of declaration. The official name for this technique is 'variable initializer'.

Constructor

The most powerful initialization technique is to provide a constructor. A constructor is a special method that is called automatically each time an object is created. The definition of a constructor is similar to a regular method except for two small differences: the name of the constructor must be the same as the name of the enclosing class and no return type is allowed. The client code does not invoke the constructor directly, instead the client creates objects using the new operator and passes any needed arguments inside the parentheses after the type name. The constructor is called automatically as part of the object creation and initialization process.

Constructor overloading

A class can supply multiple constructors with different parameter lists. It is even possible to write a constructor that takes no arguments. The no argument constructor is sometimes called the 'default constructor'. The client code chooses the constructor to call by passing the appropriate parameters when an object is created.

Constructor initializer

One constructor can call another constructor in the same class. The syntax is :this(...) placed after the constructor signature but before the constructor body. The number and type of arguments passed determine which constructor version gets called. This technique is called a "constructor initializer". The main benefit is that it allows common initialization code to be placed in one constructor and any other constructors can simply make a call rather than repeating the code.

Compiler generated constructor

If a class does not define any constructors, then the compiler automatically generates a constructor that takes no arguments and has an empty body. Conversely, if a class defines even one constructor, then the compiler assumes the class designer is taking control of construction and does not create a constructor.

Initialization order

The various initialization options are executed in a well defined order. First, all fields are set to their default values. Second, the variable initializers are executed in order of their declaration. Finally, the constructor is run.