3.4 Constructors

1. Exam Points

• Call a constructor by passing into the correct arguments.
• Default values of different data types when calling the default constructor of a class.
• All the constructors in this course will be declared public.
• For a parameter of a reference type, pass into an object of that type.
• It is good practice to declare attributes as private, and then add public methods to access them.

2. Knowledge Points

(1) Constructors

• A constructor is a special method inside a class.
• A class contains constructors that are called to create objects.
  
• About constructors:
  • A constructor has the same name as the class in which it is declared.
  • A constructor has no return type.
  • A constructor is called when an object is created.
  • A constructor can be overloaded(重载), meaning there can be multiple constructors with different signatures inside a class.
  • The purpose of a constructor: to initialize attributes or do other initialization work.
  • A constructor sets the initial state of an object but setting initial values to its attributes.
• Example:
  
  

(2) The Default Constructor

• When no constructor is written, Java provides a no-parameter constructor, and the instance variables are set to default values according to the data type of the attribute. This constructor is called the default constructor.
• The default values are:
  • int attribute: 0.
  • double attribute: 0.0.
  • boolean attribute: false.
  • reference type (ex. String): null.
• The default constructor is gone if you declare any constructor.

(3) Pass Arguments(实际参数) to Parameters(形式参数)

• For a parameter of a primitive type, pass into a value of that type.
• For a parameter of a reference type, pass into an object of that type.

3. Exercises

• Practice
• Solution