30天代码day4 Class vs. Instance

Class

A blueprint defining the charactaristics and behaviors of an object of that class type. Class names should be written in CamelCase, starting with a capital letter.

class MyClass{
    ...
}

Each class has two types of variables: class variables and instance variables; class variables point to the same (static) variable across all instances of a class, and instance variables have distinct values that vary from instance to instance.

Class Constructor

Creates an instance of a class (i.e.: calling the Dog constructor creates an instance of Dog). A class can have one or more constructors that build different versions of the same type of object. A constructor with no parameters is called a default constructor; it creates an object with default initial values specified by the programmer. A constructor that takes one or more parameters (i.e.: values in parentheses) is called a parameterized constructor. Many languages allow you to have multiple constructors, provided that each constructor takes different types of parameters; these are called overloaded constructors.

class Dog{ // class name
    static String unnamed = "I need a name!"; // class variable
    int weight; // instance variable
    String name; // instance variable
    String coatColor; // instance variable
    
    Dog(){ // default constructor
        this.weight = 0;
        this.name = unnamed;
        this.coatColor = "none";
    }
    Dog(int weight, String color){ // parameterized constructor
        // initialize instance variables
        this.weight = weight; // assign parameter's value to instance variable
        this.name = unnamed; 
        this.coatColor = color; 
    }
    Dog(String dogName, String color){ // overloaded parameterized constructor
        // initialize instance variables
        this.weight = 0;
        this.name = dogName; 
        this.coatColor = color; 
    }
}

Method

A sort of named procedure associated with a class that performs a predefined action. In the sample code below, returnType will either be a data type or if no value need be returned. Like a constructor, a method can have or more parameters.

returnType methodName(parameterOne, ..., parameterN){
    ...
    return variableOfReturnType; // no return statement if void
}

Most classes will have methods called getters and setters that get (return) or set the values of its instance variables. Standard getter/setter syntax:

class MyClass{
    dataType instanceVariable;
    ...
    void setInstanceVariable(int value){
        this.instanceVariable = value;
    }
    dataType getInstanceVariable(){
        return instanceVariable;
    }
}

Structuring code this way is a means of managing how the instance variable is accessed and/or modified.

Parameter

A parenthetical variable in a function or constructor declaration (e.g.: in int methodOne(int x), the parameter is int x).

Argument

The actual value of a parameter (e.g.: in methodOne(5), the argument passed as variable x is 5).

Additional Language Resources

Python Class and Instance Variables