In the table below, each cell contains a description of how the thing in the first column of the row relates to the thing at the head of the column.
| Class | Object | Instance | Instance variable | Constructor | Instance method | |
|---|---|---|---|---|---|---|
| Class | Classes define the structure and behavior of objects that are instances of that class. | Classes can have many instances. | Classes define the instance variables that each instance of the class will have. | Classes define the constructor or constructors used to create and initialize instances of a the class. | Classes define the instance (non-static) methods that can be called on instances of the class. | |
| Object | Each object is an instances of a class. | “Object” is another word for “instance”. | Each object has its own value for each instance variable. | An object is created and initialized by invoking a constructor. | Objects have methods that can be invoked on them via the dot operator. | |
| Instance | An instance is a particular object with a structure defined by its class. | “Instance” is another word for “object”. | Each instances contains its own copy of the class’s instance variables. | An instance created and initialized by calling a constructor. | Instances supports the methods defined in the class. | |
| Instance variable | Instance varables are declared (i.e. given their types and names) at the top level of a class to define a piece of data associated with each instance of the class. | Instance variables stores the data associated with an object and can be accessed with the dot operator applied to an object value. | Each instance variable represents part of the state of an instance. | Instance variables are usually initialized in the code of a constructor. | Instance variables can be accessed by code in instance methods. | |
| Constructor | Constructors are declared at the top level of a class to specify how to create instances of that class. | A constructor always produces a new object when invoked. | A constructor creates and initializes an instance of the class it is part of. | Constructors have the job of initializing instance variables so objects are ready to use after the are constructed. | A constructor is a bit like a method in that it contains code and has access to an object’s instance variables but rather than a return type and a name just has a name which matches the class name. | |
| Instance method | Instance methods are declared at the top level of a class to define the behaviors that can be invoked on instances of the class. | Instance methods provide the behaviors of objects and are invoked on objects via the dot operator. | Instance methods can only be invoked if you have an instance to invoke them on. | Instance methods can access instance variables on the object they were invoked on just by their plain name, e.g. x, or using the notation this.x. | Instance methods cannot run until the object has been created but can be called from within the code of the constructor. |