Objects: Instances of Classes

A class describes what information objects of that class can store and what actions they can perform. It is the plan rather than the thing built from the plan, in the same way that a blueprint describes a kind of building without being the building.

• B is not correct: one specific object with stored data is an instance of a class, such as the building named "North Hall", rather than the definition every such object is built from.
• C is not correct: a single attribute value is one piece of data an object knows about itself, while a class describes which attributes every object of that kind can store.
• D is not correct: a method call asks an object to perform a behavior, which Topic 1.14 covers; a class instead defines the kind of object that receives such a call.

The class describes the kind, and each object is one concrete instance of that kind. Building is a class, while a specific building named "North Hall" is an object of that class.

• A is not correct: an object is not a keyword; private is the word written before the instance variables shown here, and it marks data kept inside the object.
• C is not correct: a blueprint with no stored data describes the class instead, and an object is one instance that holds its own values for the declared variables.
• D is not correct: a primitive type stores a simple value directly, while an object is an instance of a class that a reference variable can locate through an object reference.

In Java, attributes are stored in instance variables. The declaration private String name; gives each object a place to keep its own name, so the name is data the object knows about itself.

• B is not correct: a behavior is an action an object can perform, such as reporting its total number of rooms or changing its name, not the stored value itself.
• C is not correct: a class is the definition of a type of object, while the stored name is one piece of data held inside one such object.
• D is not correct: an instance is a whole object built from the class, and the name is a single attribute stored inside that object rather than the object itself.

A Building object can report its total number of rooms and can change its name. Those are behaviors. The stored facts, such as the building number and the floor count, are the attributes the object keeps.

• A is not correct: an attribute is data the object knows about itself, such as the stored counts, rather than the act of reporting a value to whoever asked.
• C is not correct: a class name identifies the kind of object, so Building is the class name while reporting is one of the actions its objects perform.
• D is not correct: an instance variable value is stored data, and reporting that stored data is an action carried out on it rather than the data itself.

Read a declaration from left to right. The word private marks data kept inside the object, the type states what kind of value can be stored, and the identifier names the attribute.

• B is not correct: public int floorCount() is written in the form of a method, and methods express behaviors rather than the attributes an object stores.
• C is not correct: Building floorCount; gives the variable a class type, so it could hold an object reference to a Building rather than a count of floors.
• D is not correct: void floorCount() is also written as a method, so it describes an action; attributes are stored in instance variables instead.

A behavior is an action that an object can perform, such as reporting a total number of rooms. The other three members are instance-variable declarations, and instance variables hold attributes rather than actions.

• A is not correct: private String name; declares an instance variable, so it stores the name attribute rather than describing an action the object performs.
• B is not correct: private int rooms; declares an instance variable holding a stored count, and stored data is an attribute rather than a behavior.
• D is not correct: private int floors; declares another instance variable, so it records data the object knows about itself rather than an action it can carry out.

One building might have number 101, name "North Hall", three floors, and twenty rooms per floor. Another Building object can store different values in variables that carry those same names.

• A is not correct: both objects are instances of the same class, so the class they are built from is the one thing that cannot differ between them.
• C is not correct: int is a type in the Java language, and what it means is fixed rather than something each object decides for itself.
• D is not correct: the Java language is the same for every program, so it is not part of the information one object remembers about itself.

A blueprint describes a kind of building; it is not the building itself. Building is the class, and "North Hall", Science Center, and Library Building are objects of that class.

• A is not correct: the class is Building, the plan that describes the kind, while "North Hall" names one particular thing built from that plan.
• C is not correct: a behavior is an action such as reporting a total number of rooms, whereas "North Hall" identifies one object rather than an action.
• D is not correct: a data type keyword such as int states what kind of value a variable may store, not the identity of a single object.

An attribute is data that an object knows about itself, and a behavior is an action that an object can perform. A Building object stores a name and can also change its name.

• A is not correct: the two labels are swapped: the stored name is data the object knows, and changing that name is an action the object carries out.
• C is not correct: a class is the definition of a type of object, while these two are a stored attribute and an action described inside such a class.
• D is not correct: an object is a specific instance of a class, so a stored name and a renaming action are parts of an object rather than objects themselves.

Attributes are stored in instance variables. A declaration such as private int roomsPerFloor; gives every future object one place to keep that value, and each Building object receives its own set of these variables.

• B is not correct: an instance variable does not create a shared object, because each object created from the class receives its own separate set of these variables.
• C is not correct: a method call is how a behavior is requested, and a method has a name; an instance variable stores one attribute instead of performing an action.
• D is not correct: declaring an instance variable uses words that already exist, such as private and a type name, so nothing is added to the Java language.

One building might store number 101 and name "North Hall", while another Building object stores different values in variables with the same names. The class plan is shared, but the stored values are not.

• A is not correct: objects of a class share the plan rather than the values, and each one can store different data in variables that carry the same names.
• C is not correct: a behavior is an action written as a method, so it is not something held inside a variable whose type is int.
• D is not correct: an attribute is data stored in an instance variable, while a behavior is written as a method, so the two are different kinds of member.

The class describes the kind, and each object is one concrete instance of that kind. The section lists Building as the class, with Science Center among the objects belonging to it.

• A is not correct: only Building is a class, since Science Center is one instance built from that plan rather than a second plan of its own.
• C is not correct: the roles are reversed here: the plan is Building, and Science Center is one particular object built from that plan.
• D is not correct: a behavior is an action an object performs, such as reporting a total number of rooms, and neither of these two names an action.