Class Creation · ⁨การสร้างคลาส⁩

Learning Objective 3.1.A: Represent the design of a program by using natural language or creating diagrams that indicate the classes in the program and the data and procedural abstractions found in each class by including all attributes and behaviors.

• 3.1.A.1 Abstraction is the process of reducing complexity by focusing on the main idea. By hiding details irrelevant to the question at hand and bringing together related and useful details, abstraction reduces complexity and allows one to focus on the idea.
• 3.1.A.2 Data abstraction provides a separation between the abstract properties of a data type and the concrete details of its representation. Data abstraction manages complexity by giving data a name without referencing the specific details of the representation. Data can take the form of a single variable or a collection of data, such as in a class or a set of data.
• 3.1.A.3 An attribute is a type of data abstraction that is defined in a class outside any method or constructor. An instance variable is an attribute whose value is unique to each instance of the class. A class variable is an attribute shared by all instances of the class.
• 3.1.A.4 Procedural abstraction provides a name for a process and allows a method to be used only knowing what it does, not how it does it. Through method decomposition, a programmer breaks down larger behaviors of the class into smaller behaviors by creating methods to represent each individual smaller behavior. A procedural abstraction may extract shared features to generalize functionality instead of duplicating code. This allows for code reuse, which helps manage complexity.
• 3.1.A.5 Using parameters allows procedures to be generalized, enabling the procedures to be reused with a range of input values or arguments.
• 3.1.A.6 Using procedural abstraction in a program allows programmers to change the internals of a method (to make it faster, more efficient, use less storage, etc.) without needing to notify method users of the change as long as the method signature and what the method does is preserved.
• 3.1.A.7 Prior to implementing a class, it is helpful to take time to design each class including its attributes and behaviors. This design can be represented using natural language or diagrams.

Learning Objective 3.2.A: Explain the social and ethical implications of computing systems.

• 3.2.A.1 System reliability refers to the program being able to perform its tasks as expected under stated conditions without failure. Programmers should make an effort to maximize system reliability by testing the program with a variety of conditions.
• 3.2.A.2 The creation of programs has impacts on society, the economy, and culture. These impacts can be both beneficial and harmful. Programs meant to fill a need or solve a problem can have unintended harmful effects beyond their intended use.
• 3.2.A.3 Legal issues and intellectual property concerns arise when creating programs. Programmers often reuse code written by others and published as open source and free to use. Incorporation of code that is not published as open source requires the programmer to obtain permission and often purchase the code before integrating it into their program.

Learning Objective 3.3.A: Develop code to designate access and visibility constraints to classes, data, constructors, and methods.

• 3.3.A.1 Data encapsulation is a technique in which the implementation details of a class are kept hidden from external classes. The keywords public and private affect the access of classes, data, constructors, and methods. The keyword private restricts access to the declaring class, while the keyword public allows access from classes outside the declaring class.
• 3.3.A.2 In this course, classes are always designated public and are declared with the keyword class.
• 3.3.A.3 In this course, constructors are always designated public.
• 3.3.A.4 Instance variables belong to the object, and each object has its own copy of the variable.
• 3.3.A.5 Access to attributes should be kept internal to the class in order to accomplish encapsulation. Therefore, it is good programming practice to designate the instance variables for these attributes as private unless the class specification states otherwise.
• 3.3.A.6 Access to behaviors can be internal or external to the class. Methods designated as public can be accessed internally or externally to a class, whereas methods designated as private can only be accessed internally to the class.

Learning Objective 3.4.A: Develop code to declare instance variables for the attributes to be initialized in the body of the constructors of a class.

• 3.4.A.1 An object's state refers to its attributes and their values at a given time and is defined by instance variables belonging to the object. This defines a has-a relationship between the object and its instance variables.
• 3.4.A.2 A constructor is used to set the initial state of an object, which should include initial values for all instance variables. When a constructor is called, memory is allocated for the object and the associated object reference is returned. Constructor parameters, if specified, provide data to initialize instance variables.
• 3.4.A.3 When a mutable object is a constructor parameter, the instance variable should be initialized with a copy of the referenced object. In this way, the instance variable does not hold a reference to the original object, and methods are prevented from modifying the state of the original object.
• 3.4.A.4 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.
• 3.4.A.5 The default value for an attribute of type int is 0. The default value of an attribute of type double is 0.0. The default value of an attribute of type boolean is false. The default value of a reference type is null.

Learning Objective 3.5.A: Develop code to define behaviors of an object through methods written in a class using primitive values and determine the result of calling these methods.

• 3.5.A.1 A void method does not return a value. Its header contains the keyword void before the method name.
• 3.5.A.2 A non-void method returns a single value. Its header includes the return type in place of the keyword void.
• 3.5.A.3 In non-void methods, a return expression compatible with the return type is evaluated, and the value is returned. This is referred to as return by value.
• 3.5.A.4 The return keyword is used to return the flow of control to the point where the method or constructor was called. Any code that is sequentially after a return statement will never be executed. Executing a return statement inside a selection or iteration statement will halt the statement and exit the method or constructor.
• 3.5.A.5 An accessor method allows objects of other classes to obtain a copy of the value of instance variables or class variables. An accessor method is a non-void method.
• 3.5.A.6 A mutator (modifier) method is a method that changes the values of the instance variables or class variables. A mutator method is often a void method.
• 3.5.A.7 Methods with parameters receive values through those parameters and use those values in accomplishing the method's task.
• 3.5.A.8 When an argument is a primitive value, the parameter is initialized with a copy of that value. Changes to the parameter have no effect on the corresponding argument.

Learning Objective 3.6.A: Develop code to define behaviors of an object through methods written in a class using object references and determine the result of calling these methods.

• 3.6.A.1 When an argument is an object reference, the parameter is initialized with a copy of that reference; it does not create a new independent copy of the object. If the parameter refers to a mutable object, the method or constructor can use this reference to alter the state of the object. It is good programming practice to not modify mutable objects that are passed as parameters unless required in the specification.
• 3.6.A.2 When the return expression evaluates to an object reference, the reference is returned, not a reference to a new copy of the object.
• 3.6.A.3 Methods cannot access the private data and methods of a parameter that holds a reference to an object unless the parameter is the same type as the method's enclosing class.

Learning Objective 3.7.A: Develop code to define behaviors of a class through class methods.

• 3.7.A.1 Class methods cannot access or change the values of instance variables or call instance methods without being passed an instance of the class via a parameter.
• 3.7.A.2 Class methods can access or change the values of class variables and can call other class methods.

Learning Objective 3.7.B: Develop code to declare the class variables that belong to the class.

• 3.7.B.1 Class variables belong to the class, with all objects of a class sharing a single copy of the class variable. Class variables are designated with the static keyword before the variable type.
• 3.7.B.2 Class variables that are designated public are accessed outside of the class by using the class name and the dot operator, since they are associated with a class, not objects of a class.
• 3.7.B.3 When a variable is declared final, its value cannot be modified.

Learning Objective 3.8.A: Explain where variables can be used in the code.

• 3.8.A.1 Local variables are variables declared in the headers or bodies of blocks of code. Local variables can only be accessed in the block in which they are declared. Since constructors and methods are blocks of code, parameters to constructors or methods are also considered local variables. These variables may only be used within the constructor or method and cannot be declared to be public or private.
• 3.8.A.2 When there is a local variable or parameter with the same name as an instance variable, the variable name will refer to the local variable instead of the instance variable within the body of the constructor or method.

Learning Objective 3.9.A: Develop code for expressions that are self-referencing and determine the result of these expressions.

• 3.9.A.1 Within an instance method or a constructor, the keyword this acts as a special variable that holds a reference to the current object—the object whose method or constructor is being called.
• 3.9.A.2 The keyword this can be used to pass the current object as an argument in a method call.
• 3.9.A.3 Class methods do not have a this reference.