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Unit 5: Writing Classes

10 dot points across 10 inquiry questions. Click any dot point for a focused answer with worked past exam questions where available.

How does an accessor method return an object's data, and what is the toString method?

Topic 5.4 Accessor Methods: write accessor (getter) methods, including a toString method, that return information about an object's state without changing it.
A focused answer to AP CSA Topic 5.4, covering accessor (getter) methods, the non-void return type, why accessors do not change state, the toString method and how println uses it, and returning computed values, with a fully worked example.
10 min answer →

What are the parts of a Java class, and how do instance variables, constructors and methods fit together?

Topic 5.1 Anatomy of a Class: identify the parts of a class definition (the class header, private instance variables, constructors and methods) and explain encapsulation through access modifiers.
A focused answer to AP CSA Topic 5.1, covering the class header, private instance variables, constructors and methods, the meaning of public and private, encapsulation, and how the pieces combine into a working class, with a fully worked example.
10 min answer →

How does a constructor initialise a new object's instance variables, and what happens if you omit one?

Topic 5.2 Constructors: write constructors that initialise an object's instance variables from parameters, including overloaded and default constructors, and explain the default initial values.
A focused answer to AP CSA Topic 5.2, covering constructor syntax, initialising instance variables from parameters, the no-argument constructor, overloading, default values when a variable is not set, and the difference between a parameter and an instance variable, with a fully worked example.
11 min answer →

How do comments, preconditions and postconditions document a method's behavior?

Topic 5.3 Documentation with Comments: write comments that document a method, including its precondition and postcondition, to describe what the method assumes and guarantees.
A focused answer to AP CSA Topic 5.3, covering line and block comments, what makes a useful comment, preconditions and postconditions, why they form a contract for a method, and how they guide both callers and implementers, with a fully worked example.
10 min answer →

What ethical and social responsibilities come with designing and using computing systems and data?

Topic 5.10 Ethical and Social Implications of Computing Systems: explain the ethical and social responsibilities of programmers, including data privacy, intellectual property, system impact and responsible design.
A focused answer to AP CSA Topic 5.10, covering data privacy and security, intellectual property and licensing, the social impact of software, bias and fairness, and the responsibilities a programmer carries when designing classes that store and process data, with a worked design scenario.
10 min answer →

How does a mutator method change an object's state, and how can it validate the new value?

Topic 5.5 Mutator Methods: write mutator (setter) methods, usually void, that change an object's instance variables, including methods that validate or constrain the new value before assigning it.
A focused answer to AP CSA Topic 5.5, covering mutator (setter) methods, the void return type, changing instance variables from a parameter, validating a new value before assigning, and why mutators protect encapsulated data, with a fully worked example.
10 min answer →

What is the scope of a local variable, and how do public and private control access to members?

Topic 5.8 Scope and Access: distinguish local variable scope from instance variable scope, and use the access modifiers public and private to control where a class member can be used.
A focused answer to AP CSA Topic 5.8, covering the scope of local variables and parameters versus instance variables, variable shadowing, the public and private access modifiers, and how access control supports encapsulation, with a fully worked example.
10 min answer →

How do static variables and methods belong to the class rather than to individual objects?

Topic 5.7 Static Variables and Methods: declare and use static variables and methods, which belong to the class as a whole rather than to any one object, and explain how they differ from instance members.
A focused answer to AP CSA Topic 5.7, covering the static keyword, class-level variables shared by all objects, static methods called on the class, why static methods cannot use instance variables, and counting objects with a static field, with a fully worked example.
11 min answer →

What does the this keyword refer to, and when do you need it inside a method or constructor?

Topic 5.9 this Keyword: use the implicit parameter this to refer to the current object, disambiguate an instance variable from a parameter of the same name, and pass the current object to another method.
A focused answer to AP CSA Topic 5.9, covering what this refers to, using this to distinguish an instance variable from a shadowing parameter, calling another method on the current object with this, and passing this as an argument, with a fully worked example.
10 min answer →

How do you design a method with parameters and a return value, and how does it use the object's own data?

Topic 5.6 Writing Methods: design and implement methods that take parameters, perform a computation possibly using the object's instance variables, and return a result or perform an action.
A focused answer to AP CSA Topic 5.6, covering the method header (modifiers, return type, name, parameters), how a method uses instance variables and parameters, returning a value versus void, calling one method from another, and tracing a method, with a fully worked example.
11 min answer →