Unit 3: Class Creation

Original practice questions written to the published framework for this section. No expert has reviewed them, and they are not taken from any official exam. Try each one before you open the answer.

1. Question 1Foundational

   In Java, what is the name of the special method that runs when an object is created with new and initializes its instance variables?

   1. AThe constructor
   2. BThe main method
   3. CAn accessor method
   4. DA mutator method

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   Answer: A. The constructor

   A constructor shares the class name, has no return type, and runs exactly once when new creates the object. Accessors and mutators are ordinary methods called later on an existing object, and main is only the program's entry point.

   Key insight: A constructor has the class name and no return type, which is what makes it run on object creation.

2. Question 2Foundational

   Which access modifier should instance variables use so that code outside the class cannot read or change them directly?

   1. Aprivate
   2. Bpublic
   3. Cstatic
   4. Dfinal

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   Answer: A. private

   Declaring instance variables private is the encapsulation convention the exam rubrics expect, forcing outside code to go through accessor and mutator methods. Static controls whether a variable belongs to the class rather than to each object, and final prevents reassignment without hiding the value.

   Key insight: Encapsulation means private state with public methods that control how it is read and changed.

3. Question 3Intermediate

   A Java class Student has a private instance variable String name. Which method header declares a standard accessor for it?

   1. Apublic String getName()
   2. Bpublic void getName(String name)
   3. Cprivate String getName()
   4. Dpublic void setName(String name)

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   Answer: A. public String getName()

   An accessor is public so other classes can call it, takes no parameters, and returns the type of the variable it exposes. A void return type cannot hand back the value, and the last choice is a mutator, which changes the variable instead of reporting it.

   Key insight: Accessors return a value and take nothing; mutators take a value and return nothing.

4. Question 4Intermediate

   A Java class needs one variable that counts how many objects of the class have been created and is shared by every object. How should that variable be declared?

   1. AAs a static variable
   2. BAs a private instance variable
   3. CAs a local variable inside the constructor
   4. DAs a parameter of the constructor

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   Answer: A. As a static variable

   A static variable belongs to the class, so a single copy is shared and can be incremented in the constructor to count objects. An instance variable would give every object its own separate counter, and a local variable would vanish when the constructor finished.

   Key insight: Static means one copy per class; instance means one copy per object.

5. Question 5Advanced

   In a Java class Dog with a private instance variable String name, the constructor public Dog(String name) { name = name; } leaves that instance variable null after a Dog is constructed. What explains this and corrects it?

   1. AThe parameter shadows the field, so the constructor must use this.name = name
   2. BString objects cannot be assigned inside a constructor
   3. CThe constructor must declare a return type before it can assign a field
   4. DThe instance variable must be declared static to be assigned

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   Answer: A. The parameter shadows the field, so the constructor must use this.name = name

   Inside the constructor the identifier name refers to the parameter, so name = name assigns the parameter to itself and leaves the field untouched. Writing this.name = name names the instance variable explicitly, or the parameter could simply be renamed.

   Key insight: When a parameter shares a field's name, only this makes the field reachable.