Understanding object references is an important part of AP Computer Science A. In this interactive practice set, you will work with Topics 1.12–1.14: object creation, object references, using object references, and calling instance methods.
Key Concepts to Remember Before You Start:
References vs. Primitive Values:
Primitive variables store values such as integers and doubles. An object
variable stores a reference to an object. The reference can
be used to access the object’s instance variables and call its instance methods.
Aliasing:
When two reference variables refer to the same object, they are aliases.
A change to that object’s state through one reference can be observed through
the other reference.
null References:
A reference variable containingnulldoes not currently refer
to an object. Calling an instance method using anullreference
causes aNullPointerExceptionat runtime.
Object Creation:
Thenewkeyword creates an object by invoking a constructor.
The resulting reference can be assigned to an object variable.
Reference Equality:
For object references, the==operator determines whether two
references refer to the same object. It does not compare the objects’
instance-variable values.
Test your object-reference and memory-tracing skills with these 10 practice questions. Select an answer to see the explanation and understand the reasoning behind each result.
Topics 1.12–1.14 Practice
Question 1 of 10 |
Score: 0
1. Which of the following best describes what is stored in an object
reference variable in Java?
Correct Answer: B
An object variable stores a reference to an object.
The reference allows the program to access the object’s state and call
its instance methods.
2. Consider the following code segment:
String s1 = “Java”;
String s2 = s1;
s2 = “Python”;
System.out.println(s1);
String s2 = s1;
s2 = “Python”;
System.out.println(s1);
What is printed?
Correct Answer: B
Initially,
s2 receives the same reference value as
s1. Both variables refer to the String containing
"Java".When s2 = "Python"; executes, only s2
is reassigned. The value of s1 remains unchanged, so
s1 still refers to "Java".
3. Consider the following code segment:
Car c1 = new Car(“Red”);
Car c2 = c1;
c1.setColor(“Blue”);
System.out.println(c2.getColor());
Car c2 = c1;
c1.setColor(“Blue”);
System.out.println(c2.getColor());
Assuming
setColor changes the car’s color instance
variable, what is printed?
Correct Answer: B
c1 and c2 refer to the same
Car object. This is called aliasing.When c1.setColor("Blue") changes the object’s state,
that same changed state is visible when the object is accessed through
c2.
4. Which exception occurs when a program attempts to call an instance
method using a reference variable whose value is
null?
Correct Answer: C
A
NullPointerException occurs at runtime when code
attempts to use a reference whose value is null in a
context that requires an object.
5. What is the default value of an instance variable whose type is
an object reference when no explicit value is assigned?
Correct Answer: C
An instance variable whose type is a reference type has a default
value of
null when no other value is assigned.This is different from local variables, which must be explicitly
initialized before they are used.
6. Consider the following code segment:
String str = null;
System.out.println(str.length());
System.out.println(str.length());
What happens when this code is executed?
Correct Answer: C
str contains null, so it does not refer to
a String object. Calling length() requires a String object,
resulting in a NullPointerException at runtime.
7. Which of the following correctly creates a new
Person object and assigns its reference to a variable
named p?
Correct Answer: B
The
new keyword creates an object and invokes the
appropriate constructor.The general form is:ClassName variableName = new ClassName(arguments);
8. Consider the following code segment:
Rectangle r1 = new Rectangle(10, 5);
Rectangle r2 = new Rectangle(10, 5);
Rectangle r3 = r1;
System.out.println((r1 == r2) + ” ” + (r1 == r3));
Rectangle r2 = new Rectangle(10, 5);
Rectangle r3 = r1;
System.out.println((r1 == r2) + ” ” + (r1 == r3));
What is printed?
Correct Answer: C
The two
copies the reference value from
new expressions create two separate
Rectangle objects.Therefore, r1 == r2 is false.Then:Rectangle r3 = r1;copies the reference value from
r1 into r3.
Both variables therefore refer to the same object, making
r1 == r3 true.
9. What happens to an object when there are no longer any references
that allow the program to access it?
Correct Answer: C
When an object is no longer reachable through any active reference,
it becomes eligible for garbage collection.Java’s garbage collector can later reclaim the memory associated with
such an object. The exact time at which garbage collection occurs is
not specified by the Java language.
10. Consider the following method header:
public void modifyPoint(Point p).
If this method changes the coordinates of p internally,
what happens to the original Point object passed to the method?
Correct Answer: A
Java is strictly pass-by-value.When an object reference is passed to a method, the parameter receives
a copy of the reference value. The parameter and the original variable
therefore refer to the same object.If the method changes the object’s instance state, that change is
visible through the original reference as well.
Topics 1.12–1.14 Practice Complete!
0 / 10
Understanding Object References and Aliasing
Aliasing is an important concept in AP CSA. When two reference variables refer to the same object, modifying that object’s state through one reference can affect what is observed through the other reference.
When tracing Java code, pay close attention to whether a statement creates a
new object with new or simply copies an existing reference value.
Continue Your AP CSA Practice
You can continue reviewing the remaining Unit 1 object concepts with Topic 1.15: String Methods .
For the complete current AP CSA framework, visit the AP CSA Unit 1: Using Objects and Methods practice hub.