Lesson 3.6: Methods: Passing and Returning Object References
Lesson 3.6: Methods: Passing and Returning Object References
🕑 35–40 min·
8 Practice Questions·
4 Mastery Questions·
Output Predictor + Bug Hunt
What You'll Learn
- Explain how object references are passed to methods
- Distinguish mutating through a parameter vs reassigning the parameter reference
- Identify aliasing and predict its effects
- Apply EK 3.6.A.3: same-class access to private fields
- Explain what is returned when a method returns an object reference
Key Vocabulary
| Term | Definition |
|---|---|
| object reference | Variable storing the memory address of an object, not the object itself |
| alias | Two reference variables pointing to the SAME object in memory |
| mutable object | Object whose state can be changed after creation (has setters) |
| immutable object | Object whose state cannot be changed after creation (e.g., String in Java) |
| same-class access | A method can access private data of a parameter if the parameter is the SAME type as the enclosing class (EK 3.6.A.3) |
| null | A reference that points to no object; calling a method on null causes NullPointerException |
CED EK 3.6.A.1–3.6.A.3
When an object reference is passed, the parameter receives a COPY of the reference -- both the caller and parameter point to the SAME object (EK 3.6.A.1). A method CAN change a mutable object's state. When a method returns an object reference, the caller receives a reference to the SAME object (EK 3.6.A.2). A method CANNOT access private data of a parameter unless the parameter is the same type as the enclosing class (EK 3.6.A.3).
How Object References Are Passed
Mutating a Passed Object
public class Baker {
public void fillJar(Jar j) {
j.addCookies(10); // changes the SAME jar
}
}
Jar myJar = new Jar(5);
new Baker().fillJar(myJar);
System.out.println(myJar.getCookies()); // 15
j is a copy of the reference -- points to the SAME jar. Calling j.addCookies(10) mutates the shared object.
Aliasing
Two Variables, One Object
Jar jar1 = new Jar(5); Jar jar2 = jar1; // alias! jar2.addCookies(10); System.out.println(jar1.getCookies()); // 15, not 5
jar2 = jar1 makes both point to the same Jar. Changes through jar2 are visible through jar1.
AP Trap: Object Parameter vs Primitive Parameter
Primitives: copy of value -- caller unaffected. Objects: copy of reference -- SAME object is reachable; state CAN be changed.
AP Trap: Reassigning Parameter Reference Doesn't Affect Caller
public void replaceJar(Jar j) {
j = new Jar(100); // replaces local copy only
}
Jar myJar = new Jar(5);
replaceJar(myJar);
// myJar still has 5 cookies
Reassigning j to a new Jar replaces the LOCAL reference copy. myJar in the caller is unaffected. This is DIFFERENT from calling a mutator method on j.
AP Trap: Cannot Access Another Class's Private Data
public class Baker {
public void inspect(Jar j) {
System.out.println(j.cookies); // COMPILE ERROR
}
}
EK 3.6.A.3: Baker != Jar. Baker cannot access Jar's private fields directly.
Same-Class Private Access (EK 3.6.A.3)
Valid Same-Class Access
public class Point {
private int x; private int y;
public boolean equals(Point other) {
return this.x == other.x && this.y == other.y;
// other.x is valid: other is also a Point
}
}
A method inside Point CAN access other.x and other.y because other is also a Point.
Real-World Connection: Passing a BankAccount to a transfer() method lets it call account.withdraw() -- mutating the same account. Powerful but requires careful design.
Summary
- When an object reference is passed, the parameter is a COPY of the reference; both point to the SAME object (EK 3.6.A.1).
- A method CAN change a mutable object passed as a parameter by calling its mutators.
- Reassigning the parameter reference does NOT affect the caller's variable.
- Returned reference points to the SAME object (EK 3.6.A.2).
- Same-class: method can access private fields of a same-type parameter. Different class: compile error (EK 3.6.A.3).
Practice Questions
MCQ 1
After
change(myBox) where change calls b.setVal(99) and myBox=Box(1), result?⚠ Predict the answer before reading the options.
A 1
B 0
C Compile error
D 99
D is correct. b is a copy of reference to same Box. setVal(99) mutates shared object.
MCQ 2
What is printed?
Box a = new Box(5); Box b = a; b.setVal(20); System.out.println(a.getVal());
A 5
B 20
C 0
D Compile error
B is correct. b=a is an alias. b.setVal(20) changes shared object. a.getVal()=20.
MCQ 3
public static void reset(Box b) { b = new Box(0); } called with Box(42). What does myBox.getVal() return?⚠ Predict the answer before reading the options.
A 0
B 42
C NullPointerException
D Compile error
B is correct. Reassigning b (local ref copy) does NOT affect myBox. Still Box(42).
MCQ 4
Inside a Rectangle method,
other.width where width is private and other is Rectangle parameter. TRUE?
A Compile error
B Valid: same-class access to same-type parameter private field (EK 3.6.A.3)
C Valid only if width is static
D Valid only if other is public
B is correct. EK 3.6.A.3: same-class access is permitted.
MCQ 5
w1 starts at 100, w2 at 50.
w1.transfer(w2, 30) does money -= amt; other.money += amt;. Output?
A 100 50
B 70 80
C 130 20
D Compile error
B is correct. w1=70, w2=80. Same-class private access is valid (EK 3.6.A.3).
MCQ 6
Which statement about EK 3.6.A.3 is TRUE?
A A method can access private fields of ANY object parameter
B A method can access private fields of a parameter ONLY when the parameter is the same type as the enclosing class
C Private fields can never be accessed
D Private fields become public when passed as parameters
B is correct. Same-type = allowed; different type = compile error.
MCQ 7
Which CORRECTLY distinguishes primitive vs object reference passing?
A Both pass exact copies; neither affects the original
B Primitives: copy of value; Objects: copy of reference to same object; methods CAN mutate the object
C Objects pass a copy of the entire object
D Primitives pass the original variable
B is correct. EK 3.5.A.8 (primitives) + EK 3.6.A.1 (objects).
MCQ 8
After
Dog winner = competition.getBest();, calling winner.setName("Rex");...
A Creates a new Dog; competition's dog unchanged
B Throws NullPointerException
C Changes the same Dog object the competition holds
D Compile error
C is correct. EK 3.6.A.2: returned reference points to same object.
PRACTICE WITH A GAME — CHOOSE ONE:
Output Predictor: Object References
Predict carefully -- aliasing and reference copies.
Question 1 of 3 · Score: 0
Done!
You got 0 of 3 correct.
Bug Hunt: Object References
Click the buggy line, or 'No bug'.
Question 1 of 3 · Score: 0
No bug — code is correct
Done!
You got 0 of 3 correct.
Mastery: Methods
MCQ 1
What is printed?
// doubleNode: n.setData(n.getData() * 2) new Graph().doubleNode(myNode); // myNode started as Node(7) System.out.println(myNode.getData());
A 14
B 7
C 0
D Compile error
A is correct. doubleNode receives ref copy. setData(14) mutates shared Node.
MCQ 2
After this code, what does
a.getData() print?Node a = new Node(10); Node b = a; b.setData(50); b = new Node(99);
⚠ Predict the answer before reading the options.
A 99
B 10
C 0
D 50
D is correct. b=a alias. b.setData(50) changes shared node to 50. b=new Node(99) moves b; a still has 50.
MCQ 3
public void swap(Node a, Node b) reassigns a and b. Do caller variables change?
A Yes: object references modified by called methods
B Yes, if nodes are mutable
C No: reassigning parameters changes only local copies
D No: Node is immutable
C is correct. Reassigning local reference copies does not affect caller.
MCQ 4
public boolean sameAs(Node other) { return this.data == other.data; } -- data is private. Compiles?
A No: private inaccessible
B No: == can't compare private
C Yes, but only if data is static
D Yes: same-class access (EK 3.6.A.3)
D is correct. Inside Node, private fields of a Node parameter are accessible.
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