Given the code fragment: public static void main(String[] args) { var even = List.of(1, even.add(-1); even.add(0, -2); even.add(0, -2); System.out.println(even); } What is the output?

The correct answer is D. A runtime exception is thrown. List.of() creates an immutable, fixed-size list. While add() is defined on the List interface (so the code compiles), calling it on a List.of() result throws an UnsupportedOperationException at runtime - making D correct. Why the distractors are wrong: A (compilation fails)…

Working with Arrays and Collections

Question

Given the code fragment:
public static void main(String[] args) {
var even = List.of(1,
even.add(-1);
even.add(0, -2);
even.add(0, -2);
System.out.println(even);
}
What is the output?

Options

AThe compilation fails.
B[1,-1]
C[-3,-2,-1]
DA runtime exception is thrown.

How the community answered

(31 responses)

A
3% (1)
B
10% (3)
C
13% (4)
D
74% (23)

Explanation

List.of() creates an immutable, fixed-size list. While add() is defined on the List interface (so the code compiles), calling it on a List.of() result throws an UnsupportedOperationException at runtime - making D correct.

Why the distractors are wrong:

A (compilation fails): The code compiles because var infers the type as List<Integer>, which declares add() in its interface. Immutability is a runtime constraint, not a compile-time one.
B [1,-1]: Would only be possible if the first add(-1) succeeded - it doesn't; the list is unmodifiable.
C [-3,-2,-1]: Would require all three mutation calls to succeed, which is impossible on a List.of() list.

Memory tip: Think of List.of() as "List. Off-limits." It compiles fine because the compiler only checks the interface contract, but any write operation is blocked at runtime. If you need a mutable list, use new ArrayList<>(List.of(...)).

Topics

#List.of() immutability#UnsupportedOperationException#Immutable collections#Runtime exceptions

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