Reading Bounded Wildcards

Intermediate Algorithms and Data Structures English

Also available: Deutsch

Created by Best · 16.08.2026 at 09:13 UTC

Wildcards: List<? extends Number> is a list of an unknown subtype of Number (Producer). You can read it as Number, but you cannot write to it arbitrarily (except for null).

List<? super Integer> is a Consumer: you can write Integer to it, but reading will yield Object.

Mnemonic PECS: producer extends, consumer super.

Example: sum(List<? extends Number>) adds numbers from lists of Integer or Double.

$$\mathtt{List\lt ?\ super\ Integer\gt }$$

Wildcard	add(Integer)
List<? super Integer>	type-safe
List<? extends Integer>	rejected by the compiler
## Where used

PECS in library methods (addAll, Producer/Consumer of Collections). Framework code (Spring, Guava-like) is full of ? extends / ? super; reading the signature determines whether insertion or only reading is allowed.

Depth

An upper wildcard bound describes a source of an unknown but sufficiently specific type. Values can be safely read as the supertype, but writing is not possible except for null, because the concrete subtype remains unknown.

A lower bound, on the other hand, describes a target that can accept values of a specific type. When reading, only Object is safe. This asymmetry is often summarized as producer extends, consumer super.

Wildcards express relationships between calls without binding a type name multiple times. When input and output must share the same unknown type, a named method type parameter is usually more precise.

Difficulty levels

Determine safe read and write operations.
Choose upper and lower bounds based on data flow.
Distinguish between wildcard and named method type parameter.

Pitfalls

The bound does not specify the exact element type. Therefore, with ? extends Number, you are not allowed to insert any arbitrary Number.

University approvals: 0

Tasks

Question 1

Which operation is type-safe and common on List<? extends Number>?

add(Integer.valueOf(1)) any number of times

Number x = list.get(0)

add(new Object())

list = new ArrayList()

Question 2

Which operation is type-safe with List<? extends Number>?

Add any Number

Read an element as Number

Add an Object

Convert the list to List<Number>

Question 3

Return the first element of a list of any numeric subtypes.

Hint

From List<? extends Number>, values can be read as Number. The first element is provided by list.get(0).

import java.util.List;
public class NumberLists {
  public static Number firstNumber(
      List<? extends Number> list) {
    // TODO
    return null;
  }
}

Starter code is prefilled; replace TODO blocks with your solution.

Runtime output (stdout/stderr)

1 test case will be used for grading

Run checks runtime behavior only. Final correctness is evaluated when you submit.

Card Info

Topic: Algorithms and Data Structures
Difficulty: Intermediate
Completed: 0 users

Creator

Best

BestBuddy