Understanding Multiple Wildcards in Generic Methods in Java

How does Java handle multiple wildcards in generic methods, and what are the implications for type safety?

import java.util.*;

public class TwoListsOfUnknowns {
    static void doNothing(List<?> list1, List<?> list2) { }
    
    public static void main(String[] args) {
        List<String> list1 = null;
        List<Integer> list2 = null;
        doNothing(list1, list2); // compiles fine!
    }
}

In Java, Generics allow for safer code by enforcing type checks at compile time. However, the use of wildcards can create confusion, especially when combining different generic types. This guide clarifies how Java handles multiple wildcards, highlighting the nuances in compilation and type safety.

import java.util.*;

public class LOLUnknowns1 {
    static void probablyIllegal(List<List<?>> lol, List<?> list) {
        lol.add(list); // this compiles!! how come???
    }
}

Causes

• Wildcards in generics allow for flexibility in type parameterization but can lead to confusion when utilized improperly.
• Different types in wildcards can cause unexpected compile-time validation that does not align with runtime behavior.

Solutions

• Use bounded wildcards to provide clearer intention and constraints on the types that can be passed.
• Avoid using raw types and strive for explicit typing to ensure readability and type safety.

Mistake: Assuming that List<List<?>> is interchangeable with List<List<String>>.

Solution: Recognize that while List<E> is a List<?>, List<List<E>> is not a List<List<?>> due to how Java handles generics.

Mistake: Forgetting about the implications of adding to a generic collection with wildcards.

Solution: Always be cautious with operations that may introduce type safety issues when using wildcards, especially with lists.