Understanding the Unexpected Behavior of List<String> and List<Integer> in Generics

Why are List<String> and List<Integer> behaving unexpectedly in my Java program?

List<String> stringList = new ArrayList<>();
List<Integer> integerList = new ArrayList<>();
stringList.add("test");
integerList.add(123);

In Java, generics provide a way to enforce type safety at compile time by allowing classes, interfaces, and methods to operate on types specified as parameters. However, developers may encounter unexpected behaviors with generics, particularly when working with collections like List<String> and List<Integer>. This could arise from misunderstandings in type matching, assignment, or operations across different generic types.

List<String> stringList = new ArrayList<>();
// Ensure adding correct type
stringList.add("exampleString");

List<Integer> integerList = new ArrayList<>();
// Adding integers only
integerList.add(1); // Valid
// The following would raise a compile-time error
// integerList.add("string");

Causes

• Incorrect type assignment: Attempting to assign a raw type as a parameter, such as List instead of List<String> or List<Integer>.
• Using incompatible types: Trying to add elements of one type to a collection expecting another type can lead to compile-time errors or runtime exceptions.

Solutions

• Ensure type safety by explicitly defining the generic types. For example, use List<String> for string elements only.
• Check your method signatures if they involve generics to ensure they expect the correct type parameters.

Mistake: Using raw types instead of parameterized types (e.g., using List instead of List<String>).

Solution: Always specify the type in your List declaration to leverage type safety.

Mistake: Forgetting to handle type casting when retrieving objects from a generic collection.

Solution: Use proper type casting when extracting elements from a List to avoid ClassCastException.