Code Smell: Avoid Using Object as Input or Output

Table of Contents

• read(List<Object> keyValues)
• of(Object ...)
• Other bad examples that uses Object

read(List<Object> keyValues)

read(..., ImmutableList<Object> keyValues, ...) {
  // internally it calls: Key.of(value)  
}
readWithComoundKeys(..., List<ImmutableList<Object>> compoundKeyValues, ...);

It’s very easy for clients to make mistake when call these methods. For example:

List<String> ids;
read(..., ImmutableList.of(ids), ...);

ImmutableList.of(ids) creates a list with one element: the element is a List of all these ids:

ImmutableList<List<String>> x = ImmutableList.of(ids);

As ImmutableList<List<String>> is an ImmutableList of Object, so it compiles, but would fail at run time.

• Another problem in previous method is it uses ImmutableList which makes the method not able to take other Lists, we can just use List or Collection. ##### How General/Specific in parameter or return types?
• For parameters, use the most generic types possible
• For return types, use the most specific types possible This can be refactored as:

read(..., List<Key> keyValues, ...)

• This method can take a list of singular key or compound keys and much easier to use and not likely to make mistake.

of(Object ...)

public static SomeType of(Object... objects) {
  for (Object object : objects) {
    if (object instanceof String) {
    } else if (object instanceof AnotherType) {
    } else {
      throw new IllegalArgumentException(...);
    }
  }
}

In most cases, this can be refactored as below:

public static SomeType of(List<String> objects) {}
public static SomeType of(List<AnotherType> objects) {}

Other bad examples that uses Object

public org.joda.DateTime(Object instant) {}

public boolean java.uti.Calendar.before(Object when) {
  return when instanceof Calendar
      && compareTo((Calendar)when) < 0;
}