Dealing with completeness (type coverage) in pattern labels for switch 2 – Objects, Immutability, Switch Expressions, and Pattern Matching

Now, the selector expression can be any type which means that the total pattern Vehicle v is not total anymore. While Vehicle v becomes an optional ordinary pattern, the new total pattern is case Object obj. This means that we can cover all possible values by adding the default label or the case Object obj total pattern:

return switch(o) {
  case Car car -> “You’re a car”;
  case Van van -> “You’re a van”;
  case Vehicle v -> “You’re a vehicle”;  // optional
  case Object obj -> “You’re an object”; // total pattern         
};

I think you got the idea! How about using an interface for the base type? For instance, here is an example based on the Java built-in CharSequence interface:

public static String whatAmI(CharSequence cs) {
      
  return switch(cs) {          
    case String str -> “You’re a string”;          
    case Segment segment -> “You’re a Segment”;
    case CharBuffer charbuffer -> “You’re a CharBuffer”;
    case StringBuffer strbuffer -> “You’re a StringBuffer”;
    case StringBuilder strbuilder -> “You’re a StringBuilder”;          
  };
}  

This snippet of code doesn’t compile. The error is clear: The switch expression does not cover all possible input values. But, if we check the documentation of CharSequence we see that it is implemented by 5 classes: CharBuffer, Segment, String, StringBuffer, and StringBuilder. In our code, each of these classes is covered by a pattern label, so we have covered all possible values, right? Well, yes and no … “Yes” because we cover all possible values for the moment, and “no” because anyone can implement the CharSequence interface which will break the exhaustive coverage of our switch. We can do this:

public class CoolChar implements CharSequence { … }

At this moment, the switch expression doesn’t cover the CoolChar type. So, we still need a default label or the total pattern, case CharSequence charseq as follows:

return switch(cs) {          
  case String str -> “You’re a string”;          
  …
  case StringBuilder strbuilder -> “You’re a StringBuilder”;             
  // we have created this
  case CoolChar cool -> “Welcome … you’re a CoolChar”;
  // this is a total pattern
  case CharSequence charseq -> “You’re a CharSequence”; 
  // can be used instead of the total pattern
  // default -> “I have no idea … what are you?”;          
};

Ok, let’s tackle this scenario on the java.lang.constant.ClassDesc built-in interface:

private static String whatAmI(ConstantDesc constantDesc) {
      
  return switch(constantDesc) {          
    case Integer i -> “You’re an Integer”;
    case Long l -> “You’re a Long”;
    case Float f -> ” You’re a Float”;
    case Double d -> “You’re a Double”;
    case String s -> “You’re a String”;
    case ClassDesc cd -> “You’re a ClassDesc”;
    case DynamicConstantDesc dcd -> “You’re a DCD”;          
    case MethodHandleDesc mhd -> “You’re a MethodHandleDesc”;
    case MethodTypeDesc mtd -> “You’re a MethodTypeDesc”;                       
  };      
}