ch3.md

Making Decision

Pattern Matching

void compare(Number number) {
   if(number instanceof Integer data) {
      System.out.print(data.compareTo(5));
   }
}

void firstIfFalse(Object object) {
    if(!(object instanceof String s)){
      //here then s is NOT a String
      return;
    }
    //then here it is!. Mind the return first if
    System.out.println("length of string: "+s.length());
}

Pattern Matching - interface

The compiler can't definitively state that t is not a List. Some future program could create a subclass of Throwable that does implement List.

  void notCompatible(Throwable t) {
    //type not compatible - t cannot be a string
    /*
    if(t instanceof String s) {}
    */
    //strange behaviour for interface!
    if(t instanceof List list) {
      System.out.println("that's weird");
    }
  }

Pattern Matching

Pattern Matching - subtype

Pattern variable type must be a strict subtype of the Type under test.

Number number = 5;
//this is a simple instanceof, not pattern matching
if(number instanceof Number){}  //this is OK 

//Pattern type 'Number' is the same as expression type
//pattern variable type must be a strict subtype of Number
//if(number instanceof Number n2){} //DOES NOT COMPILE!
//this is ok as Long is a strict subtype Of Number
if(number instanceof Long nL) {}  //OK

Switch

Switch Statement

The value of a case statement must be one of these:

• a constant;
• a literal value;
• a final variable (not just effectively final)

    int aValue = 5;
    final int aConstant = 6;
    switch (x){
        case 2:  //ok - literal
        case aConstant:  //ok - constant
        case aValue: //does not compile, not final
    }

WARNING: You always need the case, a part in the default case.

Multi cases

I use comma (,)to separate values.

  void workOrChillOut(DayOfWeek day) {
    //with the new switch, I can have multiple cases in the same line
    switch (day) {
      case MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY:
        System.out.println("You have to work!");
        break;
      default:
        System.out.println("Enjoy your weekend!");
    }
  }

Multi cases - default

case and default are separated by colon.

int x = 7;
switch (x){
    case 2, 3:
      System.out.println("2,3");
    case 7: default: case 8, 9:
      System.out.println("default");
}

strange switch

Switch supported types

• int,byte, short, char
• Integer, Byte, Short, Character
• enum
• String
• var (if type was previously resolved)

Switch expression

  void printItalianDayWithDefault(DayOfWeek day) {
    var result = switch (day) {
      case MONDAY -> "lunedi";
      case TUESDAY -> "martedi";
      case WEDNESDAY -> "mercoledi";
      case THURSDAY -> "giovedi";
      case FRIDAY -> "venerdi";
      default -> "weekend";
    };
  }

We can have a switch expression even without return value assigned.

    int cnt = 0;
    char c = 'a';
    //there is no return value assigned so the default is not needed!
    switch (c) {
      case 'a' -> cnt++;
      case 'b' -> cnt++;
    }
    System.out.println(cnt); //1

yield

If the switch expression returns a value, then every branch that isn't an expression must yield a value.

final int score1 = 8, score2 = 3;
char myScore = 7;
var goal = switch (myScore) {
  default -> {
    if (10 > score1) {
      yield "unknown";
    } else {
    {yield "ciao";}  //brackets not needed
    }
}

Switch expression with yield