chapter05-control_flow.jsh

// To starts, run jshell --enable-preview which is a program able to interpret Java syntax
// then cut and paste the following lines to see how it works
// To exit jshell type /exit

// # Expression and Control Flow
// Most of the control flow syntax of Java comes from C with some enhancements

// ## Variables
// if you explicitly type a variable you can declare a variable without initializing it
int x;

// but a variable can be used only after being initialized
x = 3;
System.out.println(x);

// block of code
// a variable declared in a block of code, can not be used outside that block
 {
   var value = 42;
 }
 // value can not be used here !


// ## If

// ### Test with `if`
// the construct `if` execute the block of code that follow if the condition in between
// the parenthesis is true
void oldEnough(int age) {
  if (age >= 21) {
    System.out.println("you are old enough to drink a beer");
  }
}
oldEnough(22);

// ### Test with `if ... else`
// you can append the construct `else` after an `if` to execute a block of code
// if the condition is not true
void oldEnough(int age) {
  if (age >= 21) {
    System.out.println("you are old enough to drink a beer");
  } else {
    System.out.println("too bad for you !");
  }
}
oldEnough(17);


// ## Switch

// ### Test with a `switch` statement
// There are two forms of switch, a switch statement is a switch that doesn't
// produce a value. For those, the `default` case is not mandatory
void vehicle(int wheels) {
  switch(wheels) {
    case 1 -> System.out.println("monocycle !");
    case 2 -> System.out.println("bicycle !");
    case 3, 4 -> System.out.println("car !");
    default -> {
      // if there are several lines
      System.out.println("whaat !");
    }
  }
}
vehicle(3);

// ### Test with a `switch` expression
// A switch that produces a valeur is a switch expresssion. Given that a
// value need to be produced, a `default` case is mandatory
String vehicle(int wheels) {
  return switch(wheels) {
    case 1 -> "monocycle !";
    case 2 -> "bicycle !";
    case 3, 4 -> "car !";
    default -> "whaat !";
  };
}
System.out.println(vehicle(3));

// you can switch on integers, strings and enums
int doors(String kind) {
  return switch(kind) {
    case "smart" -> 3;
    case "sedan", "hatchback" -> 5;
    default -> { throw new IllegalArgumentException(kind); }
  };
}
System.out.println(doors("sedan"));

// ### Test with a `switch` compatible with C
// You can use the C compatible switch too, using `:` instead of `->`
// (you can not mix them) but in that case don't forget to ends
// each case with a `break`.
void vehicle(int wheels) {
  switch(wheels) {
    case 1:
      System.out.println("monocycle !");
      break;
    case 2:
      System.out.println("bicycle !");
      break;
    case 3:
    case 4:
      System.out.println("car !");
      break;
    default:
      System.out.println("whaat !");
  }
}
vehicle(3);


// ## Instanceof

// ### `instanceof`
// instanceof test the class of a value at runtime, if instanceof succeeds,
// the value is stored in the variable declared as last argument
record Car(int seats) {}
record Bus(int capacity) {}
int maxPersons(Object value) {
  if (value instanceof Car car) {
    return car.seats();
  }
  if (value instanceof Bus bus) {
    return bus.capacity();
  }
  return 0;
}
System.out.println(maxPersons(new Car(4)));
System.out.println(maxPersons(new Bus(32)));

// ### `instanceof` with no variable declaration
// if you don't need the variable declaration, you can omit it
void printKind(Object value) {
  if (value instanceof Car) {
    System.out.println("it a car");
  }
  if (value instanceof Bus bus) {
    System.out.println("it a bus");
  }
}
printKind(new Car(4));


// ## Loops

// ### `while` loop
// a `while` loop execute the block of code while the condition in between parenthesis is true
void printFirstIntegers(int n) {
  var i = 0;
  while(i < n) {
    System.out.println(i);
    i++;
  }
}
printFirstIntegers(5);

// ### `for` loop
// a for loop is a convenient way to write a `while` loop in case you do a while on a variable,
// so instead of using the `while` loop above, one can write this for loop
void printFirstIntegers(int n) {
  for(var i = 0; i < n; i++) {
    System.out.println(i);
  }
}
printFirstIntegers(5);



// ### `for` loop on array or list
// Java as a special loop for iterating over the content of an array or a list,
// it using the keyword `for`, but the declared variable contains each element one by one
var list = List.of("iron man", "captain america", "black panther");
for(var value: list) {
  System.out.println(value);
}


// ### On loops
// Most of the loops can also be abstracted using higher order constructs
// if you don't understand that code now, don't panic, we will come back
// to that later

// using `IntStream.range()`
IntStream.range(0, 5).forEach(System.out::println);

// using `List.forEach()`
list.forEach(System.out::println);