chapter05-control_flow.md

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);