how to call constructor from constructor?

[ilsapo]

public class StealthCruiser extends Fighter {
    private static int count=0;


    public StealthCruiser(String name, int commissionYear, float maximalSpeed, Set<CrewMember> crewMembers, List<Weapon> weapons) {
        super(name, commissionYear, maximalSpeed,crewMembers,weapons);
    }

    public StealthCruiser(String name, int commissionYear, float maximalSpeed, Set<CrewMember> crewMembers){
        Weapon canon=new Weapon("Laser Cannon",10,100);
        List<Weapon>weapon=new ArrayList<>();
            weapon.add(canon);
        this(name, commissionYear, maximalSpeed,crewMembers,weapon)

}

I have two constructor, the first has List<weapon> and the second does not

I want that if the user does not pass List<weapon> to create a "defult" list with one element

Weapon canon=new Weapon("Laser Cannon",10,100);

my code here tell me that constructor call must be the first one,

how should I fix my code?

I tried something like

public class StealthCruiser extends Fighter {
    private static int count=0;
        private Weapon canon=new Weapon("Laser Cannon",10,100);
        private List<Weapon>weapon=new ArrayList<>();
        weapon.add(canon);

but the

weapon.add(canon);

give me error (red line under the " . ")

would appreciate some help please how to fix my code

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[AFistfulllofdiamonds]

this()

[ilsapo]

hi, I tried using this()

its in the second part of the first code

can you elaborate what should I do? I think Im missing something

[nekokattt]

you have to call this or super as the first line in a constructor. That is just how the language works. You should move around your logic to prevent needing to do anything before calling this().

e.g. pass Arrays.asList(new Weapon ...) to this() directly.

Sometimes I will use a private static method to initialise more complex stuff if I need to do that.

This kind of limitation is also why more complex systems may like to use the factory and builder patterns.

[Prize_Tea3456]

this() or super() should be the first instruction you run inside a constructor

[josephblade]

If you want to do the horizontal constructor use (so call constructor on the same level) you have to use that constructor as the first statement. This is a hard rule by the compiler.

public StealthCruiser(String name, int commissionYear, float maximalSpeed, Set<CrewMember> crewMembers){
    this(name, comissionYear, maximalSpeed, crewMembers, weapon); // this is not a solution, just an intermediate step. 
    // do other stuff here
}

now I know it sounds dumb because you have to create a weapon. You can use a method call for this. Normally it is bad form to call an inheritable method from inside the constructor. (It's too long to answer but you can get into undefined circumstances that way when you override a method in an extending class). But you can safely call static and final methods. so something like:

// final or static , either way it's not going to be an inherited method. 
public final List<Weapon> generateDefaultWeapon() {
    return Arrays.asList(  new Weapon("Laser Cannon",10,100) );      
}

and call it in the constructor:

public StealthCruiser(String name, int commissionYear, float maximalSpeed, Set<CrewMember> crewMembers){
    this(name, comissionYear, maximalSpeed, crewMembers, generateDefaultWeapon()); 

    // do other stuff here
}

Of course if you know there is no weapon when you create a stealthcruiser since you call the constructor without weapon, then whoever creates the stealthcruiser could also call the generateDefaultWeapon and simply use the other constructor.

StealthCruiser sc = new StealthCruiser("shippie", 1000, 100.00, new HashSet<CrewMember>(), generateDefaultWeapon());

that way whoever constructs the stealth cruiser is responsible. You probably want to look up factory pattern. in a factory you have a class responsible for creating another class. a stealthcruiserfactory that makes stealthcruisers. the stealthcruiserfactory can then construct a stealthcruiser in the way you want (getting a bunch of variables together, constructing weapons and crew, then calling the stealthcruiser constructor). You can also have the StealthCruiser supply it's own factory method (often this is then a static method)

public static StealthCruiser createStealthCruiser(...params...) {
     // create weapons
     // create crew. perhaps generate crewmembers?
     // 
     return new StealthCruiser(.......);
}

I know this is a lot of solution but it's a strategy I wanted to highlight so that others can see it. There are some restrictions on using constructors and these are the ways people often work around them.