使用 super() 构造函数时，未初始化空白最终字段

Question

I am admittedly a bit new to object-oriented programming and would therefore be very grateful for a little help. I have a class "Gerade" (German for straight line) and a class "Strahl" (German for ray). I would like to call the constructor of the superclass "Gerade" in the subclass "Strahl": But now I get the error. 'The blank final field p1[and p2] may not have been initialized'? What is the reason for this. I am very grateful for any answers. Thank you in advance!

public class Gerade {
    
private final Punkt p1;
private final Punkt p2;

public Gerade(Punkt p1, Punkt p2)
{
    if(p1.equals(p2)==true)
    {
    System.out.println("Die Eingabe ist ungueltig. Beide Punkte haben die selben Koordinaten.");
    p1 = null;
    p2 = null;
    }
    

    if(p1.getXkoord().equals(p2.getXkoord())==true)
    {
        if((p1.getYkoord().compareTo(p2.getYkoord()) < 0))
        {
            this.p1 = p1;
            this.p2 = p2;
        }
        else
        {
            this.p1 = p2;
            this.p2 = p1;
        }
    }    
    
    else if((p1.getXkoord().compareTo(p2.getXkoord()) > 0))
    {
        this.p2 = p1;
        this.p1 = p2;
    }
    
    else 
    {
        this.p1 = p1;
        this.p2 = p2;
    }   
}

The class "Gerade" uses certain attributes and methods of the class "Punkt". The coordinates are BigDecimals. Just so youre not confused.

public class Strahl extends Gerade 
{
    

    private final Punkt p1;
    private final Punkt p2;
    private final Punkt start;


public Strahl(Punkt a, Punkt b)
    {
        super(a, b);
        this.start = a;

    }

Answer 1

Both p1 and p2 shall be initialized after constructor of Gerade is executed. But compiler can not assure that this will happen due to complicated control flow. Look carefully at your conditions - you have to provide for the fallback if none of if conditions is satisfied

Answer 2

Since you are calling super(a, b) , there is no reason why you should redeclare them in Strahl . Instead of writing p1.equals(p2) == true , you can simply write p1.equals(p2) .

It can be something as

class Gerade {

    private final Punkt p1 = null;
    private final Punkt p2 = null;

    public Gerade(Punkt p1, Punkt p2) {
        if (p1.equals(p2)) {
            System.out.println("Die Eingabe ist ungueltig. Beide Punkte haben die selben Koordinaten.");
            p1 = null;
            p2 = null;
        }

        // ...
    }
}

class Strahl extends Gerade {

    private final Punkt start;

    public Strahl(Punkt a, Punkt b) {
        super(a, b);
        this.start = a;
    }
}

I also suggest that you use a constructor only for initialization and any business logic should be refactored into some methods.

Answer 3

You already have two Punkt instance variables in your class Gerade . So you do not need to repeat them in your class Strahl .

In fact, this is one of the problems: The final variables p1 and p2 in Strahl are not initialized anywhere!

So, simply remove them:

public class Strahl extends Gerade {
    private final Punkt start;
    public Strahl(Punkt a, Punkt b) {
        super(a, b);
        this.start = a;
    }
}

Another problem is what @KonstantinPribluda mentioned in his answer.