超类子类构造函数java

Question

I am new to Java. In the below code I am having some difficulty to understand the concepts.

class A {

    private Integer intr;

    public A(Integer intr) {
        this.inrt = intr;
    }

    public Integer getIntr() {
        return intr;
    }
}

class B extends A {

    public B(Integer intr) { //Eclipse asking me to create this constructor. 
        super(intr);
    }
}

I am getting confused why I need to create that kind of constructor as eclipse suggesting. What is in there?

Answer 1

A sub-class's constructor calls the constructor of its super-class ( A in your case). If your sub-class B has no constructor, the compiler automatically creates a constructor without any parameters. That constructor automatically calls the constructor with no parameters of the base class A .

Since your base class A has a constructor with a parameter, the compiler does not generate a constructor without parameters for it, and therefore you must explicitly create a matching constructor with a single parameter in your sub class B.

The are other alternatives to creating that constructor in B :

You can define a no parameters constructor in A :

public A()
{
   this.inrt = 0;
}

This will allow the no parameters constructor automatically generated for B to call the matching constructor in A .

Or you can explicitly define a no parameters constructor in B that calls the existing constructor of A :

public B()
{
    super(0);
}

Answer 2

If there is no constructor, there is default constructor, which also calls super's default constructor. But when you created constructor in parent, there is no default constructor in parent.

Answer 3

The only way to instantiate a A is to use the A(Integer) constructor. Unless the super class implements a nullary constructor, every subclass has to explicitely call a super constructor at the beginning of its self constructor.

But the subclass contructor does not necessarily need the exact same signature. For example, those are also valid:

public B() {
    super(42);
    // other B specific stuff
}

public B(Integer i, String s) {
    super(i);
    // other B specific stuff
}