Java Notes

Constructors - super example

Example of class without parameterless constructor

/////////////////// class without a parameterless constructor.
//  If any constructor is defined, the compiler doesn't
//  automatically create a default parameterless constructor.
class Parent {
    int _x;
    Parent(int x) {   // constructor
        _x = x;
    }
}

////////////////// class that must call super in constructor
class Child extends Parent {
    int _y;
    Child(int y) {  // WRONG, needs explicit call to super.
        super(0);
        _y = y;    
    }
}

In the example above, there is no explicit call to a constructor in the first line of constructor, so the compiler will insert a call to the parameterless constructor of the parent, but there is no parameterless parent constructor! Therefore this produces a compilation error. The problem can be fixed by changing the Child class.

////////////////// class that must call super in constructor
class Child extends Parent {
    int _y;
    Child(int y) {  // WRONG, needs explicit call to super.
        _y = y;    
    }
}

Or the Parent class can define a parameterless constructor.

/////////////////// class without a parameterless constructor.
//  If any constructor is defined, the compiler doesn't
//  automatically create a default parameterless constructor.
class Parent {
    int _x;
    Parent(int x) {   // constructor with parameter
        _x = x;
    }
    
    Parent() {        // constructor without parameters
        _x = 0;
    }
}

A better way to define the parameterless constructor is to call the parameterized constructor so that any changes that are made only have to be made in one constructor.

    Parent() {        // constructor without parameters
        this(0);
    }
}

Note that each of these constructors implicitly calls the parameterless constructor for its parent class, etc, until the Object class is finally reached.