Why Does a Java Subclass Get an \'Implicit Super Constructor is Undefined\' Error, and How Can It Be Fixed?

Implicit Super Constructor Error in Default Java Constructor

In Java, default constructors are automatically provided by the compiler for classes that do not explicitly define any constructors. However, when a base class defines a constructor, the compiler will no longer implicitly generate a default constructor for its subclasses. This can result in a "Implicit super constructor is undefined for default constructor" error.

Example:

Consider this code:

abstract public class BaseClass {
    String someString;
    public BaseClass(String someString) {
        this.someString = someString;
    }
    abstract public String getName();
}

public class ACSubClass extends BaseClass {
    public ASubClass() { // default constructor
        // super(someString);  // error: super() must be invoked in subclasses
    }
    public String getName() {
        return "name value for ASubClass";
    }
}

Here, ACSubClass does not have an explicit constructor. Instead, it relies on the default constructor provided by the compiler. However, since BaseClass defines a constructor, no default constructor exists for BaseClass. This leads to the compiler error, stating that an explicit constructor must be defined in the subclass.

Solution:

To resolve this error, one must provide an explicit constructor in the subclass that invokes the super constructor using super(parameters). Alternatively, the base class can be modified to provide a default constructor.

// Option 1: Provide an explicit constructor in the subclass
public class ACSubClass extends BaseClass {
    public ASubClass(String someString) {
        super(someString);
    }
    public String getName() {
        return "name value for ASubClass";
    }
}

// Option 2: Modify the base class to have a default constructor
abstract public class BaseClass {
    String someString;

    public BaseClass() {}  // added default constructor

    public BaseClass(String someString) {
        this.someString = someString;
    }
    abstract public String getName();
}

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