Why is this an issue?

An Externalizable class is one which handles its own Serialization and deserialization. During deserialization, the first step in the process is a default instantiation using the class' no-argument constructor. Therefore an Externalizable class without a no-arg constructor cannot be deserialized.

Noncompliant code example

public class Tomato implements Externalizable {  // Noncompliant; no no-arg constructor

  public Tomato (String color, int weight) { ... }
}

Compliant solution

public class Tomato implements Externalizable {

  public Tomato() { ... }
  public Tomato (String color, int weight) { ... }
}