Why is this an issue?

The Collection.toArray() method returns an Object[] when no arguments are provided to it. This can lead to a ClassCastException at runtime if you try to cast the returned array to an array of a specific type. Instead, use this method by providing an array of the desired type as the argument.

Note that passing a new T[0] array of length zero as the argument is more efficient than a pre-sized array new T[size].

Code examples

Noncompliant code example

public String [] getStringArray(List<String> strings) {
  return (String []) strings.toArray();  // Noncompliant, a ClassCastException will be thrown here
}

Compliant solution

public String [] getStringArray(List<String> strings) {
  return strings.toArray(new String[0]); // Compliant, the toArray method will return an array of the desired type, and we can remove the casting operation
}
public String [] getPresizedStringArray(List<String> strings) {
  return strings.toArray(new String[strings.size()]); // Compliant, but slightly less efficient than the previous example
}

Resources