Why is this an issue?

Non-abstract classes and enums with non-static, private members should explicitly initialize those members, either in a constructor or with a default value.

Noncompliant code example

class A { // Noncompliant
  private int field;
}

Compliant solution

class A {
  private int field;

  A(int field) {
    this.field = field;
  }
}

Exceptions