问当类可以通过黑盒测试进行测试时，我应该避免Di吗

EN

public interface Nand {
  int nand(int a, int b);
}

public class NandImpl implements Nand {
  public int nand(int a, int b) {
    return a * b == 0 ? 1 : 0;
  }
}

public interface And {
  int and(int a, int b);
}

public class AndImpl implements And {
  Nand nand;

  public AndImpl(Nand nand) {
    this.nand = nand;
  }

  public int and(int a, int b) {
    return nand.nand(nand.nand(a, b), nand.nand(a, b));
  }
}

public class NandImpl {

  static NandImpl singleton = null;

  protected NandImpl() {
  }

  public static NandImpl getInstance() {
    if(singleton == null) {
      singleton = new NandImpl();
    }
    return singleton;
  }

  public int nand(int a, int b) {
    return a * b == 0 ? 1 : 0;
  }
}

public class AndImpl {

  static AndImpl singleton = null;
  NandImpl nand;

  protected AndImpl() {
    nand = NandImpl.getInstance();
  }

  public static AndImpl getInstance() {
    if(singleton == null) {
      singleton = new AndImpl();
    }
    return singleton;
  }

  public int and(int a, int b) {
    return nand.nand(nand.nand(a, b), nand.nand(a, b));
  }
}