Address.java

public class Address {
  public static Builder newAddress() {
    return new Address().new Builder();
  } 
  private String street;
  ...
  public class Builder { 
    public Builder ofStreet(String initialStreet) { 
      street = validateStreet(initialStreet); 
      return this; 
    } 
    public Address build() {
      validate();
      return Address.this;
    }
  }
}