如何通過外部類構造函數訪問內部類？

Question

我試圖創建一個使用在我的主要的人的構造人與代碼

Person outerClass = new Person("Anon", new Date(06,03,1991), null); 

，但它說，它無法找到該類日期。我是否正確地填寫這個構造函數並調用了類間？

public class Person implements Cloneable 
    { 
     private String name; 
     private Date born; 
     private Date died;//null indicates still alive. 

     public Person(String initialName, Date birthDate, Date deathDate) 
     { 
      if (consistent(birthDate, deathDate)) 
      { 
       name = initialName; 
       born = new Date(birthDate); 
       if (deathDate == null) 
        died = null; 
       else 
        died = new Date(deathDate); 
      } 
      else 
      { 
       System.out.println("Inconsistent dates. Aborting."); 
       System.exit(0); 
      } 
     } 
     private class Date 
     { 
      private String month; 
      private int day; 
      private int year; //a four digit number. 

      public Date() 
      { 
       month = "January"; 
       day = 1; 
       year = 1000; 
      } 

      public Date(int monthInt, int day, int year) 
      { 
       setDate(monthInt, day, year); 
      } 

Answer 1

由於您的要求需要一個內部類，讓我們拋開有關它是否是合適的問題，爲Date是一個內部類的Person。

它找不到類Date，因爲它是私人的。然而，即使它是公開的，你仍然不能實例化一個，因爲你首先需要一個Person的實例。一種方法是從構造函數中刪除Date參數，並使用mutators。例如爲：

public class Person { 
    private final String name; 
    private Date birthDate; 

    public class Date { 
     private final int year, month, day; 

     public Date(final int year, final int month, final int day) { 
      this.year = year; 
      this.month = month; 
      this.day = day; 
     } 

     public String toString() { 
      // look at me, I can access attributes of the enclosing Person 
      return name + " is associated with the year " + year; 
     } 
    } 

    public Person(final String name) { 
     this.name = name; 
    } 

    public void setBirthDate(final Date birthDate) { 
     this.birthDate = birthDate; 
    } 

    public static final void main(final String... arguments) throws Exception { 
     final Person person = new Person("name"); 
     final Date birthDate = person.new Date(2015, 11, 9); 
     person.setBirthDate(birthDate); 
    } 
} 

然而，如果是有意義的內部類獨立存在外類的一個實例的，那麼它應該是靜態的。這將允許您在沒有現有的Person的情況下自行實例化Date。例如： -

public class Person { 
    private final String name; 
    private final Date birthDate; 

    public static class Date { 
     private final int year, month, day; 

     public Date(final int year, final int month, final int day) { 
      this.year = year; 
      this.month = month; 
      this.day = day; 
     } 

     public String toString() { 
      // I do not know about any Person 
      return "year: " + year; 
     } 
    } 

    public Person(final String name, final Date birthDate) { 
     this.name = name; 
     this.birthDate = birthDate; 
    } 

    public static final void main(final String... arguments) throws Exception { 
     final Person person = new Person("name", new Date(2015, 11, 9)); 
    } 
} 

注意，這是在功能上與聲明Date作爲自己的頂級類不同的是，現在的完全限定類名「Person.Date」結束。