One of the advantages of the static factory methods with private constructor(object creation must have been restricted for external classes to ensure instances are not created externally) is that you can create instance-controlled classes. And instance-controlled classes guarantee that no two equal distinct instances exist(a.equals(b) if and only if a==b) during your program is running that means you can check equality of objects with == operator instead of equals method, according to Effective java.

The ability of static factory methods to return the same object from repeated invocations allows classes to maintain strict control over what instances exist at any time. Classes that do this are said to be instance-controlled. There are several reasons to write instance-controlled classes. Instance control allows a class to guarantee that it is a singleton (Item 3) or noninstantiable (Item 4). Also, it allows an immutable class (Item 15) to make the guarantee that no two equal instances exist: a.equals(b) if and only if a==b. If a class makes this guarantee, then its clients can use the == operator instead of the equals(Object) method, which may result in improved performance. Enum types (Item 30) provide this guarantee.

摘自Effective Java, Joshua Bloch(第1项，第6页)

与构造函数不同，具有名称，可以阐明代码。 不需要在每次调用时创建一个新对象-对象 如果需要，可以缓存和重用。 可以返回其返回类型的子类型-特别是，Can 返回调用方不知道其实现类的对象。 这是许多框架中非常有价值和广泛使用的特性 它们使用接口作为静态工厂方法的返回类型。

标准:/ / www.javapractices.com/topic/TopicAction.do ? Id = 21

我们避免提供对数据库连接的直接访问，因为它们是资源密集型的。因此，我们使用一个静态工厂方法getDbConnection，它在低于限制时创建一个连接。否则，它将尝试提供一个“备用”连接，如果没有则会出现异常而失败。

public class DbConnection{
   private static final int MAX_CONNS = 100;
   private static int totalConnections = 0;

   private static Set<DbConnection> availableConnections = new HashSet<DbConnection>();

   private DbConnection(){
     // ...
     totalConnections++;
   }

   public static DbConnection getDbConnection(){

     if(totalConnections < MAX_CONNS){
       return new DbConnection();

     }else if(availableConnections.size() > 0){
         DbConnection dbc = availableConnections.iterator().next();
         availableConnections.remove(dbc);
         return dbc;

     }else {
         throw new NoDbConnections();
     }
   }

   public static void returnDbConnection(DbConnection dbc){
     availableConnections.add(dbc);
     //...
   }
}

静态工厂的优点之一是API可以返回对象，而不需要将它们的类设为public。这导致了非常紧凑的API。在java中，这是通过Collections类实现的，它隐藏了大约32个类，这使得它的集合API非常紧凑。

One of the advantages of the static factory methods with private constructor(object creation must have been restricted for external classes to ensure instances are not created externally) is that you can create instance-controlled classes. And instance-controlled classes guarantee that no two equal distinct instances exist(a.equals(b) if and only if a==b) during your program is running that means you can check equality of objects with == operator instead of equals method, according to Effective java.

The ability of static factory methods to return the same object from repeated invocations allows classes to maintain strict control over what instances exist at any time. Classes that do this are said to be instance-controlled. There are several reasons to write instance-controlled classes. Instance control allows a class to guarantee that it is a singleton (Item 3) or noninstantiable (Item 4). Also, it allows an immutable class (Item 15) to make the guarantee that no two equal instances exist: a.equals(b) if and only if a==b. If a class makes this guarantee, then its clients can use the == operator instead of the equals(Object) method, which may result in improved performance. Enum types (Item 30) provide this guarantee.

摘自Effective Java, Joshua Bloch(第1项，第6页)

如果类的构造函数是私有的，那么就不能从类的外部为类创建对象。

class Test{
 int x, y;
 private Test(){
  .......
  .......
  }
}

我们不能从上面的类的外部创建一个对象。所以你不能从类外访问x y。那么这门课有什么用呢? 下面是答案:FACTORY方法。 在上面的类中添加下面的方法

public static Test getObject(){
  return new Test();
}

现在你可以在这个类的外部创建一个对象。就像……

Test t = Test.getObject();

因此，通过执行类的私有构造函数返回类对象的静态方法称为FACTORY方法。