什么是静态工厂方法？

#1 楼

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);
     //...
   }
}

#2 楼

Coordinate c = Coordinate.createFromCartesian(double x, double y)

Coordinate c = Coordinate.createFromPolar(double distance, double angle)

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public class Foo{
  public Foo(boolean withBar){
    //...
  }
}

//...

// What exactly does this mean?
Foo foo = new Foo(true);
// You have to lookup the documentation to be sure.
// Even if you remember that the boolean has something to do with a Bar
// you might not remember whether it specified withBar or withoutBar.

public class Foo{
  public static Foo createWithBar(){
    //...
  }

  public static Foo createWithoutBar(){
    //...
  }
}

// ...

// This is much easier to read!
Foo foo = Foo.createWithBar();

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

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

Test t = Test.getObject();

#8 楼

//instantiating a class using constructor
Vinoth vin = new Vinoth(); 

//instantiating the class using static method
Class Vinoth{
  private Vinoth(){
  }
  // factory method to instantiate the class
  public static Vinoth getInstance(){
    if(someCondition)
        return new Vinoth();
  }
}

public static Boolean valueOf(boolean b) {
        return (b ? TRUE : FALSE);
}

public class Singleton{
    //initailzed during class loading
    private static final Singleton INSTANCE = new Singleton();

    //to prevent creating another instance of Singleton
    private Singleton(){}

    public static Singleton getSingleton(){
        return INSTANCE;
    }
}

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