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This one's an oldie. Originally from Effective Java by Josh Bloch.Use static factory methods instead of constructors when creating an object.
Reasons why:
- Method names describe the instantiation of the object much simpler than a constructor.
public class MyObject implements IObject
{
public MyObject()
{
//logic
}
public MyObject(string param1)
{
//some other logic; however, besides the documentation, no way to tell what the difference between this constructor and the previous one is
}
}
=============================================
public class MyObject implements IObject
{
private MyObject() {} //no ability to call a constructor outside the class
public static MyObject getInstanceDefault()
{
return new MyObject();
}
public static MyObject getInstanceWithName(string param1)
{
MyObject obj = new MyObject();
obj.setName(param1);
}
}
constructor instantiation:
IObject m = new MyObject();
IObject m1 = new MyObject(name);
factory method instantiation:
IObject m = MyObject.getInstanceDefault();
IObject m1 = MyObject.getInstanceWithName(name);
- Can return an object that's a subtype of the current type
public class MyObject implements IObject
{
public MyObject()
{
//logic
}
}
=============================================
public class MyObject implements IObject
{
public MyObject() {}
}
public class MyOtherObject implements IObject
{
public MyOtherObject() {}
}
public class MyObjectProvider
{
List list = new ArrayList();
public static void AddNewObject(IObject obj)
{
this.list.add(obj);
}
public static IObject getInstance()
{
if(list.Size() == 0)
throw new Exception();
else
{
IObject obj = this.list.get(0);
this.list.remove(0);
return obj;
}
}
}
constructor instantiation. Can ONLY return object of MyObject type:
IObject m = new MyObject();
factory method instantiation:
prep:
MyObjectProvider.AddNewObject(new MyObject());
MyObjectProvider.AddNewObject(new MyOtherObject());
IObject m = MyObjectProvider.getInstance(); //returns MyObject type
IObject m1 = MyObjectProvider.getInstance(); //returns MyOtherObject type
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