java – 声明一个全局变量,或在每个类中声明多次

public Long dbInsertCheckin(final String Class) {
final SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
...
}

和

public class SmashDataSource {
    final SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    final SimpleDateFormat sdf = new SimpleDateFormat("EEEE");
    final SimpleDateFormat timeFormat = new SimpleDateFormat("HHmm");
...
}

这让我想到,而不是声明“dateformat”,“sdf”,或“timeformat”或我在多个地方使用的其他人,对我来说,在我的应用程序类中全局声明这些,然后在任何地方引用它们都没有意义我想要

public class MyApp extends Application {
public final SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    public final SimpleDateFormat sdf = new SimpleDateFormat("EEEE");
    public final SimpleDateFormat timeFormat = new SimpleDateFormat("HHmm");

稍后在其他课程中引用它们：

MyApp.dateformat
Myapp.sdf

从性能/内存使用的角度来看,这会更好吗？有什么理由不这样做吗？对我来说,似乎多次声明它们会消耗更多内存,而最终声明只会消耗一次….但我不知道编译器是如何进行优化的.

(但是我可能会避免使用我的Application类,因此我的全局字段仅在需要时使用,但这是一个实现细节.)

我可能会这样做.它保持灵活性,同时将事物放在一个地方,以便它们在您的应用程序中保持一致.

public class MyGlobals
{
    private static SimpleDateFormat dateFormat;


    public static SimpleDateFormat getDateFormat()
    {
        if(dateFormat== null)
        {
            dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        }
        return dateFormat;

    }
}

然后你可以在其他类中使用它：

MyGlobals.getDateFormat();

====

以下是关于它的开发文档的一些信息：

http://developer.android.com/guide/faq/framework.html#3

For sharing complex non-persistent user-defined objects for short
duration, the following approaches are recommended:

Singleton class

You can take advantage of the fact that your application components
run in the same process through the use of a singleton. This is a
class that is designed to have only one instance. It has a static
method with a name such as getInstance() that returns the instance;
the first time this method is called, it creates the global instance.
Because all callers get the same instance, they can use this as a
point of interaction. For example activity A may retrieve the instance
and call setValue(3); later activity B may retrieve the instance and
call getValue() to retrieve the last set value.

A public static field/method

An alternate way to make data accessible across Activities/Services is
to use public static fields and/or methods. You can access these
static fields from any other class in your application. To share an
object, the activity which creates your object sets a static field to
point to this object and any other activity that wants to use this
object just accesses this static field.

A HashMap of WeakReferences to Objects

You can also use a HashMap of WeakReferences to Objects with Long
keys. When an activity wants to pass an object to another activity, it
simply puts the object in the map and sends the key (which is a unique
Long based on a counter or time stamp) to the recipient activity via
intent extras. The recipient activity retrieves the object using this
key.

Persistent Objects

Even while an application appears to continue running, the system may
choose to kill its process and restart it later. If you have data that
you need to persist from one activity invocation to the next, you need
to represent that data as state that gets saved by an activity when it
is informed that it might go away.

For sharing complex persistent user-defined objects, the following
approaches are recommended:

06002

If the shared data needs to be retained across points where the
application process can be killed, then place that data in persistent
storage like Application Preferences, SQLite DB, Files or
ContentProviders. Please refer to the Data Storage for further details
on how to use these components.