FeedReaderContract的目的是什么以及如何在OpenHelper类中定义内部类

Question

我在android中学习SQLite.对于SQLite,我正在使用developer.android.com.但是在阅读代码时我遇到了一些困惑.他们写道FeedReaderContract构造,以防止实例化FeedReaderContract类,但他们没有定义FeedReaderContract任何地方和类之间的关系FeedReaderContract和FeedEntry.

这是我所指的链接.我提供代码.如何在openhelper类中定义内部类.有人能建议我这么好吗？

例如,此代码段定义单个表的表名和列名:

public static abstract class FeedEntry implements BaseColumns 
{
    public static final String TABLE_NAME = "entry";
    public static final String COLUMN_NAME_ENTRY_ID = "entryid";
    public static final String COLUMN_NAME_TITLE = "title";
    public static final String COLUMN_NAME_SUBTITLE = "subtitle";
    ...
}

为了防止有人意外地实例化合同类,给它一个空的构造函数.

 //Prevents the FeedReaderContract class from being instantiated.
private FeedReaderContract() {}

Answer 1

我理解为：

public static class FeedReaderContract{

    // Prevents the FeedReaderContract class from being instantiated.
    private FeedReaderContract() {} 

    //The FeedEntry table definition
    public static abstract class FeedEntry implements BaseColumns {
        public static final String TABLE_NAME = "entry";
        public static final String COLUMN_NAME_ENTRY_ID = "entryid";
        public static final String COLUMN_NAME_TITLE = "title";
        public static final String COLUMN_NAME_SUBTITLE = "subtitle";
        ...
    }

    //more tables definition
}

所以你不能实例化合约，但可以访问所有内部类常量。就像示例行一样：

private static final String SQL_CREATE_ENTRIES =
"CREATE TABLE " + FeedReaderContract.FeedEntry.TABLE_NAME + " (" +
FeedReaderContract.FeedEntry._ID + " INTEGER PRIMARY KEY," //continues

在类中访问内部类的FeedEntry常量（TABLE_NAME和） 。_IDFeedReaderContract

希望能帮助到你。