该嵌套类的实际目的是什么？

Question

I am a newbie in Android programming and have little confusion in following code. This portion of code is from android developer site:

public final class FeedReaderContract {
    // To prevent someone from accidentally instantiating the contract class,
    // give it an empty constructor.
    public FeedReaderContract() {}

    /* Inner class that defines the table contents */
    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";
        ...
    }
}

I read some articles about nested classes in Java documentation and some other blogs and understood following reasons to use nested class:

1. Provides better encapsulation.
2. If a class is concerned with only one another class, then we better would make nested class of these two.

But here, I could not figure out anything to use nested class.

What exactly is the purpose of making nested class?

Answer 1

This particular nested class is used as a namespace for String constants related to its main class.

• It is abstract , so it's not intended for instantiation
• It implements BaseColumns interface, which is essentially a way to "dump" string constants into the FeedEntry class

Why did they used nested class instead of free-standing class?

Use of a nested class underscores "semantic closeness" of the inner class to its outer class. It cnveys to the readers of your code that FeedEntry can be interpreted only in the context of FeedReaderContract . Otherwise, FeedEntry is meaningless. Essentially, this is done for the human readers of code, because in this case there would be no difference to the compiler.