java - 如何将从数据库中获取的结果转换为单个字符串？

Question

我有从数据库获取标签的代码:

public void fetchTags() {  
        ContentResolver contentResolver = this.getContentResolver();  
        Cursor cursor = contentResolver.query(TagsContentProvider.CONTENT_URI,  
                 new String[]{Tags.COLUMN_TNAME}, null, null, null);  
        cursor.getCount();  
        while(cursor.moveToNext()) {  
            System.out.println(cursor.getString(cursor.getColumnIndex(Tags.COLUMN_TNAME)));         
        }  
        cursor.close();         
    }  

输出:

如何将结果转换为单个字符串，其中字符以逗号分隔？

Answer 1

使用 StringBuilder :

// ...

final String SEPARATOR = ", ";

StringBuilder builder = new StringBuilder();
String prefix = "";

while(cursor.moveToNext()) {  
    builder.append(prefix);
    prefix = SEPARATOR;
    builder.append(cursor.getString(cursor.getColumnIndex(Tags.COLUMN_TNAME)));    
}  

String output = builder.toString()

// ...

StringBuilder 允许您比普通 outputString += inputString 更快地连接 String。这与 String 对象是不可变的，而 StringBuilder 则不然。如果您想了解更多信息，请查看例如在this article .

编辑:要将创建的字符串返回给调用方法，您的方法 fetchTags() 必须定义如下:

public String fetchTags() {

    // your code

    return output;

}

您使用返回类型void定义它:public void fetchTags() { ... }。这将不允许您返回任何值。

完整的方法 fetchTags() 应该如下所示:

public String fetchTags() {
    ContentResolver contentResolver = this.getContentResolver();
    Cursor cursor = contentResolver.query(TagsContentProvider.CONTENT_URI,
        new String[] {
            Tags.COLUMN_TNAME
        }, null, null, null);
    cursor.getCount();

    final String SEPARATOR = ", ";

    StringBuilder builder = new StringBuilder();
    String prefix = "";

    while (cursor.moveToNext()) {
        builder.append(prefix);
        prefix = SEPARATOR;
        builder.append(cursor.getString(cursor.getColumnIndex(Tags.COLUMN_TNAME)));
    }

    String output = builder.toString()

    cursor.close();

    return output;
}