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Java通用投射(Java generic casting)

 我有一个泛型类A<T> ，一个子类B扩展了A<String> 。 A类有一个返回它的方法（所以A<T> ）。 问题是，如果我在B对象上调用该方法，它将返回A<String>并且我必须转换为B，但是B与A<String>相同，不是吗？ 所以我不应该投... 

I have a generic class A<T> and a subclass B extends A<String>. The class A have a method which returns this (so an A<T>). The problem is, if I call the method on a B object, it will return a A<String> and I have to cast to B, but B is the same as A<String>, isn't it ? So I shouldn't have to cast ...

原文：https://stackoverflow.com/questions/38973046

更新时间：2023-10-31 14:10

最满意答案

 如果您想要获取数组中的所有名称字段，下面的代码应该可以使用  
NSMutableArray* result=[[NSMutableArray alloc] init];

    NSString *ask = [NSString stringWithFormat:@"SELECT name FROM users"];
        sqlite3_stmt *statement;
        const char *query_statement = [ask UTF8String];
        const char *dbpath = [_databasePath UTF8String];
        if (sqlite3_open(dbpath, &_DB) == SQLITE_OK){
            {
                if (sqlite3_prepare_v2(_DB, query_statement, -1, &statement, nil) == SQLITE_OK)
                {
                    while (sqlite3_step(statement) == SQLITE_ROW)
                    {
                        NSString *nameField = [[NSString alloc] initWithUTF8String:(const char *)sqlite3_column_text(statement, 0)];

                        //add your namefield here.
                       [result addObject:nameField];
                    }

                    sqlite3_finalize(statement);
                    sqlite3_close(_DB);
                }
            }
 
 最后你可以做到  
totalStrings=[[NSMutableArray alloc] initWithArray:result];

The code below should work in case you want to get all namefields in array  
NSMutableArray* result=[[NSMutableArray alloc] init];

    NSString *ask = [NSString stringWithFormat:@"SELECT name FROM users"];
        sqlite3_stmt *statement;
        const char *query_statement = [ask UTF8String];
        const char *dbpath = [_databasePath UTF8String];
        if (sqlite3_open(dbpath, &_DB) == SQLITE_OK){
            {
                if (sqlite3_prepare_v2(_DB, query_statement, -1, &statement, nil) == SQLITE_OK)
                {
                    while (sqlite3_step(statement) == SQLITE_ROW)
                    {
                        NSString *nameField = [[NSString alloc] initWithUTF8String:(const char *)sqlite3_column_text(statement, 0)];

                        //add your namefield here.
                       [result addObject:nameField];
                    }

                    sqlite3_finalize(statement);
                    sqlite3_close(_DB);
                }
            }
 
In the end you can do 
totalStrings=[[NSMutableArray alloc] initWithArray:result];

Java通用投射(Java generic casting)

最满意答案

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