Question

我正在尝试在运行时按类型将其强制转换为对象，但这种方式不起作用。有什么聪明的方法可以代替所有选项使用instanceOf()？

public <T> void updateUser(final SQLiteDatabase db, final String key, Class<T> cls, Object newVal, String prevVal){
   ContentValues userValue = new ContentValues();
    try {
        userValue.put(key, cls.cast(newVal));
    } catch(ClassCastException e) {
    }
    db.update(mDBName, userValue, key + " = ?", new String[] {prevVal});
}

Answer 1

您展示的方法（使用Class.cast）确实有效，但是具有处理异常的开销。

使用Class.isInstance方法是一种更清晰，更简洁的方法：

public <T> void updateUser(final SQLiteDatabase db, final String key, Class<T> cls, Object newVal, String prevVal){
    ContentValues userValue = new ContentValues();
    if (cls.isInstance(newVal)) {
        // cls.cast is only necessary if `userValue` has a value type of T
        // like "Map<String, T> userValue"; if it's "Map<String, Object>" then
        // you can just use "newVal" without the cast.
        userValue.put(key, cls.cast(newVal));
        // You'll want to include "db.update" in the "if"-block,
        // since you need to update at least one field to make it a 
        // valid SQL statement.
        db.update(mDBName, userValue, key + " = ?", new String[] {prevVal});
    } else {
        // Raise some kind of error or log something?
    }
}

如何将对象转换为仅在运行时知道的类型？

1 个答案: