如何将 object 转换为仅在运行时已知的类型？

Question

I'm trying to do casting by Type to Object at runtime but it doesn't work in that way. There is some smart way to do that instead of using instanceOf() for all options?

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

The approach that you show (using Class.cast ) does work, but has the overhead of handling an exception.

A clearer and more concise way of doing this is using the Class.isInstance method:

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?
    }
}