反射和NullPointerException与静态字段/方法

Question

我试图使用反射来访问我不希望被对象继承的方法和变量。对于这些静态变量，我收到一个空指针异常，并且我无法注意到我错过的任何内容。如果您能帮忙解决这个异常，我将不胜感激:)

我是否缺少任何行，或者在需要时不使用Alien.class ?

主类:

public class Main {
public static void main(String[] args)
{
    System.out.println("Starting process of alien creation...");
    Alien alien_1 = new Alien("Bob", 5, (float) 45.3, AlienType.REACTIVE);
    Alien alien_2 = new Alien("Lilly", 7, (float) 49.8, AlienType.FRIENDLY);
    System.out.println("n" + alien_1.getName());
    alien_1.setName("Carl");
    System.out.println(alien_1.getName());
    Method getNameList = null;
    Field nameList = null;
    try {
        getNameList = Alien.class.getClass().getDeclaredMethod("getNameList");
    } catch (NoSuchMethodException e) {
        System.out.println("ERROR - Method "getNameList" does not exist!");
    } catch (SecurityException e) {
        System.out.println("ERROR - Method "getNameList" cannot be used!");
    }
    try {
        nameList = Alien.class.getClass().getDeclaredField("alienNames");
    } catch (NoSuchFieldException e) {
        System.out.println("ERROR - Field "nameList" does not exist!");
    } catch (SecurityException e) {
        System.out.println("ERROR - Field "nameList" cannot be used!");
    }
    getNameList.setAccessible(true);
    nameList.setAccessible(true);
    try {
        ArrayList<String> returnValue = (ArrayList<String>) getNameList.invoke(new Alien(), nameList);
    } catch (IllegalAccessException | IllegalArgumentException | InvocationTargetException e) {
        e.printStackTrace();
    }
}

}

外星人类:

public class Alien 
{
    public void getAndPrintType(AlienType type)
    {
        switch (type) 
        {
            case FRIENDLY : System.out.print(", type - FRIENDLY.n"); break;
            case SCARY : System.out.print(", type - SCARY.n"); break;
            case REACTIVE : System.out.print(", type - REACTIVE.n"); break;
        }
    }
    String name;
    int age;
    float weight;
    AlienType type;
    int totalAliens;
    public static ArrayList<String> alienNames = new ArrayList<String>();
    ArrayList<Integer> alienAges = new ArrayList<Integer>();
    ArrayList<Float> alienWeights = new ArrayList<Float>();
    ArrayList<AlienType> alienTypes = new ArrayList<AlienType>();
    float totalWeight;
    public Alien(String name, int age, float weight, AlienType type)
    {
        System.out.print("You have created an alien of attributes: ");
        System.out.print("name - " + name );
        System.out.print(", age - " + String.valueOf(age));
        System.out.print(", weight - " + String.valueOf(weight) + " pounds");
        getAndPrintType(type);
        this.name = name;
        this.age = age;
        this.weight = weight;
        this.type = type;
        // Global impact
        totalAliens++;
        alienNames.add(name);
        alienAges.add(age);
        alienWeights.add(weight);
        alienTypes.add(type);
    }
    public Alien() {}
    private static String getNameList()
    {
        String returnValue = "";
        for (String nameLocal : alienNames)
        {
            if (!(alienNames.get((alienNames.size() - 1)).equals(nameLocal)))
            {
                returnValue += nameLocal + ", ";
            }
            else
            {
                returnValue += nameLocal;
            }
        }
        return returnValue;
    }
}

Answer 1

问题似乎在这一行:

getNameList = Alien.class.getClass().getDeclaredMethod("getNameList");

Alien.class成为对Class<Alien>的引用，因此getClass返回Class<Alien>的运行时类，即Class.class。然后在对Class.class的引用上调用getDeclaredMethod，寻找名为getNameList的方法，这在Class类中不存在，并且抛出NoSuchMethodException，并且getNameList保持为空。

知道了所有这些，解决方案是删除对getClass的调用，这样你就可以在引用Class<Alien>时调用getDeclaredMethod:

getNameList = Alien.class.getDeclaredMethod("getNameList");