带有静态 fields/methods 的反射和 NullPointerException？

Question

我正在尝试使用反射来访问我不希望由对象继承的方法和变量。我收到这些 static 变量的空指针异常，我无法注意到我遗漏的任何内容。如果能帮助解决异常，我将不胜感激 :)

我是否遗漏了任何行，或者在需要时没有使用 Alien.class？

主要class:

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class:

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

问题似乎出在这一行：

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

Alien.class 成为对 Class<Alien> 的引用，因此 getClass returns Class<Alien> 的运行时间 class 即 Class.class.然后，您在对 Class.class 的引用上调用 getDeclaredMethod，查找名为 getNameList 的方法，这在 Class class 和 NoSuchMethodException 被抛出，并且 getNameList 保持为空。

了解所有这些，解决方案是删除对 getClass 的调用，这样您就可以在对 Class<Alien> 的引用上调用 getDeclaredMethod:

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

带有静态 fields/methods 的反射和 NullPointerException？

Reflection and NullPointerException with static fields/methods?

java

reflection

static

instance