Java - 我可以将变量修改从一个类转换为另一个类吗？

Question

Say I have in class A :

int a = 1, b = 2, c = 3;

Now, say in class B I have a function:

public int void reset(String intName, A classA) {
...
}

Within reset() if I do reset('c', classA) , then I want c in classA to be reset. If I do a reset('b', classA) , then I want b in classA to be reset and so forth. I'm not sure if this is possible, but I need this to kill off units of different classes in a game.

Answer 1

This is possible through reflection. But since reflection is slow, I advise you not to try reflection immediately. Try something else first. Why can't you just do:

someObjectOfA.a = some value;
someObjectOfA.b = some value;
someObjectOfA.c = some value;

Anyway, here is the solution using reflection:

public static void reset(String name, A a) throws NoSuchFieldException, IllegalAccessException {
    Class<A> clazz = A.class;
    Field f = clazz.getField(name);
    f.set(a, 0);
}

Note that a , b and c must be declared public or a NoSuchFieldException will be thrown.

If you don't want to expose your fields, you can add setters like this one:

public void setA(Integer value) {
    a = value;
}

Then the reset method will be:

public static void reset(String name, A a) throws IllegalAccessException, NoSuchMethodException, InvocationTargetException {
    Class<A> clazz = A.class;
    Method m = clazz.getMethod("set" + name.toUpperCase(), Integer.class);
    m.invoke(a, 0);
}

Answer 2

There are many options here, even without using reflection (ie dynamically modifying arbitrary properties of the object by names specified at runtime). The simplest is to simply have A be responsible for translating from the specified key to the corresponding variable:

public class A {
    private static int a = 1, b = 2, c = 3;

    public void reset(String key) {
        switch(key) {
        case "a": a = 0;
                  break;
        case "b": b = 0;
                  break;
        case "c": c = 0;
                  break;
        default:
                  break;
        }
    }
}

public class B {
    public int void reset(String intName, A obj) {
        obj.reset(intName);
    }
}

This leads to repetitive code though, and it's easy to make mistakes, especially as you add new fields to A .

---

A better option would be using a Map to dynamically map the property names to their values:

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

public class A {
    private static Map<String, Integer> map = new HashMap<>();
    static {
        map.put("A", 1);
        map.put("B", 2);
        map.put("C", 3);
    }

    public Map<String, Integer> getGameProperties() {
        return Collections.unmodifiableMap(map);
    }

    public void reset(String key) {
        map.put(key, 0);
    }
}

public class B {
    public int void reset(String intName, A obj) {
        obj.reset(intName);
    }
}

This handles the property declaration and key mapping in a single step, so that fixes the issue of the repetitive, error-prone code. It does potentially affect the external interface of the class now since you got rid of the individual fields and replaced them with a Map . That's of course a good reason to keep your fields private and use getters/setters instead, since getters can be updated to look at a Map without changing the external API of class A .

Answer 3

I would design my class A like this:

class A
{
   public enum UnitType { TANK, TRIKE, COMMANDO };

   public void reset (UnitType type); 

   //or even:
   public void reset (Set<UnitType> types); 

}

So class B could call:

a.reset(UnitType.TANK);

or in the Set case

a.reset(EnumSet.of(UnitType.TANK, UnitType.COMMANDO));

or

a.reset(EnumSet.allOf(UnitType.class));

You can still choose in class A how to keep track of the counts, without influencing the interface to class B

For example, class A can use a Map internally to keep track of the counts (which seems a good idea), but also fields, or any other method.

Answer 4

Why not just to have a something like setter in class A if you really want to reset it from other class?

Say I have in class A:

int a = 1, b = 2, c = 3;

public void resetA()
{
    this.a = 0;
}
public void resetB()
{
    this.b = 0;
}
public void resetC()
{
    this.c = 0;
}

or

int a = 1, b = 2, c = 3;

public void resetAll()
{
    this.c = 0;
    this.a = 0;
    this.b = 0;
}

i just can't think for now how to make a good coding for it's flexibility but it can reset you're value even from the other class if you like

Answer 5

Try this. No need to write setter method for each field.

        try
        {
            Field field = classA.getClass().getDeclaredField(intName);    
            field.setAccessible(true);
            Object value = field.get(classA);
            System.out.println(" val "+ value);
            field.set(classA, 0);//Reset to zero
            value = field.get(classA);
            System.out.println(" val "+ value);
        } catch (Exception e)
        {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }