如何使用名称来访问Java中的变量？

Question

I am trying to write a method in a class which could be invoked several times, each time modifying one of the class' fields. However, I need to new the object and set the field's value to it if I want to modify it, but if do this inside the method, the reference seem to be lost and the field left unchanged after the calling.

Public class A {
    private Immutable a;  //Immutable class
    private Immutable b;
    public void modify(Immutable k,int v) {
        k=new Immutable(v);  //Now I am trying to pass 
                             //a and b as parameters but they remain unchanged
    }
}

Is there any way to pass the name of the field into the method (eg, change the method modify(Immutable k, int v) to modify(String kName, int v) , then use the name of the field to access it?

Thanks for any inputs!

Answer 1

If you need to access a variable by a String key, you should use a Map .

Map<String, Immutable> _vars = new HashMap<String, Immutable>();

public void modify(String key, int v) {
  _vars.put(key, new Immutable(v);
}

Answer 2

What I understand is that you're trying to create a new Immutable given an integer (v). In your modify method right now, k is a temporary reference. Setting the value "k =" in here, only affects the reference stored here in this method, not whatever reference you called modify with.

You have client code like this currently:

A a = new A();
Immutable k = new Immutable(x);
a.modify(k, y);

and you're hoping that k will be changed. What you really want instead of the 3rd line is:

k = new Immutable(y);

Assuming that things are really more complicated, then I would need more information to help you further.

Answer 3

Java does not support Call-by-name or call-by-reference, only Call-by-value. Your k variable (the method parameter) is completely independent from any variable used outside of the class (if there was one at all).

You could use Reflection to support passing "a" or "b" (or a Field object), but you should not.

Better have two methods:

public void setA(int v) {
    this.a = new Immutable(v);
}

public void setB(int v) {
    this.b = new Immutable(v);
}

If it is more complicated than a single constructor call, factor the common part out to a common method.

Answer 4

Use a value holder.

public class ValueHolder<T> {
    private T value ;
    public ValueHolder(T value) {
        this.value = value ;
    }
    public T get() { 
        return value ; 
    }
    public void set(T value) { 
        this.value = value; 
    }
    public static <V> ValueHolder<V> make(V value) {
        return new ValueHolder<V>(value);
    }
}
public class Processor {
    private Inmutable a ;
    private Inmutable b ;
    public void modify(ValueHolder<Inmutable> k, int v) {
        k.set(new Inmutable(v));
    }
}

Once that is done you can get the instance you just created from the value holder.

Processor processor = new Processor();
ValueHolder<Inmutable> holder = ValueHolder.make(k);
processor.modify(holder, value);
k = holder.get() ;