java: how can i set a class's field by a field name that stored in a variable

Question

Hi. how can i set a field in a class that it's name is dynamic and stored in a string variable ?

public class test {

    public String a1;
    public string a2;  

    public test(String key) {
        this.key='found';  <--- error
    } 

}

Answer 1

You have to use reflection:

• Use Class.getField() to get a Field reference. If it's not public you'll need to call Class.getDeclaredFields() instead
• Use AccessibleObject.setAccessible to gain access to the field if it's not public
• Use Field.set() to set the value, or one of the similarly-named methods if it's a primitive

Here's an example which deals with the simple case of a public field. For the more normal situation of private fields, you would need to loop through the declared fields. A nicer alternative would be to use properties, if possible.

import java.lang.reflect.Field;

class DataObject
{
    // I don't like public fields; this is *solely*
    // to make it easier to demonstrate
    public String foo;
}

public class Test
{
    public static void main(String[] args)
        // Declaring that a method throws Exception is
        // likewise usually a bad idea; consider the
        // various failure cases carefully
        throws Exception
    {
        Field field = DataObject.class.getField("foo");
        DataObject o = new DataObject();
        field.set(o, "new value");
        System.out.println(o.foo);
    }
}