Java: How can I access a class's field by a name stored in a variable?

Question

How can I set or get a field in a class whose 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.getDeclaredField() 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. 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);
    }
}