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);     } }