Modifying final fields in Java

Question

Let's start with a simple test case:

import java.lang.reflect.Field;  public class Test {   private final int primitiveInt = 42;   private final Integer wrappedInt = 42;   private final String stringValue = "42";    public int getPrimitiveInt()   { return this.primitiveInt; }   public int getWrappedInt()     { return this.wrappedInt; }   public String getStringValue() { return this.stringValue; }    public void changeField(String name, Object value) throws IllegalAccessException, NoSuchFieldException {     Field field = Test.class.getDeclaredField(name);     field.setAccessible(true);     field.set(this, value);     System.out.println("reflection: " + name + " = " + field.get(this));   }    public static void main(String[] args) throws IllegalAccessException, NoSuchFieldException {     Test test = new Test();      test.changeField("primitiveInt", 84);     System.out.println("direct: primitiveInt = " + test.getPrimitiveInt());      test.changeField("wrappedInt", 84);     System.out.println("direct: wrappedInt = " + test.getWrappedInt());      test.changeField("stringValue", "84");     System.out.println("direct: stringValue = " + test.getStringValue());   } } 

Anybody care to guess what will be printed as output (shown at the bottom as to not spoil the surprise immediately).

The questions are:

1. Why do primitive and wrapped integer behave differently?
2. Why does reflective vs direct access return different results?
3. The one that plagues me most - why does String behave like primitive int and not like Integer?

Results (java 1.5):

reflection: primitiveInt = 84 direct: primitiveInt = 42 reflection: wrappedInt = 84 direct: wrappedInt = 84 reflection: stringValue = 84 direct: stringValue = 42 

Answer 1

Compile-time constants are inlined (at javac compile-time). See the JLS, in particular 15.28 defines a constant expression and 13.4.9 discusses binary compatibility or final fields and constants.

If you make the field non-final or assign a non-compile time constant, the value is not inlined. For instance:

private final String stringValue = null!=null?"": "42";