Modifying final fields in Java

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";

In my opinion this is even worse: A colleague pointed to the following funny thing:

@Test public void  testInteger() throws SecurityException,  NoSuchFieldException, IllegalArgumentException, IllegalAccessException  {      
    Field value = Integer.class.getDeclaredField("value");      
    value.setAccessible(true);       
    Integer manipulatedInt = Integer.valueOf(7);      
    value.setInt(manipulatedInt, 666);       
    Integer testInt = Integer.valueOf(7);      
    System.out.println(testInt.toString());
}

By doing this, you can change the behaviour of the whole JVM you are running in. (of course you can change only the values for the values between -127 and 127)