Classes not extending java.lang.Object

Question

While you create a user defined class in Java, you do not specify it as extending Object. But still the class is an Object. How does this work? How does javac or the JVM inject all properties of a class to the user defined class?

Answer 1

If you don't actually write extends Object, the compiler inserts it for you.

EDIT: Apparently I caused some confusion about whether there is actually an insertion of code going on. I wasn't entirely sure myself so I ran a little experiment: create the following class in file test.java:

public class test {}

and compile it, then run

javap -c test

to disassemble the bytecode. Look what comes out:

Compiled from "test.java"
public class test extends java.lang.Object{
public test();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."":()V
   4:   return

}

So yes, the compiler does actually insert extends java.lang.Object (or the bytecode equivalent) into the class.