Why java.lang.Object class methods are native?

These are some methods in java.lang.Object class, I was going through...

 protected native Object clone() throws CloneNotSupportedException;

 public final native Class getClass();

 public native int hashCode();

 public final native void notify();

 private static native void registerNatives();

 public final native void wait(long timeout) throws InterruptedException;

why are these methods define as native? Please share reference and further reading docs if possible.

Source : java.lang.Object