Why Object class have static block?

What you are looking at is just all the method and field declarations. Since the static block is somewhat like a method, you will just see the empty declaration of a static-initalizer.

If you look at the OpenJDK source code for java.lang.Object on line 40, the code actually says this

public class Object {

     private static native void registerNatives();
     static {
         registerNatives();
     }

A simple explanation of the static block is that the block only gets called once, no matter how many objects of the type you create.

If you want more information from the command line, javap -verbose java.lang.Object outputs this

  static {};
    descriptor: ()V
    flags: ACC_STATIC
    Code:
      stack=0, locals=0, args_size=0
         0: invokestatic  #16                 // Method registerNatives:()V
         3: return
      LineNumberTable:
        line 41: 0
        line 42: 3
}

Or, less verbose javap -c java.lang.Object

  static {};
    Code:
       0: invokestatic  #16                 // Method registerNatives:()V
       3: return

If you want to read about what registerNatives() does, you can read this post.

What does the registerNatives() method do?