Why Object class have static block?

Question

I want to just know about why Object,String etc. have static{} block at the end.what is the use of static block in Object Class.

Open the cmd prompt and type

javap java.lang.Object

Answer 1

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();
     }

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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.

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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
}

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

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

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

What does the registerNatives() method do?