Constructors called due to static initialization

Question

I am going through Thinking in Java by Bruce Eckel 4th Edition. In the chapter Initialization & Cleanup, page : 189 the first bullet point in the second para mentions:

Even though it doesn't explicitly use the static keyword the constructor is actually a static method.

I have the following piece of code:

class Bar {
    Bar() {
        System.out.println("Bar Creation");
    }
}

class Foo {
    static int x = 10;
    static Bar b = new Bar();

    Foo() {
        System.out.println("Foo Creation");
    }
}

public class Test { 
    public static void main(String[] args) {
        System.out.println(Foo.x);
    }
}

If what it says is true The constructor for Foo should have been called. I don't see that happening with the following piece of code.

The output is:

Bar Creation
10

Can someone clarify what does it mean?

I have tried my best to quote the book. I don't think the parts previous to that statement or after that have much relevance to this statement in context of the question.

Thanks,

Gudge

Thanks for all your comments. I understand perfectly well that unless object is created the constructor won't be called. Then what does the statement in the book mean. — gudge, May 12 '13 at 14:39

Kevin Reid · Accepted Answer · 2013-05-12T14:42:28.190

There's no reason for Foo() to be called just because you mentioned the class. Static initializers like static Bar b = new Bar(); are called when the class is loaded; static methods are called by your code.

I would guess that what the book means is that constructors are like static methods in that the dispatch is static: that is, there is no way to inherit and override a constructor, and a call site for a constructor, or a static method, always refers to some specific class determined at compile time.

(This staticness of constructors is the motivation for “factory” objects which construct objects in their instance methods.)

sanbhat · Answer 2 · 2013-05-12T14:35:40.070

Simply because you haven't created new instance of Foo to access static field x of it

If you had created new Foo() the constructor code would have got called. Since x is a static member field, its not required to create the instance of the holder class to access that field.

Another way of accessing x is new Foo().x, but the new Object creation is unnecessary.

And the reason why "Bar Creation" got printed is, because A static field declared by Foo is assigned. See JLS 12.4.1 for the initialization specs

Peter Lawrey · Answer 3 · 2013-05-12T14:26:40.993

A constructor is called after a new object is created. I wouldn't call it a static method as it must have an instance and that instance is accessible via default this

public class Main {
    public Main() {
        // static methods do not have a `this`
        System.out.println("Main called" + this.getClass());
    }

    public static void main(String... ignore) {
        new Main();
    }
}

If you decompile the byte code, you can see static methods

$ javap -c -p -cp . Main
Compiled from "Main.java"
public class Main {
  public Main();
    Code:
       0: aload_0       
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       7: new           #3                  // class java/lang/StringBuilder
      10: dup           
      11: invokespecial #4                  // Method java/lang/StringBuilder."<init>":()V
      14: ldc           #5                  // String Main called 
      16: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      19: aload_0       
      20: invokevirtual #7                  // Method java/lang/Object.getClass:()Ljava/lang/Class;
      23: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/Object;)Ljava/lang/StringBuilder;
      26: invokevirtual #9                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      29: invokevirtual #10                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      32: return        

  public static void main(java.lang.String...);
    Code:
       0: new           #11                 // class Main
       3: dup           
       4: invokespecial #12                 // Method "<init>":()V
       7: pop           
       8: return        
}

Note: static methods have a different modifier.

score 1 · Answer 4 · edited May 12 '13 at 14:24

1

Unless you call new() on Foo class, constructor will not be called. As new keyword internally calls the constructor. You are simply accessing a static field of the class using its name for that you don't need to create an instance.

edited May 12 '13 at 14:24

Sean Owen

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answered May 12 '13 at 14:20

Juned Ahsan

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score 0 · Answer 5 · answered May 12 '13 at 14:27

0

You're newer newing up Foo, instead you're newing up Bar as a static field in Foo. You can access static variables in a class without ever constructing an instance of it.

answered May 12 '13 at 14:27

jontejj

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score 0 · Answer 6 · answered May 12 '13 at 14:31

0

The constructor is only called when you use the new keyword. Since you don't do new Foo(), the constructor isn't called.

answered May 12 '13 at 14:31

Constructors called due to static initialization

6 Answers6