Strange behaviour of static variables [duplicate]

Question

I have class AbstractsAndInterfaces:

public static AbstractsAndInterfaces instance = new AbstractsAndInterfaces();
private static final int DELTA = 5;

private static int BASE = 7;

private int x;
public AbstractsAndInterfaces()
{
    //System.out.println(BASE);
    //System.out.println(DELTA);
    x = BASE + DELTA;
}
public static int getBASE()
{
    return BASE;
}

   /**
 * @param args the command line arguments
 */
public static void main(String[] args) {

    System.out.println(AbstractsAndInterfaces.instance.x);
}

Why does it print 5?

Why BASE variable is uninitialized?

Answer 1

That's because :

private static final int DELTA = 5; will be a compile-time constant. So, 5 will already be available (initialized) when the class gets initialized (once it is loaded).

The first line that gets executed is : public static AbstractsAndInterfaces instance = new AbstractsAndInterfaces();

So, now you will go to :

public AbstractsAndInterfaces()
{
    //System.out.println(BASE);
    //System.out.println(DELTA);
    x = BASE + DELTA; // same as x= 0+5 i.e default value of fields + constant value 5
}

In the above code, DELTA will be 5 but BASE will be 0 as it is not final. Changing BASE to final will change the result to 12.

EDIT :

Sample code to show the OP's scenario with byte code.

public class Sample {
    static Sample s = new Sample();
    static final int x = 5;
    static int y = 10;

    public Sample() {
        int z = x + y;
        System.out.println(z);
    }

    public static void main(String[] args) {

    }

}

In the above code, when the program is run, the output will be 5 and not 10. Now let us look at the byte-code.

The byte code of constructor looks like this :

p

ublic Sample();
   descriptor: ()V
   flags: ACC_PUBLIC
   Code:
     stack=2, locals=2, args_size=1
        0: aload_0
        1: invokespecial #24                 // Method java/lang/Object."<init>
:()V 
        4: iconst_5       ------->           // Here 5 is used directly as it is a compile time constant
        5: getstatic     #20   ------->      // Field y:I
        8: iadd
        9: istore_1
       10: getstatic     #25                 // Field java/lang/System.out:Ljav
/io/PrintStream;
       13: iload_1
       14: invokevirtual #31                 // Method java/io/PrintStream.prin
ln:(I)V
       17: return

Byte code for static init block :

static {}; descriptor: ()V flags: ACC_STATIC Code: stack=2, locals=0, args_size=0 0: new #1 // class Sample 3: dup 4: invokespecial #15 // Method "":()V 7: putstatic #18 // Field s:LSample; 10: bipush 10 12: putstatic #20 // Field y:I 15: return LineNumberTable: line 3: 0 line 5: 10 line 1: 15 LocalVariableTable: Start Length Slot Name Signature

If you check carefully, x is not being initialized in the static init of class. Only y is being set as it is static. But when you look at the constructor, you will see that the constant 5 is directly being used (iconst_5) and pushed onto the stack.

Now if you add the following code in main() :

public static void main(String[] args) {
    System.out.println(Sample.x);
    System.out.println(Sample.y);
}

The byte code for main() will be :

   public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #25                 // Field java/lang/System.out:Ljav
a/io/PrintStream;
         3: iconst_5 -->                      // No "x" but a constant value instead of "x"
         4: invokevirtual #31                 // Method java/io/PrintStream.prin
tln:(I)V
         7: getstatic     #25                 // Field java/lang/System.out:Ljav
a/io/PrintStream;
        10: getstatic     #20                 // Field y:I --> but "y" is being fetched
        13: invokevirtual #31                 // Method java/io/PrintStream.prin
tln:(I)V
        16: return
      LineNumberTable:
        line 13: 0
        line 14: 7
        line 15: 16
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      17     0  args   [Ljava/lang/String;
}

Again observe that x is being used as iconst_5whereas y is being fetched/referred indirectly.