Incrementing an Integer variable doesn't affect another referencing the same object

Question

I always understood static variables to share one instance whenever they were referenced. I wanted to put this to the test but the results were different than I had expected.

static Integer counter = 0;
static Integer test = counter;

public static void main(String args[]) {
     counter++;
     System.out.println("counter: " + counter);
     System.out.println("test: " + test);
}

output:

counter: 1

test: 0

Since test references counter I thought that when I increment counter then test will automatically be incremented as well. However, it seems test is referencing 0 from somewhere, question is, where?

Answer 1

Since test references counter

This assumption is false. In Java, you cannot reference variables. What is stored in a variable is a value. That value can either be a primitive type value or a reference type value. In the case of primitives, the value is the value of the primitive. In the case of reference types,

The reference values (often just references) are pointers to these objects, and a special null reference, which refers to no object.

In

int a = 0;
int b = a;

the variable a is evaluated to produce a value, 0, and that value is stored in b.

In

Integer a = 0;
Integer b = a;

0 is converted to an Integer through Integer.valueOf(int) and the value, a reference to an Integer object is stored in a. Then a is evaluated, producing the value of that reference to an Integer object, and that value is stored in b.

This is the same concept discussed in

• Is Java "pass-by-reference" or "pass-by-value"?

Also, the fact that the variables are static is irrelevant.

The only way around this is to update test manually i.e.

counter++; 
test = counter;

Answer 2

Here is my understanding.

• Step 1: When you say Integer counter = 0;what you're actually doing is
  Integer counter = new Integer(0); Here you created an Integer object whose value is 0 and counter is reffering to it.

• Step 2: Integer test = counter;

  Now test is also referring to the same Integer object we created in step 1.

• Step 3: Here comes the tricky part. When you do counter++; in your main method, Java Auto-boxing and Auto-unboxing features are implementing the below code for you.

  counter = Integer.valueOf(counter.intValue() + 1);

As Integer class is immutable in Java, when the value is incremented from 0 to 1, the valueOf() method is creating a new Integer object and storing the new value 1 in it instead of changing the old object's value (See below for Java's valueOf() method implementation). As you're referring the new object with counter, it de-referenced the old Integer object whose value is 0. But the reference variable test is still holding the old Integer object. This is the reason test printing the old value.

Java's valueOf() method from library class Integer

638     public static Integer More ...valueOf(int i) {
639         assert IntegerCache.high >= 127;
640         if (i >= IntegerCache.low && i <= IntegerCache.high)
641             return IntegerCache.cache[i + (-IntegerCache.low)];
642         return new Integer(i); //See, new object is created here.
643     }

See below for detailed diagrammatic explanation.

Solid line implies reference variable is still holding the object.

Dotted line implies reference variable is no more holding the object.

Answer 3

test doesn't reference counter. It has the same initial value. int is a primitive.