为什么每次乘法后我的变量的值都没有更新？

Question

Why isn't value of my variable poc updated after every multiplication done within System.out.println(); ?

public static void main(String[] args) {
    byte poc = 0b0001;

    System.out.println("The value of byte " + poc + " when multiplied by 0010 is: " + poc*0b0010);
    System.out.println("The value of byte " + poc + " when multiplied by 0010 is: " + poc*0b0010);
    System.out.println("The value of byte " + poc + " when multiplied by 0010 is: " + poc*0b0010);
    System.out.println("The value of byte " + poc + " when multiplied by 0010 is: " + poc*0b0010);
    System.out.println("The value of byte " + poc + " when multiplied by 0010 is: " + poc*0b0010);
    System.out.println("The value of byte " + poc + " when multiplied by 0010 is: " + poc*0b0010);
    System.out.println("The value of byte " + poc + " when multiplied by 0010 is: " + poc*0b0010);
    System.out.println("The value of byte " + poc + " when multiplied by 0010 is: " + poc*0b0010);
}

ps This is not homework. I'm new to Java, and I'm trying to grasp the concept of pass by value. I do not understand why isn't value of poc updated every time it is multiplied within println statement. For example, in the following code variable poc is updated every time.

public static void main(String[] args) {
        byte poc = 0b0001;

        System.out.println("The value of byte " + poc + " when incremented by 1 is: " + ++poc);
        System.out.println("The value of byte " + poc + " when incremented by 1 is: " + ++poc);
        System.out.println("The value of byte " + poc + " when incremented by 1 is: " + ++poc);
        System.out.println("The value of byte " + poc + " when incremented by 1 is: " + ++poc);
        System.out.println("The value of byte " + poc + " when incremented by 1 is: " + ++poc);
        System.out.println("The value of byte " + poc + " when incremented by 1 is: " + ++poc);
        System.out.println("The value of byte " + poc + " when incremented by 1 is: " + ++poc);
        System.out.println("The value of byte " + poc + " when incremented by 1 is: " + ++poc);
    }

Why is updated in the second piece of code, but not in the first one?

Answer 1

This really has nothing to do with pass-by-value ^†

poc*0b0010 is not a statement that assigns the result of poc*0b0010 to poc , it is just an expression which returns the result of the multiplication.

++poc is an expression that both increments poc (assignment) and returns the result of the assignment.

Note that you can use *= ( a compound assignment operator ) to do this by surrounding poc*0b0010 in brackets to create a Parenthesized Expression within the string concatenation:

System.out.println("The value of byte " + poc 
  + " when multiplied by 0010 is: " + (poc*=0b0010));

See The Java Tutorials > Assignment, Arithmetic, and Unary Operators for a full tutorial of arithmetic and assignment.

Also see JLS: 15.15.1. Prefix Increment Operator ++ and 15.17.1. Multiplication Operator *

^† for information about pass-by-value see the section "Passing Primitive Data Type Arguments" in this java tutorial

Answer 2

The reason that this is happening is because poc*0b0010 only returns a value, while ++poc increments the value of poc AND then returns the updated value of poc .

Essentially ++poc is the shorthand for poc = poc + 1; Notice how the result of poc + 1 is assigned to poc.

However, poc*0b0010 is just arithmetic being performed & returned, but not assigned to anything.

If you used poc *= 0b0010 then the expression would be equivalent to poc = poc * 0b0010 .

Just a final note: This behavior doesn't actually have anything to do with pass-by-value vs pass-by-reference, but instead has to do with assignment and the difference between arithmetic and unary operations.