Assignment Expressions

Assignment Expressions (Eck 2.5)

Expressions – syntactically correct combination of variables, literals, operators, and method calls that evaluate to a value with a specific type. The following meta language shows where expressions are found within Java statements

Assignment expressions update the value of a variable or array element.

Assignment Expression Statement Metalanguage

The following shows the meta language for an assignment expressions statement. As discussed in this section, the = assignment operator is a regular operator and is not limited to an assignment expression statement; however, the = operator is used most often in an assignment expression statement.

<variable> = <exp>;  // The assignment operator is part of expressions

Assignment Operator Precedence

The bolded descriptions in the following table are the operators discussed in this section, listed in order from highest precedence (evaluated first) to lowest precedence (evaluated last):

Operators on the same line have the same precedence. When operators of the same precedence are strung together in the absence of parentheses, unary operators and assignment operators are evaluated right-to-left, while the remaining operators are evaluated left-to-right. For example, A*B/C means (A*B)/C, while A=B=C means A=(B=C).

double unitPrice = 4.35;
double totalPrice = 100 * unitPrice // 434.999999999999994
// binary to decimal: no exact representation for 4.35
// similar to 10/3 is 3.3333333...
// floating point numbers are fuzzy

The = Assignment Operator

One thing to remember is that assignment is an operator, not a statement. This means that Java allows you to include assignments in expressions as follows. The precedence of the assignment operator is low and it groups from right to left. I suggest you use the assignment operator as an assignment statement.

int i1,i2,i3;
i1 = 1;
i2 = i3 = i1; // after this i1, i2, i3 contain 1
i3 = (i1 = i3) + i2;  // after this i3 contains 2, i1,i2 contain 1

The += Style Assignment Operators

Adding a value to a variable is a common action in programming - for example, x = x + 10; adds 10 to x. Java has a special assignment statement that provides a short cut for this technique. Every Java operator that applies to two operands, except for the relational operators, can be used in the += style assignment operator.

x -= y;     // same as:   x = x - y;
x *= y;     // same as:   x = x * y;
x /= y;     // same as:   x = x / y;
x %= y;     // same as:   x = x % y;
q &&= p;    // same as:   q = q && p;  (for booleans q and p)

Increment and Decrement Operators

Adding 1 or subtracting 1 to a variable is a common action in programming. The ++ and -- are shortcut Java operators for incrementing and decrementing a variable. The ++ and -- operators can be used as either prefix or postfix. The difference of prefix and postfix is explained in the next section. You will use this shortcut quite often in for loops.

x++; // same as x = x + 1
++x; // same as x = x + 1
x--; // same as x = x - 1
--x; // same as x = x - 1

Expressions with Side Effects

Most of the time expressions simply evaluate to a value; however, sometimes expressions have side effects. The term side effect means the expression changes the value of some variable. A simple assignment statement (which is really an assignment expression) such as the following has side effects.

i1 = 1;

In this case the side effect is obvious. Likewise the following expressions have clearly defined side effects.

x++
y--
x -= y;     // same as:   x = x - y;
x *= y;     // same as:   x = x * y;
x /= y;     // same as:   x = x / y;
x %= y;     // same as:   x = x % y;
q &&= p;    // same as:   q = q && p;  (for booleans q and p)

However, you can create expressions with side effects that are not as obvious. The following is an example copied from the Oracle website.

int a = 9;
a += (a = 3);  // a is 12 after evaluating this expression
int b = 9;
 b = b + (b = 3);  // b is 12 after evaluating this expression

Java Expression Statements

Recall that a Java block is a sequence of statements enclosed within curly braces {}. Java has declaration statements, which we began studying in Primitive Types, expression statements, two of which are discussed in this section, method calls, which we begin studying in Methods, and object creation expression statement, which we begin studying in Simple Objects. Java expression statements consist of the following.

• Assignment expression
  • x = 4.4;
• Increment/decrement expression
  • i++;
• Method Calls
  • System.out.println(“Gusty”);
• Object Creation Expression
  • Person p = new Person(“Gusty”,22);

Swapping Variables Pattern

Programming often entails moving the contents of one variable to another. A common problem is swapping the contents of two variables, e.g., x and y. You cannot perform two consecutive assignment statements, x = y; followed by y = x;. You have to introduce a temporary variable. The following code sequence does not swap x and y.

int x = 1;
int y = 2;
x = y;  // x is now 2
y = x;  // y is now 2

The pattern for swapping the contents of two variables is given as follows.

public class SwappingVariablesPattern {
   public static void main(String[] args) {
      int x = 1;
      int y = 2;
      int t = x; // t is 1
      x = y;     // x is 2
      y = t;     // y is 1
   }
}