The Nuts and Bolts of the Java Language |

The character-counting program uses several operators including`=`

,`!=`

,`++`

, and`+`

highlighted in this listing:Operators perform some function on either one or two operands. Operators that require one operand are calledclass Count { public static void main(String[] args) throws java.io.IOException { int count=0; while (System.in.read()!=-1) count++; System.out.println("Input has "+count+" chars."); } }unary operators. For example,`++`

is a unary operator that increments the value of its operand by one. Operators that require two operands arebinary operators--the`=`

operator is a binary operator that assigns the value from its right-hand operand to its left-hand operand.Java's unary operators can use either prefix or postfix notation. Prefix notation means that the operator appears

beforeits operand:Postfix notation means that the operator appearsoperator opafterits operand:All of Java's binary operators use infix notation, which means that the operator appearsop operatorbetweenits operands:In addition to performing the operation, an operator also returns a value. The value and its type depends on the operator and the type of its operands. For example, the arithmetic operators (perform basic arithmetic operations such as addition and subtraction) return numbers--typically the result of the arithmetic operation. The data type returned by the arithmetic operators depends on the type of its operands: if you add two integers, you get an integer back. An operation is said toop1 operator op2evaluate toits result.It's useful to divide Java's operators into these categories: arithmetic, relational and conditional, bitwise and logical, and assignment.

## Arithmetic Operators

The Java language supports various arithmetic operators--including`+`

(addition),`-`

(subtraction),`*`

(multiplication),`/`

(division), and`%`

(modulo)--on all floating point and integer numbers. For example, you can use this Java code to add two numbers:or this code to compute the remainder that results from dividingaddThis + toThis`divideThis`

by`byThis`

:divideThis % byThisThis table summarizes Java's binary arithmetic operations:

Operator Use Description `+`

`op1 + op2`

Adds op1andop2`-`

`op1 - op2`

Subtracts op2fromop1`*`

`op1 * op2`

Multiplies op1byop2`/`

`op1 / op2`

Divides op1byop2`%`

`op1 % op2`

Computes the remainder of dividing op1byop2

Note:The Java language extends the definition of the operator`+`

to include string concatenation. The example program uses`+`

to concatenate "`Input has`

", the value of`count`

, and "`chars.`

" Note that this operation automatically coerces the value`count`

to a`String`

.You'll see more about this in Arrays and Strings.System.out.println("Input has " + count + " chars.");

The

`+`

and`-`

operators have unary versions which set the sign of the operand:

Operator Use Description `+`

`+op`

Indicates a positive value `-`

`-op`

Arithmetically negates `op`

In addition, there are two short cut arithmetic operators,

`++`

which increments its operand by one, and`--`

which decrements its operand by one. The character-counting example uses`++`

to increment the`count`

variable each time it reads a character from the input source:Note that thecount++;`++`

operator appears after its operand in this example. This is the postfix version of the operator.`++`

also has a prefix version in which`++`

appears before its operand. Both the prefix and postfix versions of this operator increment the operand by one. So why are there two different versions? Because each version evaluates a different value:`op++`

evaluates to the value of the operandbeforethe increment operation and`++op`

evaluates the value of the operandafterthe increment operation.In the character-counting program suppose that count is say, 5, before this statement is executed:

After the statement is executed the value ofcount++;`count`

is 6. No surprises there. However, the statement`count++`

evaluates to 5. In the same scenario the prefix version of`++`

:also sets++count;`count`

to 6, however the statement`++count`

does not evaluate to 5 like the postfix version of`++`

, but 6. This difference is unimportant in the character-counting program but is critical in situations where the value of the statement is used for a computation, for flow control, or for something else. For example, the following loop will execute one less time if you change`count++`

to`++count`

:You'll learn more about flow control in Control Flow Statements.do { . . . } while (count++ < 6);Similarly,

`--`

also has prefix and postfix versions which function in the same way as`++`

.

Operator Use Description `++`

`op++`

Increments `op`

by 1; evaluates to value before incrementing`++`

`++op`

Increments `op`

by 1; evaluates to value after incrementing`--`

`op--`

Decrements `op`

by 1; evaluates to value before decrementing`--`

`--op`

Decrements `op`

by 1; evaluates to value after decrementing

## Relational and Conditional Operators

Relational operators compare two values and determine the relationship between them. For example,`!=`

returns`true`

if the two operands are unequal. The character-counting program uses`!=`

to determine whether the value returned by`System.in.read`

is not equal to -1.This table summarizes Java's relational operators:

Operator Use Return `true`

if`>`

`op1 > op2`

`op1`

is greater than`op2`

`>=`

`op1 >= op2`

`op1`

is greater than or equal to`op2`

`<`

`op1 < op2`

`op1`

is less than`op2`

`<=`

`op1 <= op2`

`op1`

is less than or equal to`op2`

`==`

`op1 == op2`

`op1`

and`op2`

are equal`!=`

`op1 != op2`

`op1`

and`op2`

are not equalOften the relational operators are used with another set of operators, the conditional operators, to construct more complex decision making expressions. One such operator is

`&&`

which performs theboolean andoperation. For example, you can use two different relational operators along with`&&`

to determine if both relationships are true. The following line of code uses this technique to determine if an array index is between two boundaries--that is, to determine if the index is both greater than 0 and less than`NUM_ENTRIES`

(which is a previously defined constant value):Note that in some instances, the second operand to a conditional operator may not be evaluated. Consider this statement:0 < index && index < NUM_ENTRIESIf((count > NUM_ENTRIES) && (System.in.read() != -1))`count`

is less than`NUM_ENTRIES`

, the left-hand operand for`&&`

evaluates to false. The`&&`

operator will return true only ifbothoperands are true. So in this situation, the return value of`&&`

can be determined without evaluating the right-hand operand. In a case such as this, Java will not evaluate the right-hand operand. Thus,`System.in.read`

won't get called and a character will not be read from standard input.There are three conditional operators:

Operator Use Returns `true`

if`&&`

`op1 && op2`

`op1`

and`op2`

are both true`||`

`op1 || op2`

either `op1`

or`op2`

is true`!`

`! op`

`op`

is falseThe operator

`&`

can be used as a synonym for`&&`

if boths of its operands are boolean. Similarly,`|`

is a synonym for`||`

if both of its operands are boolean.## Bitwise Operators

The bitwise operators allow you to perform bit manipulation on data. This table summarizes the bitwise and logical operators available in the Java language.

Operator Use Operation `>>`

`op1 >> op2`

shift bits of `op1`

right by distance`op2`

`<<`

`op1 << op2`

shift bits of `op1`

left by distance`op2`

`>>>`

`op1 >>> op2`

shift bits of `op1`

right by distance`op2`

(unsigned)`&`

`op1 & op2`

bitwise `and`

`|`

`op1 | op2`

bitwise `or`

`^`

`op1 ^ op2`

bitwise `xor`

`~`

`~op2`

bitwise complement The three shift operators simply shift the bits of the left-hand operand over by the number of positions indicated by the right-hand operand. The shift occurs in the direction indicated by the operator itself. For example:

shifts the bits of the integer 13 to the right by one position. The binary representation of the number 13 is 1101. The result of the shift operation is 1101 shifted to the right by one position--110 or 6 in decimal. Note that the bit farthest to the right falls off the end into the bit bucket. A right shift of one bit is equivalent to, but more efficient than, dividing the left-hand operand by two. A left shift of one bit is equivalent to multiplying by two.13 >> 1;The bitwise and operation performs the "and" function on each parallel pair of bits in each operand. The "and" function sets the resulting bit to one if both operands are 1.

`op1`

`op2`

Result 0 0 0 0 1 0 1 0 0 1 1 1 Suppose you were to "and" the values 12 and 13:

The result of this operation is 12. Why? Well, the binary representation of 12 is 1100, and the binary representation of 13 is 1101. The "and" function sets the resulting bit to one if both operand bits are 1, otherwise, the resulting bit is 0. So, if you line up the two operands and perform the "and" function, you can see that the two high-order bits (the two bits farthest to the left of each number) of each operand are 1. Thus the resulting bit in the result is also 1. The low-order bits evaluate to 0 because either one or both bits in the operands are 0:12 & 13The1101 & 1100 ------ 1100`|`

operator performs the inclusive or operation and`^`

performs the exclusive or operation. Inclusive or means that if either of the two bits are 1 then the result is 1. The following table shows the results of your inclusive or operations:

`op1`

`op2`

Result 0 0 0 0 1 1 1 0 1 1 1 1 Exclusive or means that if the two operand bits are different the result is one, otherwise the result is 0. The following table shows the results of your exclusive or operation.

`op1`

`op2`

Result 0 0 0 0 1 1 1 0 1 1 1 0 And finally, the complement operator inverts the value of each bit of the operand: if the operand bit is 1 the result is 0 and if the operand bit is 0 the result is 1.

Among other things, bitwise manipulations are useful for managing sets of boolean flags. Suppose for example, that you had several boolean flags in your program that indicated the state of various components in your program: is it visible, is it draggable, and so on. Rather than define a separate boolean variable to hold each flag, you could define a single variable,

`flags`

, for all of them. Each bit within`flags`

would represent the current state of one of the flags. You would then use bit manipulations to set and get each flag.First, set up constants that indicated the various flags for your program. These flags should each be a different power of two to ensure that the "on" bit didn't overlap with another flag. Define a variable,

`flags`

, whose bits would be set according to the current state of each flag. The following code sample initializes`flags`

to 0 which means that all flags are false (none of the bits are set).To set the "visible" flag when something became visible you would use this statement:final int VISIBLE = 1; final int DRAGGABLE = 2; final int SELECTABLE = 4; final int EDITABLE = 8; int flags = 0;To test for visibility, you could then write:flags = flags | VISIBLE;flags & VISIBLE## Assignment Operators

You use the assignment operator,`=`

, to assign one value to another. The character-counting program uses`=`

to initialize`count`

with this statement:In addition to the basic assignment operator, Java provides several short cut assignment operators that allow you to perform an arithmetic, logical, or bitwise operation and an assignment operation all with one operator. Specifically, suppose you wanted to add a number to a variable and assign the result back into the variable, like this:int count=0;You can shorten this statement using the short cut operatori = i + 2;`+=`

.The two previous lines of code are equivalent.i += 2;This table lists the shortcut assignment operators and their lengthy equivalents:

Operator Use Equivalent to `+=`

`op1 += op2`

`op1 = op1 + op2`

`-=`

`op1 -= op2`

`op1 = op1 - op2`

`*=`

`op1 *= op2`

`op1 = op1 * op2`

`/=`

`op1 /= op2`

`op1 = op1 / op2`

`%=`

`op1 %= op2`

`op1 = op1 % op2`

`&=`

`op1 &= op2`

`op1 = op1 &= op2`

`|=`

`op1 |= op2`

`op1 = op1 | op2`

`^=`

`op1 ^= op2`

`op1 = op1 ^ op2`

`<<=`

`op1 <<= op2`

`op1 = op1 << op2`

`>>=`

`op1 >>= op2`

`op1 = op1 >> op2`

`>>>=`

`op1 >>>= op2`

`op1 = op1 >>> op2`

The Nuts and Bolts of the Java Language |