3.5. Operators
The usual arithmetic operators +, -, *, / are used in Java for addition, subtraction, multiplication, and division. The / operator denotes integer division if both arguments are integers, and floating-point division otherwise. Integer remainder (sometimes called modulus) is denoted by %. For example, 15 / 2 is 7, 15 % 2 is 1, and 15.0 / 2 is 7.5.
Note that integer division by 0 raises an exception, whereas floating-point division by 0 yields an infinite or NaN result.
There is a convenient shortcut for using binary arithmetic operators in an assignment. For example,
x += 4;
is equivalent to
x = x + 4;
(In general, place the operator to the left of the = sign, such as *= or %=.)
Note
One of the stated goals of the Java programming language is portability. A computation should yield the same results no matter which virtual machine executes it. For arithmetic computations with floating-point numbers, it is surprisingly difficult to achieve this portability. The double type uses 64 bits to store a numeric value, but some processors use 80-bit floating-point registers. These registers yield added precision in intermediate steps of a computation. For example, consider the following computation:
double w = x * y / z;
Many Intel processors compute x * y, leave the result in an 80-bit register, then divide by z, and finally truncate the result back to 64 bits. That can yield a more accurate result, and it can avoid exponent overflow. But the result may be different from a computation that uses 64 bits throughout. For that reason, the initial specification of the Java virtual machine mandated that all intermediate computations must be truncated. The numeric community hated it. Not only can the truncated computations cause overflow, they are actually slower than the more precise computations because the truncation operations take time. For that reason, the Java programming language was updated to recognize the conflicting demands for optimum performance and perfect reproducibility. By default, virtual machine designers are now permitted to use extended precision for intermediate computations. However, methods tagged with the strictfp keyword must use strict floating-point operations that yield reproducible results.
For example, you can tag main as
public static strictfp void main(String[] args)
Then all instructions inside the main method will use strict floating-point computations. If you tag a class as strictfp, then all of its methods must use strict floating-point computations.
The gory details are very much tied to the behavior of the Intel processors. In the default mode, intermediate results are allowed to use an extended exponent, but not an extended mantissa. (The Intel chips support truncation of the mantissa without loss of performance.) Therefore, the only difference between the default and strict modes is that strict computations may overflow when default computations don't.
If your eyes glazed over when reading this note, don't worry. Floating-point overflow isn't a problem that one encounters for most common programs. We don't use the strictfp keyword in this book.
3.5.1. Increment and Decrement Operators
Programmers, of course, know that one of the most common operations with a numeric variable is to add or subtract 1. Java, following in the footsteps of C and C++, has both increment and decrement operators: n++ adds 1 to the current value of the variable n, and n-- subtracts 1 from it. For example, the code
int n = 12; n++;
changes n to 13. Since these operators change the value of a variable, they cannot be applied to numbers themselves. For example, 4++ is not a legal statement.
There are actually two forms of these operators; you've just seen the postfix form of the operator that is placed after the operand. There is also a prefix form, ++n. Both change the value of the variable by 1. The difference between the two only appears when they are used inside expressions. The prefix form does the addition first; the postfix form evaluates to the old value of the variable.
int m = 7; int n = 7; int a = 2 * ++m; // now a is 16, m is 8 int b = 2 * n++; // now b is 14, n is 8
We recommend against using ++ inside expressions because this often leads to confusing code and annoying bugs. (Of course, while it is true that the ++ operator gives the C++ language its name, it also led to the first joke about the language. C++ haters point out that even the name of the language contains a bug: "It should really have been called ++C, because we only want to use a language after it has been improved.")
3.5.2. Relational and boolean Operators
Java has the full complement of relational operators. To test for equality you use a double equal sign, ==. For example, the value of
3 == 7
is false.
Use a != for inequality. For example, the value of
3 != 7
is true.
Finally, you have the usual < (less than), > (greater than), <= (less than or equal), and >= (greater than or equal) operators.
Java, following C++, uses && for the logical "and" operator and || for the logical "or" operator. As you can easily remember from the != operator, the exclamation point ! is the logical negation operator. The && and || operators are evaluated in "short circuit" fashion: The second argument is not evaluated if the first argument already determines the value. If you combine two expressions with the && operator, expression1 && expression2 and the truth value of the first expression has been determined to be false, then it is
impossible for the result to be true. Thus, the value for the second expression is not calculated. This behavior can be exploited to avoid errors. For example, in the expression
x != 0 && 1 / x > x + y // no division by 0
the second part is never evaluated if x equals zero. Thus, 1 / x is not computed if x is zero, and no divide-by-zero error can occur.
Similarly, the value of expression1 || expression2 is automatically true if the first expression is true, without evaluating the second expression.
Finally, Java supports the ternary ?: operator that is occasionally useful. The expression
condition ? expression1 : expression2
evaluates to the first expression if the condition is true, to the second expression otherwise. For example,
x < y ? x : y
gives the smaller of x and y.
3.5.3. Bitwise Operators
When working with any of the integer types, you have operators that can work directly with the bits that make up the integers. This means that you can use masking techniques to get at individual bits in a number. The bitwise operators are
& ("and") | ("or") ^ ("xor") ~ ("not")
These operators work on bit patterns. For example, if n is an integer variable, then
int fourthBitFromRight = (n & 0b1000) / 0b1000;
gives you a 1 if the fourth bit from the right in the binary representation of n is 1, and 0 otherwise. Using & with the appropriate power of 2 lets you mask out all but a single bit.
Note
When applied to boolean values, the & and | operators yield a boolean value. These operators are similar to the && and || operators, except that the & and | operators are not evaluated in "short circuit" fashion. That is, both arguments are evaluated before the result is computed.
There are also >> and << operators which shift a bit pattern to the right or left. These operators are convenient when you need to build up bit patterns to do bit masking:
int fourthBitFromRight = (n & (1 << 3)) >> 3;
Finally, a >>> operator fills the top bits with zero, unlike >> which extends the sign bit into the top bits. There is no <<< operator.
Caution
The right-hand side argument of the shift operators is reduced modulo 32(unless the left-hand side is a long, in which case the right-hand side is reduced modulo 64). For example, the value of 1 << 35 is the same as 1 << 3 or 8.
C++ Note
In C/C++, there is no guarantee as to whether >> performs an arithmetic shift(extending the sign bit) or a logical shift(filling in with zeroes). Implementors are free to choose whatever is more efficient. That means the C/C++ >> operator is really only defined for non-negative numbers. Java removes that ambiguity.
3.5.4. Mathematical Functions and Constants
The Math class contains an assortment of mathematical functions that you may occasionally need, depending on the kind of programming that you do.
To take the square root of a number, use the sqrt method:
double x = 4; double y = Math.sqrt(x); System.out.println(y); // prints 2.0
Note
There is a subtle difference between the println method and the sqrt method. The println method operates on an object, System.out, defined in the System class. But the sqrt method in the Math class does not operate on any object. Such a method is called a static method. You can learn more about static methods in Chapter 4.
The Java programming language has no operator for raising a quantity to a power: You must use the pow method in the Math class. The statement
double y = Math.pow(x, a);
sets y to be x raised to the power a (xa). The pow method's parameters are both of type double, and it returns a double as well.
The Math class supplies the usual trigonometric functions（Math类也提供了常用的三角函数）:
Math.sin
Math.cos
Math.tan
Math.atan
Math.atan2
and the exponential function（指数函数） with its inverse, the natural logarithm（自然对数）, as well as the decimal logarithm:
Math.exp
Math.log
Math.log10
Finally, two constants denote the closest possible approximations to the mathematical constants π and e:
Math.PI
Math.E
Tip
You can avoid the Math prefix for the mathematical methods and constants by adding the following line to the top of your source file:
import static java.lang.Math.*;
For example:
System.out.println("The square root of u03C0 is " + sqrt(PI));
We discuss static imports in Chapter 4.
Note
The methods in the Math class use the routines in the computer's floating-point unit for fastest performance. If completely predictable results are more important than fast performance, use the StrictMath class instead. It implements the algorithms from the "Freely Distributable Math Library" fdlibm, guaranteeing identical results on all platforms. See www.netlib.org/fdlibm for the source code of these algorithms. (Whenever fdlibm provides more than one definition for a function, the StrictMath class follows the IEEE 754 version whose name starts with an "e".)
3.5.5. Conversions between Numeric Types
It is often necessary to convert from one numeric type to another. Figure 3.1 shows the legal conversions.
The six solid arrows in Figure 3.1 denote conversions without information loss. The three dotted arrows denote conversions that may lose precision. For example, a large integer such as 123456789 has more digits than the float type can represent. When the integer is
converted to a float, the resulting value has the correct magnitude but it loses some precision.
int n = 123456789; float f = n; // f is 1.23456792E8
When two values are combined with a binary operator (such as n + f where n is an integer and f is a floating-point value), both operands are converted to a common type before the operation is carried out.
- If either of the operands is of type double, the other one will be converted to a double.
- Otherwise, if either of the operands is of type float, the other one will be converted to a float.
- Otherwise, if either of the operands is of type long, the other one will be converted to a long.
- Otherwise, both operands will be converted to an int.
3.5.6. Casts
In the preceding section, you saw that int values are automatically converted to double values when necessary. On the other hand, there are obviously times when you want to consider a double as an integer. Numeric conversions are possible in Java, but of course
information may be lost. Conversions in which loss of information is possible are done by means of casts. The syntax for casting is to give the target type in parentheses, followed by the variable name. For example:
double x = 9.997; int nx = (int) x;
Now, the variable nx has the value 9 because casting a floating-point value to an integer discards the fractional part.
If you want to round a floating-point number to the nearest integer (which in most cases is the more useful operation), use the Math.round method:
double x = 9.997; int nx = (int) Math.round(x);
Now the variable nx has the value 10. You still need to use the cast (int) when you call round. The reason is that the return value of the round method is a long, and a long can only be assigned to an int with an explicit cast because there is the possibility of information loss.
Caution
If you try to cast a number of one type to another that is out of the range for the target type, the result will be a truncated number that has a different value. For example, (byte) 300 is actually 44.
C++ Note
You cannot cast between boolean values and any numeric type. This convention prevents common errors. In the rare case when you want to convert a boolean value to a number, you can use a conditional expression such as b ? 1 : 0.
3.5.7. Parentheses and Operator Hierarchy
Table 3.4 on the following page shows the precedence of operators. If no parentheses are used, operations are performed in the hierarchical order indicated. Operators on the same level are processed from left to right, except for those that are right-associative, as indicated in the table. For example, && has a higher precedence than ||, so the expression
a && b || c
means
(a && b) || c
Since += associates right to left, the expression
a += b += c
means
a += (b += c)
That is, the value of b += c (which is the value of b after the addition) is added to a.
C++ Note
Unlike C or C++, Java does not have a comma operator（Java中没有逗号运算符）. However, you can use a comma-separated list of expressions in the first and third slot of a for statement.
3.5.8. Enumerated Types
Sometimes, a variable should only hold a restricted set of values. For example, you may sell clothes or pizza in four sizes: small, medium, large, and extra large. Of course, you could encode these sizes as integers 1, 2, 3, 4, or characters S, M, L, and X. But that is an errorprone setup. It is too easy for a variable to hold a wrong value (such as 0 or m).
You can define your own enumerated type whenever such a situation arises. An enumerated type has a finite number of named values. For example:
enum Size { SMALL, MEDIUM, LARGE, EXTRA_LARGE };
Now you can declare variables of this type:
Size s = Size.MEDIUM;
A variable of type Size can hold only one of the values listed in the type declaration, or the special value null that indicates that the variable is not set to any value at all.
We discuss enumerated types in greater detail in Chapter 5.