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The tables below group the Java operators according to similar
actions. For newcomers to Java, you can just scan these tables for
now, especially for operations dealing with class and objects, and
then refer back to them as you encounter more of the language.
Most of the operators are fairly straightforward but there can
be subtle details in their implementation that you must understand.
For example, you must know the difference in the pre-
and post-increment operations with regard to when the operation
executes versus when the value of the operand value returns.
We will return to many of these operators as the course proceeds.
General Properties
of Operators
In Java, an expression
carries out some operation or operations that are directed by
a list of allowed operators. An operator acts upon one, two or
three operands. Here are some general properties of operators:
Operands
An operand can be:
-
a numeric variable - integer,
floating point or character
-
any primitive type variable -
numeric and boolean
-
reference variable to an object
-
a literal - numeric value, boolean
value, or string.
-
an array element, "a[2]"
-
char
primitive, which in numeric operations is treated as an unsigned
two byte integer
The operator is unary if it acts on a single operand;
binary if it requres two operands. The conditional
operator is the only ternary operator in Java.
Each operator places specific requirements on the operand types
allowed. For example, the subtractive operator "- "
in x=a-b; requires
that a and b variables be numeric types. The assignment
operator "=" then requires that x also be a numeric
type. (If a and b were wider types than x, a casting
operation would also be required.)
Returned Value
A value is "returned" at the completion of an operation.
The following statements use the assignment
operator "=" and the addition
operator "+"
and result in x holding the value 3 and y holding the value
8. The entire expression y= x+5 could be used in another expression:
int
x=3;
int y;
int z = (y=x+5) * 4;
which results in y holding 8 and z holding 32. The assignment
operator "=" in the expression " y=x+5"
produces a new value for y and also returns the value of
y to be used in the expression for z.
Effects on Operands
In most of the operations, the operands themselves are not changed.
However, for some operators, the operand(s) do undergo a change:
-
Assignment
operators: " x=y"
replaces the value of the first operand with that of the second.
The other assignment operators, " *=,
+=, etc" will also replace the value of the first
operand but only after using its initial value in the particular
operation indicated by the symbol before the =
sign.
- Increment &
decrement operators: The operand is incremented
or decremented, before or after the operand's value is returned,
depending on whether it is a pre- or post- increment of decrement.
If an operand is changed by the operation, note
that if the statement holding that expression is processed again,
e.g. in a loop, the resulting value can be different for each
pass.
Expression
Evaluation
The operands of an operator are always evaluated
left to right. For example, in
x
= a + b;
the first "+"
operator will determine the value of a
and then b.
Do not get this rule confused with the precedence
and associativity rules.
For example, in the expression
x
= a + b * c;
the first "+"
operator still first determines its left operand ("a"
in this case) and then its right operand. But in this case the
right operand consists of the expression "b*c".
The multiplication operator "*"
has a higher precedence than the additive "+".
Precedence can be overridden with parentheses, e.g.
x
= (a + b) * c;
will force the addition of b
to a, and then
this sum is multiplied by c.
Although the precedence ratings, which are similar
to those in C/C++, were chosen for the most "natural"
ordering of the operator evaluations, it never hurts to use the
parentheses if you are unsure of the precedence and don't have
the table handy.
When the operations in an expression all have the
same precedence rating, the associativity rules determine
the order of the operations. For most operators, the evaluation
is done left to right, e.g.
x
= a + b - c;
Here, addition and subtraction have the same precedence
rating and so a
and b are added
and then from this sum c
is subtracted. Again, parentheses can be used to overrule the
default associativity, e.g.
x
= a + (b - c);
However, the assignment and unary operators, are
associated right to left, e.g.
x
+= y -= -~4;
is equivalent to
x
+= (y -= (-(~4)));
or in long hand,
int
a = ~4;
a = -a;
y = y - a;
x = x + y;
Other
Operator Tricks
Finally, here are some other miscellaneous notes
about operators. As indicated in the last example above, assignment
operations can be chained:
x
= y = z = 4;
with the evaluations proceeding right to left.
Assignments can be combined into other operations,
to compact the code, e.g.
if(
(x = 5) == b) y = 10;
which first sets x
to 5, then returns that value for the test of b.
You should not overuse this technique else it makes the code unreadable.
Occasionally, though, it can be a neat approach.
Tables
of Java Operators
|
Assignment
Operators
x
operation= y
is equivalent to
x
= x operation y
x and y must be numeric or
char types except for "=", which allows x and y also to
be object references. In this case, x must be of the same
type of class or interface as y. If mixed floating-point
and integer types, the rules for mixed types in expressions
apply.
|
|
=
|
Assignment
operator.
x
= y;
y is evaluated and x set to this value.
The value of x is then returned. |
|
+=,
-=, *=, /=, %=
|
Arithmetic operation and then assignment,
e.g.
x
+= y;
is equivalent to
x
= x + y;
|
|
&=, |=, ^= |
Bitwise
operation and then assignment, e.g.
x
&= y;
is equivalent to
x
= x & y; |
| <<=,
>>=, >>>= |
Shift
operations and then assignment, e.g.
x
<<= n;
is equivalent to
x
= x << n; |
Arithmetic
Operators
x and y are numeric or
char types. If mixed floating-point and integer types,
then floating-point arithmetic used and a floating-point
value returned. If mixed integer types, the wider type
is returned. If double and float mixed, double is returned.
|
|
x
+ y
|
Addition |
|
x - y |
Subtraction |
| x
* y |
Multiplication |
| x
/ y |
Division
If FP arithmetic and y = 0.0, then infinity returned if x
is not zero, NaN if x is zero.
ArthmeticException thrown if x & y are integer types and
y is zero. |
| x
% y |
Modulo
- remainder of x/y returned.
If FP arithmetic and y = 0.0 or infinity,
then NaN returned
ArthmeticException thrown if x & y are integer types and
y is zero. |
| -x
|
Unary
minus
Negation of x value |
Increment
& Decrement Operators
x and y are numeric (FP & integer)
or char types. |
|
x++
|
Post-increment : add 1 to the value.
The value is returned before the increment is made,
e.g.
x
= 1;
y = x++;
Then y will hold 1 and x will hold 2
|
|
x-- |
Post-decrement : subtract 1 from the value.
The value is returned before the decrement is made,
e.g. :
x
= 1;
y = x--;
Then y will hold 1 and x will
hold 0.
|
| ++x |
Pre-increment : add 1 to the value.
The value is returned after the increment is made,
e.g.
x
= 1;
y = ++x;
Then y will hold 2 and x will
hold 2.
|
| --x |
Pre-decrement : subtract 1 from the value.
The value is returned after the decrement is made,
e.g.
x
= 1;
y = --x;
Then y will hold 0 and x will
hold 0.
|
|
Boolean
Operators
x and y are boolean types.
x and y can be expressions that result in a boolean value.
Result is a boolean true
or false
value.
|
| x
&& y |
Conditional
AND |
If both
x and y are true, result is true.
If either x or y are false, the result is false
If x is false, y is not evaluated. |
| x
& y |
Boolean
AND |
If both
x and y are true,the result is true.
If either x or y are false, the result is false
Both x and y are evaluated before the test. |
| x
|| y |
Conditional
OR |
If either
x or y are true, the result is true.
If x is true, y is not evaluated. |
| x
| y |
Boolean
OR |
If either
x or y are true, the result is true.
Both x & y are evaluated before the test. |
| !x
|
Boolean
NOT |
If x
is true, the result is false.
If x is false, the result is true. |
| x
^ y |
Boolean
XOR |
If x
is true and y is false, the result is true.
If x is false and y is true, the result is true.
Otherwise, the result is false.
Both x and y are evaluated before the test. |
|
Comparison
Operators
x and y are numeric
or char types only except for "==" and "!="
operators, which can also compare references. If mixed types,
then the narrower type converted to wider type. Returned
value is boolean true
or false.
|
|
x <
y
|
Is x
less than y ? |
| x
<= y |
Is x
less than or equal to y ? |
| x
> y |
Is
x greater than y ? |
| x
>= y |
Is x
greater than or equal to y ? |
| x
== y |
Is x
equal to y ? |
| x
!= y |
Is x
not equal to y ? |
|
Bitwise
Operators
x and y are integers.
If mixed integer types, such as int
and long,
the result will be of the wider type.
Note: Operations on byte
and short types may give unexpected
results since operands are promoted to integers during intermediate
operations. For example,
byte x = (byte)0xFF;
x >>>= 1;
will result in 0xFF in x rather
than 0x7F. That is because the
operation is carried out on a signed integer rather than
simply on 8 bits. Here the signed byte is promoted to the
signed integer 0xFFFFFFFF.
Use of an integer would go as follows:
int i = 0xFF;
i >>>= 1;
This results in 0x7F
in the variable i.
|
|
~x
|
Compliment |
Flip each bit, ones
to zeros, zeros to ones |
| x
& y |
AND
|
AND
each bit a with corresponding bit in b |
| x
| y |
OR |
OR each
bit in a with corresponding bit in b |
| x
^ y |
XOR |
XOR
each bit in x with corresponding bit in y |
| x
<< y |
Shift
left |
Shift
x to the left by y bits. High order bits lost.
Zero bits fill in right bits. |
| x
>> y |
Shift
Right -
Signed |
Shift
x to the right by y bits. Low order bits lost.
Same bit value as sign (0 for positive numbers, 1 for negative)
fills in the left bits. |
| x
>>> y |
Shift
Right -
Unsigned |
Shift
x to the right by y bits. Low order bits lost.
Zeros fill in left bits regardless of sign. |
| Class
and Object Operators |
| x
instanceof c |
Class
Test Operator |
The first operand must be an object reference.
c is the name of a class or interface.
If x is an instance of type c or a subclass of c, then true
returned.
If x is an instance of interface type c or a sub-interface,
then true
is returned.
Otherwise, false
is returned.
|
|
new
c(args)
|
Class
Instantiation |
Create
an instance of class c using constructor c(args) |
|
"."
|
Class
Member Access |
Access
a method or field of a class or object :
o.f
- field access for object o
o.m()
- method access for object o |
|
()
|
Method
Invocation |
Parentheses
after a method name invokes
(i.e. calls) the code for the method, e.g.
o.m()
o.m(x,y) |
|
(c)
|
Object
Cast |
Treat
an object as the type of class or interface c:
c
x=(c)y;
Treat y as an instance of class or interface c |
| + |
String
Concatenation |
This binary operator will concatenate one
string to another. E.g.
String
str1 = "abc";
String str2 = "def";
String str3 = str1 + str2
results in str3
holding "abcdef".
For mixed operands, if either a or b in
(a + b) is a string, concatenation to a string will occur.
Primitives will be converted to strings and the toString()
methods of objects will be called.
(This is the only case of operator overloading
in Java.)
Note that the equivalence operator "+="
will also perform string concatenation.
|
| [] |
Array
Element Access |
In Java, arrays are classes. However, the
bracket operators work essentially the same as in the C/C++.
To access a given element of an array, place
the number of the element as an int
value (long
values cannot be used in Java arrays) into the brackets,
e.g.
float
a = b[3];
int n = 5;
char c=c[n];
where b
is a float
array and c
is a char array.
|
| Other
Operators |
| x=boolean?y:x |
Conditional
Operator |
The
first operand - boolean - is a boolean variable or
expression.
First this boolean operand is evaluated. If it is true
then the second operator evaluated and x is set to that value.
If the boolean operator is false,
then the third operand is evaluated and x is set to that value. |
| (primitive
type) |
Type
Cast |
To assign a value of one primitive numeric
type a more narrow type, e.g. long to int, an explicit cast
operation is required, e.g.
long
a = 5;
int
b = (int)a;
|
|
Operator
Precedence
The larger the number,
the higher the precedence.
|
| 1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
|
=
*=
/=
%=
+=
-=
<<=
>>=
>>>=
&=
^=
|=
|
?: |
|| |
&& |
| |
^ |
&
|
==
!=
|
<
<=
>
>= |
<<
>>
>>> |
+
- |
*
/
% |
new
(type) |
++x
--x
+x
-x
~
! |
.
[]
(args)
x++
x-- |
Notes:
- (type)
refers to the casting operator
- "." is the object member access
operator
- []
is the array access operator
- (args)
indicates the invocation of a method.
- In column 11, the + and - refer to binary
addition and subtraction. Also, the + refers tothe string
append operator. Whereas in column 14, the + & - refer
to the unary operations +x
and -x
specify the sign of the value.
- |, ^,
and &
refer to both the bitwise and boolean operators.
|
|
Operator
Associativity
The following operators
have Right to Left associativity. All other operators
(see precedence table above) are
evaluated left to right.
|
|
=
*=
/=
%=
+=
-=
<<=
>>=
>>>=
&=
^=
|=
|
?:
new
(type cast)
++x
--x
+x
-x
~
! |
References & Web Resources
Latest update: May 22, 2008
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