chap 2 : Operators and Assignments (scjp/ocjp)

Operators and
Assignments

Ben Abdallah Helmi Architect J2EE

Evaluation Order
1. int [] a = { 4, 4 };
2. int b = 1;
3. a[b] = b = 0;
the value 0 is first assigned to the variable called
b then the value 0 is assigned into the last
element of the array a .
.

Overview of the Java Operators


The Unary Operators
The increment and decrement operators: ++ and -The unary plus and minus operators: + and -

The bitwise inversion operator: ~
The boolean complement operator: !

The cast: ()

The Increment and Decrement Operators: ++ and --


The Unary Plus and Minus Operators: + and 1. x = -3;
2. y = +3;
3. z = -(y + 6);
the value of z is initialized to −9.


The Bitwise Inversion Operator: ~
For example, applying this operator to a byte
containing 00001111 would result in the value
11110000. The same simple logic applies, no
matter how many bits there are in the value
being operated on. This operator is often used
in conjunction with shift operators
(<<, >>, and >>>) to perform bit
manipulation, for example, when driving I/O
ports.

The Boolean Complement Operator: !
1. Vector v = new Vector();
2. v.add ("Hello");
3. String s = (String)v.get(0);


The Arithmetic Operators


The Multiplication and Division
Operators: * and /
1. int a = 12345, b = 234567, c, d;
2. long e, f;
3.
4. c = a * b / b; // this should equal a, that is, 12345
5. d = a / b * b; // this should also equal a
6. System.out.println("a is " + a +
7. "nb is " + b +
8. "nc is " + c +
9. "nd is " + d);
10.
11. e = (long)a * b / b;
12. f = (long)a / b * b;
13. System.out.println(
14. "ne is " + e +
15. "nf is " + f);
The output from this code is
a is 12345
b is 234567
c is -5965
d is 0
e is 12345
f is 0

The Modulo Operator: %


The Addition and Subtraction
Operators: + and -

To prepare for the Certification Exam questions, and to use the + operator effectively in your
own programs, you should understand the following points:
•

•

•
•

For a + expression with two operands of primitive numeric type, the result
o Is of a primitive numeric type.
o Is at least int, because of normal promotions.
o Is of a type at least as wide as the wider of the two operands.
o Has a value calculated by promoting the operands to the result type and then
performing the addition using that type. This might result in overflow or loss of
precision.
For a + expression with any operand that is not of primitive numeric type,
o At least one operand must be a String object or literal; otherwise, the expression is
illegal.
o Any remaining non-String operands are converted to String, and the result of the
expression is the concatenation of all operands.
To convert an operand of some object type to a String, the conversion is performed by
invoking the toString() method of that object.
To convert an operand of a primitive type to a String, the conversion is performed by a static
method in a container class, such as Integer.toString(). J2EE
Ben Abdallah Helmi Architect

Arithmetic Error Conditions
In exceptional conditions, the following rules apply:

•

Integer division by zero, including modulo (%) operation, results in an ArithmeticException.

•

No other arithmetic causes any exception. Instead, the operation proceeds to a result, even
though that result might be arithmetically incorrect.

•

Floating-point calculations represent out-of-range values using the IEEE 754 infinity, minus
infinity, and Not a Number (NaN) values. Named constants representing these are declared
in both the Float and Double classes.

•

Integer calculations, other than division by zero, that cause overflow or a similar error
simply leave the final, typically truncated bit pattern in the result. This bit pattern is derived
from the operation and the number representation and might even be of the wrong sign.
Because the operations and number representations do not depend upon the
platform, neither do the result values under error conditions.

These rules describe the effect of error conditions, but some additional significance is associated
with the NaN values. NaN values are used to indicate that a calculation has no result in
ordinary arithmetic, such as some calculations Architect J2EE infinity or the square root of a
Ben Abdallah Helmi involving
negative number.

Arithmetic Promotion of Operands

The diagram in Figure 2.2 shows the process by which a byte is shifted right. First the
byte is promoted to an int, which is done treating the byte as a signed quantity.
Next, the shift occurs, and 0 bits are indeed propagated into the top bits of the
result—but these bits are not part of the original byte. When the result is cast
down to a byte again, the high-order bits of that byte appear to have been created
by a signed shift right, rather than an unsigned one. This is why you should
generally not use the logical right-shift operator with operands smaller than an int:
It is unlikely to produce the result you expected.

The Ordinal Comparisons Operators:
<, <=, >, and >=
int p = 9;
int q = 65;
int r = -12;
float f = 9.0F;
char c = ‘A’;
the following tests all return true:
p<q
f<q
f <= c
c>r
c >= q

Notice that arithmetic promotions are applied when these operators are used.
This is entirely according to the normal rules discussed in Chapter 4. For
example, although it would be an error to attempt to assign, say, the float
value 9.0F to the char variable c, it is perfectly acceptable to compare the two.
To achieve the result, Java promotes the smaller type to the larger type; hence
the char value ‘A’ (represented by the Unicode value 65) is promoted to a float
65.0F. The comparison is then performed on the resulting float values.

The instanceof Operator
The instanceof operator tests the class of an object at runtime. The left argument can be any
object reference expression, usually a variable or an array element, whereas the right operand
must be a class, interface, or array type. You cannot use a java.lang.Class object reference
or a String representing the name of the class as the right operand. A compiler error results if
the left operand cannot be cast to the right operand.

If the left argument of the instanceof operator is a null value, the instanceof test simply returns
false; it does not cause an exception.

The Equality Comparison Operators:
== and !=
The operators == and != test for equality and
inequality, respectively, returning a boolean value. For primitive
types, the concept of equality is quite straightforward and is subject to
promotion rules so that, for example, a float value of 10.0 is considered
equal to a byte value of 10. For variables of object type, the “value” is
taken as the reference to the object—typically, the memory address.
You should not use these operators to compare the contents of
objects, such as strings, because they will return true if two references
refer to the same object, rather than if the two objects have equivalent
value. Object comparisons compare the data of two objects, whereas
reference comparisons compare the memory locations of two objects.

To achieve a content or semantic comparison, for example, so that two
different Double objects containing the value 1234 are considered
equal, you must use the equals() method rather than the == or !=
operator.

The Bitwise Operators
The bitwise operations calculate each bit of their
results by comparing the corresponding bits of
the two operands on the basis of these three
rules:
• For AND operations, 1 AND 1 produces 1. Any
other combination produces 0.
• For XOR operations, 1 XOR 0 produces 1, as does
0 XOR 1. (All these operations are commutative.)
Any other combination produces 0.
• For OR operations, 0 OR 0 produces 0. Any other
combination produces 1.

Boolean Operations
• For AND operations, true AND true produces
true. Any other combination produces false.
• For XOR operations, true XOR false produces
true, and false XOR true produces true.Other
combinations produce false.
• For OR operations, false OR false produces
false. Any other combination produces true.


The Short-Circuit Logical Operators

The main difference between the & and && and between the | and || operators is that the right operand might not be
evaluated in the latter cases. We will look at how this happens in the rest of this section. This behavior is based on two
mathematical rules that define conditions under which the result of a boolean AND or OR operation is entirely determine
by one operand without regard for the value of the other:
•
For an AND operation, if one operand is false, the result is false, without regard to the other
operand.
•
For an OR operation, if one operand is true, the result is true, without regard to the other
operand.
To put it another way, for any boolean value X,
•
false AND X = false
•
true OR X = true
1. if (s != null) {
2. if (s.length() > 20) {
3. System.out.println(s);
4. }
5. }
However, the same operation can be coded very succinctly like this:
1. if ((s != null) && (s.length() > 20)) {
2. System.out.println(s);
3. }

//first example:
1. int val = (int)(2 * Math.random());
2. boolean test = (val == 0) || (++val == 2);
3. System.out.println("test = " + test + "nval = " + val);
//second example:
1. int val = (int)(2 * Math.random());
2. boolean test = (val == 0) | (++val == 2);
3. System.out.println("test = " + test + "nval = " + val);
The first example will sometimes print
test = true
val = 0
and sometimes it will print
test = true
val = 2
The second example will sometimes print
test = true
val = 1
and sometimes it will print
test = true
val = 2

The Conditional Operator
The conditional operator—?:— (also known as a ternary operator, because it takes three operands) provides a way to code
simple conditions (if/else) into a single expression. The (boolean) expression to the left of the ? is evaluated. If true, the
result of the whole expression is the value of the expression to the left of the colon; otherwise, it is the value of the
expression to the right of the colon. The expressions on either side of the colon must be assignment-compatible with
the result type.
For example, if a, b, and c are int variables, and x is a boolean, then the statement a = x
? b : c; is directly equivalent to the textually longer version:
1. if (x) {
2. a = b;
3. }
4. else {
5. a = c;
6. }
Of course x, a, b, and c can all be complex expressions if you desire.
Here are the points you should review for handling conditional operators in an exam question, or to use them properly in a
program. In an expression of the form a = x ? b : c;
•
The types of the expressions b and c should be compatible and are made identical through
conversion.
•
The type of the expression x should be boolean.
•
The types of the expressions b and c should be assignment-compatible with the type of a.
•
The value assigned to a will be b if x is true or will be c if x is false.
Now that we have discussed the conditional (ternary) operator, only one group of operators remains: the assignment
operators.

The Assignment Operators
1. byte x = 2;
2. x += 3;
If this had been written using the longhand approach
1. byte x = 2;
2. x = (byte)(x + 3);
The assignment operators in Java are considered to be operators
because they have a resulting value. So, given three int variables
a, b, and c, the statement a = b = c = 0; is entirely legal. It is
executed from right to left, so that first 0 is assigned into the
variable c. After it has been executed, the expression c = 0 takes
the value that was assigned to the left side—that is, 0. Next, the
assignment of b takes place, using the value of the expression to
the right of the equal sign—again, 0. Similarly, that expression
takes the value that was assigned, so finally the variable a is also
set to 0.

chap 2 : Operators and Assignments (scjp/ocjp)

More Related Content

What's hot (19)

Similar to chap 2 : Operators and Assignments (scjp/ocjp) (20)

More from It Academy (20)

Recently uploaded (20)

chap 2 : Operators and Assignments (scjp/ocjp)