Java Reference
In-Depth Information
Java syntax: Variable declaration
type variable ;
Java syntax: Assignment statement
variable = expression ;
Example : double price ;
Additionally: The type of the variable is the same as
or wider than the type of the expression .
Purpose : To declare that variable variable
is needed and that it can hold values of type
type . By variable , we mean the name of the
variable.
Example : price= 8.99 ;
Purpose : To assign a value to a variable. To execute
the assignment, evaluate the expression and store its
value in the variable .
variable before the assignment is performed.
// legal: double is wider than int
price= 4;
// illegal: int is narrower than double
quantity= 5.0;
Expressions with variables
We can write expressions not only with literals (for example, 4 , 6.2 , and
true ), but with variables. Here is one such expression: price + 4.0 . When an
expression with a variable is evaluated, the value currently associated with the
variable is used. For example, if quantity has the value 8 and price has the
value 3.99 , the value of the expression
quantity * price
is 31.92 . If the value of a variable changes, later evaluation of the same expres-
sion yields a different value.
Below, we show a sequence of assignments to int variables v1 and v2 . As
each assignment is executed, one after the other, the new values of the variables
are as shown in the comments to the right of the assignment. The values are the
numbers that appear after the colons.
v1= 5; // v1: 5 v2: ?
v2= 3; // v1: 5 v2: 3
v1= v1 + 1; // v1: 6 v2: 3
v1= v1 + 2 + v1; // v1: 14 v2: 3
v2= v1; // v1: 14 v2: 14
v1= v2 + v1 + 1; // v1: 29 v2: 14
In mathematics, short variable names like x and y are common. In comput-
er programs, longer names tend to be used in order to give some indication of
their meaning, like price and taxIsIncluded . There is a tension between using
short names, in order to keep a program compact, and long names, in order to
provide more understanding. Our conventions for names are discussed in Sec.
13.1, and we recommend that you read the beginning of that section now.
The following convention is followed by Java programmers: a variable
Lesson page
13-2 discusses
naming con-
ventions in
detail.
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