Java Reference
In-Depth Information
It starts the program with three strings:
First num, alpha
, and
53
. Since
First num
is a
string, it is enclosed in double quotes. Note that
53
is actually treated as a string. You can use
"53"
instead of
53
in the command line.
When the
main
method is invoked, the Java interpreter creates an array to hold the com-
mand-line arguments and pass the array reference to
args
. For example, if you invoke a
program with
n
arguments, the Java interpreter creates an array like this one:
args =
new
String[n];
The Java interpreter then passes
args
to invoke the
main
method.
Note
If you run the program with no strings passed, the array is created with
new String[0]
.
In this case, the array is empty with length
0
.
args
references to this empty array.
Therefore,
args
is not
null
, but
args.length
is
0
.
7.13.2 Case Study: Calculator
Suppose you are to develop a program that performs arithmetic operations on integers. The
program receives an expression in one string argument. The expression consists of an inte-
ger followed by an operator and another integer. For example, to add two integers, use this
command:
VideoNote
Command-line argument
java Calculator 2 + 3
The program will display the following output:
2 + 3 = 5
FigureĀ 7.12 shows sample runs of the program.
The strings passed to the main program are stored in
args
, which is an array of strings. The
first string is stored in
args[0]
, and
args.length
is the number of strings passed.
Here are the steps in the program:
1. Use
args.length
to determine whether the expression has been provided as three
arguments in the command line. If not, terminate the program using
System.exit(1)
.
2. Perform a binary arithmetic operation on the operands
args[0]
and
args[2]
using the
operator in
args[1]
.
Add
Subtract
Multiply
Divide
F
IGURE
7.12
The program takes three arguments (
operand1 operator operand2
) from
the command line and displays the expression and the result of the arithmetic operation.
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