More on Arrays
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Passing arrays to or from methods
Arrays of objects
Command line arguments
Variable length parameter lists
Two dimensional arrays
Reading for this lecture: L&L 7.3 – 7.7
1
Arrays as Parameters to Methods
• An entire array can be passed as a parameter to
a method
• Like any other object, a reference to the array is
passed, making the formal and actual parameters
aliases of each other
• Therefore, changing an array element within the
method changes that array element in the original
• An individual array element can be passed to a
method as well, in which case the type of the
formal parameter is the same as the element type
2
Arrays as Return Values from Methods
• An entire array can be returned by a method as
its return value
• Like any other object, a reference to the array is
passed
• If the array is declared inside the method as a
local variable, then the returned reference is the
only remaining reference to the array object
• An individual array element can be returned by a
method as well, in which case the type of the
returned value is the same as the element type
3
Arrays of Objects
• The elements of an array can be object references
• The following declaration reserves space to store 5
references to String objects
String[] words = new String[5];
• It does NOT create the String objects themselves
• Initially an array of objects holds null references
• Each object stored in an element of an array must
be instantiated separately
4
Arrays of Objects
• The words array when initially declared:
words
-
• A reference to words.length is OK (= 5)
• However, the following reference will throw a
NullPointerException:
System.out.println(words[0].length());
5
Arrays of Objects
• To create some String objects and store
them in elements of the array:
words[0] = new String(“friendship”);
words[1] = “loyalty”;
words[2] = “honor”;
“friendship”
words
“loyalty”
“honor”
6
Arrays of Objects
• String objects can be created using literals
• The following declaration creates an array
object called verbs with a length of 4 and
fills it with references to four String objects
created using string literals
String[] verbs = {"play", "work", "eat", "sleep"};
7
Arrays of Objects
• To use one of the methods of an object element of an
array:
verbs[2].equals(“eat”);
// true
• To pass one of the object elements of an array as a
parameter to a method:
“eat”.equals(verbs[2]);
// true
• To return an element of an array:
public String methodName(String [] verbs)
{
return verbs[2];
// “eat”
}
8
Command-Line Arguments
• Your program’s main method is defined as:
public static void main(String [] args)
• The signature of the main method indicates that it
takes an array of String objects as a parameter
• These values come from command-line arguments
that are provided when the interpreter is invoked
• In Dr Java interactions pane, this invocation of the
JVM passes three String objects as arguments to
the main method of StateEval:
> java StateEval pennsylvania texas arizona
9
Command Line Arguments
• These strings are stored at indexes 0-2
in the array args for the main method
• The array args will contain:
args[0]  “pennsylvania”
args[1]  “texas”
args[2]  “arizona”
• Code in main can print the arguments:
for (String arg : args)
System.out.println(arg);
10
Variable Length Parameter Lists
• Suppose we want to create a method to processes
a different amount of data from one invocation to
the next
• For example, let's define a method called average
that returns the average of a variable number of
integer parameters
// one call to average three values
mean1 = average (42, 69, 37);
// another call to average seven values
mean2 = average (35, 43, 93, 23, 40, 21, 75);
11
Variable Length Parameter Lists
• Using special syntax in a formal parameter
list, we can define a method to accept any
number of parameters of the same type
Indicates a variable length parameter list
public double average (int ... list)
{
// see next slide
element
}
array
type
name
• The parameters are automatically put into
an array for easy processing in the method
12
Variable Length Parameter Lists
public double average (int ... list)
{
double result = 0.0;
if (list.length != 0)
{
int sum = 0;
for (int num : list)
sum += num;
result = (double)sum / list.length;
}
return result;
}
13
Variable Length Parameter Lists
• A method can accept individual parameters
and a varying number of parameters
• The individual parameters must come first
in the formal arguments
• The varying number of parameters must
come last in the formal arguments
• A single method can accept only one set of
varying parameters
14
Two-Dimensional Arrays
• A one-dimensional array stores a list of elements
• A two-dimensional array can be thought of as a
table of elements, with rows and columns
one
dimension
two
dimensions
15
Two-Dimensional Arrays
• A two-dimensional array is declared specifying the
size of each dimension separately:
int[][] scores = new int[12][50];
• An array element is referenced using two indexes:
int value = scores[3][6]
• Each dimension subdivides the previous one into
the specified number of elements
• Each dimension has its own length constant
scores.length == 12
// 12 arrays of 50
scores[anyInteger0Through11].length == 50
16
Two-Dimensional Arrays
• A two-dimensional array is an array of arrays
• The array stored in one row can be specified
using one index:
int [] scoreRow;
scoreRow = scores[1];
// 50 int’s
• scoreRow gets initialized from scores[1]
scoreRow.length == 50
// for all n >= 0 && n < 50
scoreRow[n] == scores[1][n]
17