More on Arrays

1
More on Arrays

• Passing arrays to or from methods
• Arrays of objects
• Command line arguments
• Variable length parameter lists
• Two dimensional arrays
• Reading for this lecture LL 7.3 7.7

2
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

3
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

4
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 String5
• 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

5
Arrays of Objects

• The words array when initially declared

• A reference to words.length is OK ( 5)
• However, the following reference will throw a
  NullPointerException
• System.out.println(words0.length())

6
Arrays of Objects

• To create some String objects and store them in
  elements of the array

words0 new String(friendship) words1
loyalty words2 honor
7
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"
8
Arrays of Objects

• To use one of the methods of an object element of
  an array
• verbs2.equals(eat) // true
• To pass one of the object elements of an array as
  a parameter to a method
• eat.equals(verbs2) // true
• To return an element of an array
• public String methodName(String verbs)
• return verbs2 // eat

9
Command-Line Arguments

• Your programs 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
• gt java StateEval pennsylvania texas arizona

10
Command Line Arguments

• These strings are stored at indexes 0-2 in the
  array args for the main method
• The array args will contain
• args0 ? pennsylvania
• args1 ? texas
• args2 ? arizona
• Code in main can print the arguments
• for (String arg args)
• System.out.println(arg)

11
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)
12
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
• The parameters are automatically put into an
  array for easy processing in the method

public double average (int ... list) // see
next slide
13
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
14
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

15
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

16
Two-Dimensional Arrays

• A two-dimensional array is declared specifying
  the size of each dimension separately
• int scores new int1250
• An array element is referenced using two indexes
• int value scores36
• 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
• scoresanyInteger0Through11.length 50

17
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 scores1 // 50 ints
• scoreRow gets initialized from scores1
• scoreRow.length 50
• // for all n gt 0 n lt 50
• scoreRown scores1n