Arrays and Strings

1
Arrays and Strings

• CSCI 392
• Adapted from Dannelly

2
Declaring an Array Variable

• Two versions of the same declaration
• int intlist
• int intlist
• Declaring a two-dimensional array
• int twodimarray
• int twodimarray

3
Creating Space for an Array

• Given the declaration
• int myarray
• You must also tell the VM to create space
• myarray new int100
• You can do both steps in one line
• int myarray new int100
• You can also wait until runtime to set the array
  size
• int runtimeSize 100
• int myarray new intruntimeSize

4
Initializing Array Values

• Version One
• int one_to_four new int 1,2,3,4
• Version Two
• int one_to_four 1,2,3,4
• An Array of Strings
• String names "Bob","Joe","Sue","Fred"
• Computing Initial Values at Runtime
• int examscores gradeExam(1),gradeExam(2)

5
Using Arrays

• Array Variables are Objects. So, they have their
  own class methods and instance properties. For
  example
• myarray.length
• Arrays.sort(myarray)
• Array indexes must be of type int.
• Using an index out of bounds throws
  ArrayIndexOutOfBoundsException

6
Copying Arrays

• copy method prior to JDK 1.6
• System.arraycopy (original, 0,
• copy, 0,
• original.length)
• Copy method JDK 1.6 or later
• intArray2 Arrays.copyOf (intArray,
  intArray.length)

7
import java.util.Arrays public class Array_copy
public static void main(String args) int
intArray 1,2,3 int intArray2
4,5,6 int count 0 while (count ! 3)
System.out.println ("intArray "
intArraycount) count count 1
count 0 while (count ! 3)
System.out.println ("intArray2 "
intArray2count) count count 1
count 0 intArray2 Arrays.copyOf
(intArray, intArray.length) while (count !
3) System.out.println ("intArray2 "
intArray2count) count count 1

intArray 1 intArray 2 intArray 3 intArray2
4 intArray2 5 intArray2 6 intArray2 1 intArray2
2 intArray2 3
8
Array Methods

• import java.util.Arrays
• // compare the contents of two arrays
• if (Arrays.equals (array1, array2))
• System.out.println ("contents are the same")
• // quick sort an array
• Arrays.sort (myarray)
• // initialize the first ten positions with a 1
• Arrays.fill (myarray, 0, 9, 1)
• // fill entire array with value of 1
• Arrays.fill (myarray, 1)

9
char ! String

• public class samCopy2
• public static void main(String args)
• char copy1 't', 'e', 's','t'
• char copy2 'b', 'e', 's', 't'
• System.out.println ("copy1 "
  String.valueOf(copy1))
• System.out.println ("copy2 " copy2)

copy1 test copy2 C_at_8f4fb3
10
char ! String

• This code yields a compiler error.
• class string_test_2
• static void main(String args)
• String myvar "bob"
• myvar0 'B' // error myvar is not an
  array

11
String Methods

• Compare two strings
• String name1, name2
• ...
• if ( name1.compareTo(name2) lt 0 ) // like C
• Convert an integer into a string
• String mystring String.valueOf (myint)
• Convert a string into an integer
• int myint Integer.parseInt (mystring)
• Append
• name " "
• name first last

12
More String Methods

• Extract a Substring
• String s1 name.substring(0, 4)
• Remove spaces at the end
• name name.trim()
• Does the string contain a number
• String str1
• ...
• char c str1.charAt (0) // get first char
• if (Character.isDigit(c)) // test that char
• System.out.print ("str1 starts with a number")