Java
provides a data structure, the array, which stores a fixed-size sequential collection of elements of
the same type.
An
array is used to store a collection of data, but it is often more useful to
think of an array as a collection of variables
of
the same type.
Instead
of declaring individual variables, such as number0, number1, ..., and number99,
you declare one array variable
such
as numbers and use numbers[0], numbers[1], and ..., numbers[99] to represent
individual variables.
This
tutorial introduces how to declare array variables, create arrays, and process
arrays using indexed variables.
Declaring Array Variables:
To
use an array in a program, you must declare a variable to reference the array,
and you must specify the type of array
the
variable can reference. Here is the syntax for declaring an array variable:
dataType[] arrayRefVar; // preferred way.
or
dataType
arrayRefVar[]; // works but not preferred way.
Note: The style dataType[] arrayRefVar is preferred. The style dataType arrayRefVar[] comes from the C/C++
language
and was adopted in Java to accommodate C/C++ programmers.
Example:
The
following code snippets are examples of this syntax:
double[] myList; // preferred way.
or
double myList[]; // works
but not preferred way.
Creating Arrays:
You
can create an array by using the new operator with the following syntax:
arrayRefVar
= new dataType[arraySize];
The
above statement does two things:
It
creates an array using new dataType[arraySize];
It
assigns the reference of the newly created array to the variable arrayRefVar.
Declaring
an array variable, creating an array, and assigning the reference of the array
to the variable can be combined
in
one statement, as shown below:
dataType[] arrayRefVar = new dataType[arraySize];
Alternatively
you can create arrays as follows:
dataType[] arrayRefVar = {value0, value1, ..., valuek};
The
array elements are accessed through the index. Array indices are 0-based; that is, they start from 0 to
arrayRefVar.length-1.
Example:
Following
statement declares an array variable, myList, creates an array of 10 elements
of double type, and assigns its
reference
to myList.:
double[] myList = new double[10];
Following
picture represents array myList. Here myList holds ten double values and the
indices are from 0 to 9.
Processing Arrays:
When
processing array elements, we often use either for loop or foreach loop because
all of the elements in an array are
of
the same type and the size of the array is known.
Example:
Here
is a complete example of showing how to create, initialize and process arrays:
public
class TestArray {
public
static void main(String[] args) {
double[] myList = {1.9, 2.9, 3.4, 3.5};
// Print
all the array elements
for (int i = 0; i < myList.length; i++) {
System.out.println(myList[i] + " ");
}
//
Summing all elements
double total = 0;
for (int i = 0; i < myList.length; i++) {
total += myList[i];
}
System.out.println("Total
is " + total);
//
Finding the largest element
double max = myList[0];
for (int i = 1; i < myList.length; i++) {
if (myList[i] > max) max = myList[i];
}
System.out.println("Max
is " + max);
}
}
This
would produce following result:
1.9
2.9
3.4
3.5
Total is 11.7
Max is 3.5
The foreach Loops:
JDK
1.5 introduced a new for loop, known as foreach loop or enhanced for loop,
which enables you to traverse the
complete
array sequentially without using an index variable.
Example:
The
following code displays all the elements in the array myList:
public
class TestArray {
public
static void main(String[] args) {
double[] myList = {1.9, 2.9, 3.4, 3.5};
// Print
all the array elements
for (double element: myList) {
System.out.println(element);
}
}
}
This
would produce following result:
1.9
2.9
3.4
3.5
Passing Arrays to Methods:
Just
as you can pass primitive type values to methods, you can also pass arrays to
methods. For example, the following
method
displays the elements in an int array:
public
static void printArray(int[] array) {
for (int i = 0; i < array.length; i++) {
System.out.print(array[i] + " ");
}
}
You
can invoke it by passing an array. For example, the following statement invokes
the printArray method to display
3,
1, 2, 6, 4, and 2:
printArray(new int[]{3, 1, 2, 6, 4, 2});
Returning an Array from a Method:
A
method may also return an array. For example, the method shown below returns an
array that is the reversal of
another
array:
public
static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1; i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
The Arrays Class:
The
java.util.Arrays class contains various static methods for sorting and
searching arrays, comparing arrays, and filling
array
elements. These methods are overloaded for all primitive types.
SN
Methods with Description
1 public static int binarySearch(Object[] a, Object key)
Searches
the specified array of Object ( Byte, Int , double etc) for the specified value
using the binary search
algorithm.
The array must be sorted prior to making this call. This returns index of the
search key, if it is
contained
in the list; otherwise, (-(insertion point + 1).
2 public static boolean equals(long[] a, long[] a2)
Returns
true if the two specified arrays of longs are equal to one another. Two arrays
are considered equal if
both
arrays contain the same number of elements, and all corresponding pairs of
elements in the two arrays are
equal.
This returns true if the two arrays are equal. Same method could be used by all
other premitive data
types
( Byte, short, Int etc.)
3 public static void fill(int[] a, int val)
Assigns
the specified int value to each element of the specified array of ints. Same
method could be used by
all
other premitive data types ( Byte, short, Int etc.)
4 public static void sort(Object[] a)
Sorts
the specified array of objects into ascending order, according to the natural
ordering of its elements.
Same method could be used by all other
premitive data types ( Byte, short, Int etc.)
0 comments:
Post a Comment