Array is one of the most important data structure in any programming
language, at same time different programming languages implement and treat
array differently. Any one who is done some programming knows value of array and
importance of knowing about array type, using them in there program. Together with linked list, array forms a set of basic data-structures. Though Java offers excellent Collection API
and some of collection classes like ArrayList and
HashMap , they are internally based on array. If you are coming from C or C++ background then you will find some difference about how array behaves there and how it does in Java, most notable difference between array in C and array in Java is bounds checking. C compiler doesn't check if program is accessing valid array index, while in Java, JVM does that and throws ArrayIndexOutOfBoundException, if program tries to access invalid array index. In this Java article, we will take a look on
array in Java, both primitive and object arrays. It's a collection of important
things about Java array and there properties.
Java Array
101
1) Array in Java is object and array instance is also created using new
operator. Array.length gives length of array.
for example for following java array:
int[] intArray
= new int[10];
System.out.println(intArray.length)
Output: 10
Array.length denotes it’s capacity, because
each index is initialized with default value, as soon as array is created.
2) Array index in Java starts with zero. Negative indexes are invalid in
Java. Java will throw ArrayIndexOutOfBoundException ,if you
try to access an Array with invalid index which could mean
negative index, index greater than or equal to length of array in Java.
3) Array are fixed length data
structure. Once declared, you can not change length of Array in Java.
4) Array are stored at contiguous memory location in Java Heap. So if you
are creating a large array it is possible that you might have enough space in
heap but still Java throw OutOfmemoryError
because of requested sized might not be available on contiguous memory
location.
5) Different type of Array in Java have different types representing them
for example in below example intArray.getClass() will be
different than floatArray.getclass()
int[] intArray = new int[10];
float[]
floatArray = new float[10];
6) You can not store a double value in an array of int, that would result
in compilation error.
int[] intArray
= new int[10];
intArray[5]=1.2;
//compilation error
if you try to do this operation in runtime, Java will throw ArrayStoreException
7) In Java Array can be created by different ways. here are few examples
of creating array in Java
int[]
intArray; //creating
array without initializing or specifying size
int
intArray1[]; //another
int[] reference variable can hold reference of an integer array
int[]
intArray2 = new int[10];
//creating array by specifying size
int[]
intArray3 = new int[]{1,2,3,4}; //creating and
initializing array in same line.
you have choice to initialize java array while creating it alternatively
you can initialize array later by using for
loop. If you have noticed brackets which is used to denote array can be placed
either side of array variable.
First approach is convenient for creating multiple arrays in Java e.g.
int[] array1, array2;
here both array1 and array2 are
integer array, while in second approach you need to place bracket twice like
int array1[], array2[];
though not much difference it just matter of style, I read int[] as int
array so first approach fits perfect there.
8) If not initialized explicitly array elements in are initialized with
default value of Type used to declare Java array. For example in case of an uninitialized integer array there element will
have default value 0, for uninitialized boolean array it would false and for an
Object array it would be null.
9) You can access element of Array by using [] operator. Since array
index starts at zero [0] returns first element and [length-1] returns
last element from array in Java. For loop is a convenient way to iterate over entire
array
in Java. You can use for loop to initialize entire array be accessing each
index or you can update/retrieve array elements. Java 5 also provides enhanced
for loop which will take care of array indexes by themselves and prevent ArrayIndexOutOfBoundException in Java. Here
is an example of iterating array in Java using for loop.
Traditional
approach
int[] numbers
= new int[]{10, 20,
30, 40,
50};
for (int i = 0;
i < numbers.length; i++) {
System.out.println("element at index " + i + ": " + numbers[i]);
}
Output:
element at index 0: 10
element at index 1: 20
element at index 2: 30
element at index 3: 40
element at index 4: 50
Enhanced
for loop approach
for(int i:
numbers){
System.out.println(i);
}
Output:
10
20
30
40
50
As you see in case case enhanced for loop we don't need to check for
indexes and its an excellent way if you want to access all element of array one
by one but at the same time since you don't have access to index you can modify
any element of Java array.
10) In Java you can easily convert
an array into ArrayList.
ArrayList is index based Java collection class which is backed up array.
advantage of ArrayList is that it can resize itself. resizing of ArrayList is
just creating a larger array and copying content to that array as you can not
change size of array in Java. check how to convert Array to ArrayList in Java
for more details.
11) Java API Also provide several convenient method to operate on Java
array via java.util.Arrays class. By using Arrays class you
can sort an array in Java and you can also perform binary search in Java.
12) java.lang.System class provides utility method for
copying elements of one array into another. System.arrayCopy is very
powerful and flexible method of copying contents from one array to another. You
can copy entire array or sub-array based on your need.
Syntax of System.arraycopy:
public static void
arraycopy(Object src, int srcPos, Object dest, int destPos, int
length)
As you see arraycopy() method allow us to specify indexes and length which gives
your flexibility to copy sub-array
and store it on desired location of destination or target array. here is
an example of copying first three elements of source array into target array in
Java:
public static void
main(String args[]) {
int[]
source = new int[]{10,
20, 30,
40, 50};
int[]
target = new int[5];
System.out.println("Before copying");
for(int i:
target){
System.out.println(i);
}
System.arraycopy(source,
0, target, 0, 3);
System.out.println("after copying");
for(int i:
target){
System.out.println(i);
}
}
Output:
Before copying
0
0
0
0
0
after copying
10
20
30
0
0
You can see before copying all elements of target array were zero(default
value of int variable) and after copying first 3 elements represent first 3 items
of source array.
13) Java also supports multi-dimensional arrays, which can be very useful
to represent 2D and 3D based data like rows and columns or matrix.
multi-dimensional array in Java are also referred as array of array because in
each index of first array another array is stored of specified length. here is
an example of creating multi-dimensional array in Java:
int[][]
multiArray = new int[2][3];
This array has two rows and 3 column or you can visualize it as array of
3 arrays with length 2. Here is how you can initialize multi-dimensional array
in Java:
int[][]
multiArray = {{1,2,3},{10,20,30}};
System.out.println(multiArray[0].length);
System.out.println(multiArray[1].length);
Alternatively you can also initialize multi-dimensional array
individually by using for loop or manually. e.g.
multiArray[0] = new int[]{40,50,60}; will replace value of multiArray[0].
14) Array is extremely fast data-structure and one should use it if you already know number of elements going to be stored.
That's all about Array in Java. As you see Java array are very powerful
way of storing elements and provides fastest access if you know the index. Though it also has limitation e.g. once created you can not change the size of array, but if you ever need a dynamic array, consider using java.util.ArrayList class. It provides dynamic re-size functionality and auto re-size itself. Its
also very easy to convert an Array into ArrayList and operate on that.
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