Arrays in Java

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What are Java Array?

Array is an
Indexed Collection of
Fixed Number of
Homogeneous/Similar Data Elements.
int[ ] x = new int[5];

Example:



Valid Array Examples:

int[ ] arr1=new int[5];
int arr2[ ]=new int[5];
int [ ]arr3=new int[5];

int[ ] arr4 = new int[ ]{10,20,30,40,50};
int[ ] arr5 = {10,20,30,40,50};

Arrays can be initialized when they are declared. The array will automatically be created large enough to hold the number of elements you specify in the array initializer. There is no need to use new.

public static void main(String args[]) {
   int month_days[ ];
    month_days = new int[12];
    month_days[0] = 31;
    month_days[1] = 28;
    month_days[2] = 31;
    month_days[3] = 30;
    month_days[4] = 31;
    month_days[5] = 30;
    month_days[6] = 31;
    month_days[8] = 30;
    month_days[9] = 31;
    month_days[10] = 30;
    month_days[11] = 31;
     System.out.println("April has " + month_days[3] + " days.");
     }
}

Advantage of Java Array
1. Code Optimization: It makes the code optimized, we can retrieve or sort the data easily.
2. Random access: We can get any data located at any index position.

Disadvantage of Java Array
1. Size is fixed(Static Array): We can store only fixed size of elements in the array. It doesn't grow its size at runtime. To solve this problem, collection(Dynamic Array) framework is used in java.
2. Similar Data Type: To solve this problem, we use Object Array.


class TestArray{ 
public static void main(String args[]){ 

//1. int array:
                                int a[ ]=new int[5];         //declaration and instantiation 
                                a[0]=10;              //initialization 
                                a[1]=20; 
                                a[2]=30; 
                                a[3]=40; 
                                a[4]=50; 
                                System.out.println(a.getClass().getName()); //Corresponding class name
                                System.out.println(a.length); //size or length of array

//print all the values of array: use for loop
                                for(int i=0;i<a.length;i++){//length is the property of array 
                                System.out.println(a[i]);
                                }
//2. double array:
                                double d[ ]=new double[3];
                                d[0]=10.12;
                                d[1]=11.22;
                                d[2]=45.86;
                               
//3. char array:
                                char c[ ]=new char[3];
                                c[0]='a';
                                c[1]='7';
                                c[2]='$';
                               
//4. boolean array:
                                boolean b[ ]=new boolean[2];
                                b[0]=true;
                                b[1]=false;
                               
//5. String array:
                                String s[ ]=new String[2];
                                s[0]="Hello";
                                s[1]="World";
                               
//6. Object array: (Object is a class) --> used to store different data types values.
                                Object ob[ ]=new Object[7];
                                ob[0]="Navdeep";
                                ob[1]=28;
                                ob[2]=17.23;
                                ob[3]="India";
                                ob[4]=true;
                                ob[5]='M';
                                ob[6]="01/01/2018";
}}

ARRAY Creation:

Every Array in Java is an Object only. Hence, we can create Arrays by using new operator.
Ex:
int[ ] a=new int[3];

For every Array type corresponding classes are available and these classes are part of Java Language and not available to the programmer level.
 Ex:
int[ ] a=new int[3];
System.out.println(.getClass().getName());  [I

Array Type
Corresponding Class Name
int[ ]
[I
int[ ][ ]
[[I
byte[ ]
[B
short[ ]
[S
long[ ]
[J
float[ ]
[F
double[ ]
[D
char[ ]
[C
boolean[ ]
[Z
String[ ]
[Ljava.lang.String;
String[ ][ ]
[[Ljava.lang.String;
Object[ ]
[Ljava.lang.Object;
Object[ ][ ]
[[Ljava.lang.Object;


Loopholes in Array Creation:
1) At the time of Array creation compulsory we should specify the size, otherwise we will get compile time error.
int[ ]  x = new  int[ ];      //Invalid
int[ ]  x = new  int[6];    //Valid

2) It is legal to have an array with size 0 in java.
int[ ]  x = new  int[0];

public static void main(String[ ] args)
Sopln(args.length); //Output: 0

3) If we are trying to specify array size with some Negative int value then we will get Runtime Exception saying “NegativeArraySizeException”
int[ ]  x = new  int[-6];

4) If we are trying to initialize more than array size then we will get Runtime Exception saying “ArrayIndexOutOfBoundException”
int[ ]  x = new  int[4];
x[4] = 200; //“ArrayIndexOutOfBoundException”

5) To specify Array size allowed Data types are:
byte, short, char and int.
If we are trying to specify any other types then we will get Compile Time Error.
int[ ]  x = new  int[10];
int[ ]  x = new  int[‘a’];
byte b = 20;
int[ ]  x = new  int[b];
short s = 40
int[ ]  x = new  int[s];

6) The maximum allowed Array size in java is 2147483647 which is the maximum value of int data type.
int[ ]  x = new  int[2147483647]; Valid {We get Runtime exception if sufficient heap memory not available}
int[ ]  x = new  int[2147483648]; {CE: int number too large}

Passing Java Array to a Method

We can also pass arrays to methods just as we can pass primitive type values to methods.
Example:- 
public class ArrayMethods{
public static void main(String args[])
     {
     int x[ ] = { 1, 2, 3 };
     data(x);
     }
public static void data(int y[ ])
     {
             System.out.println(y[0]);
             System.out.println(y[1]);
             System.out.println(y[2]);
     }
}


Multidimensional Array(arrays of arrays)



These are other Multidimensional arrays representation of other data types.
The main Advantage of this Multidimensional arrays is Memory Utilisation will be improved.

Array Declaration
1. One Dimensional Array Declaration
int[ ] x;   //This one is Recommended because variable is clearly separated from the data type.
int [ ]x; //Valid
int x[ ]; //Valid

Ø  At the time of Declaration we can’t specify the size, else we will get Compile Time Error.
int[6] x; //Invalid
int[ ] x; //Valid

2. Two Dimensional Array Declaration
int[ ][ ]  x;
int  [ ][ ]x;
int  x[ ][ ];
int[ ]  [ ]x;
int[ ]  x[ ];
int   [ ]x[ ];
All above are Valid Declaration.

Question: Which of the following are valid?
1. int[ ] a,b;
2. int[ ] a[ ],b;
3. int[ ] a[ ],b[ ];
4. int[ ] [ ]a,b;
5. int[ ] [ ]a,b[ ];
6. int[ ] [ ]a,[ ]b;

Ans: 1 to 5 are Valid, 6 is Invalid.
Explanation:-
1. a - 1, b - 1
2. a - 2, b - 1
3. a - 2, b - 2
4. a - 2, b - 2
5. a - 2, b - 3
6. Compile time Error

Conclusion:
1. If we want to dimension before the variable that facility is applicable only for 1st variable in a declaration.
2. If we are trying to apply for next variable or remaining variables we will get Compile time Error.

int[ ]  [ ]a, [ ]b, [ ]c;

Here only [ ]a is Valid
[ ]b, [ ]c is Invalid.

3. Three-Dimensional Array Declaration
int[ ][ ][ ]  a;
int  [ ][ ][ ]a;
int  a[ ][ ][ ] ;
int[ ]  [ ][ ]a;
int[ ]  a[ ][ ];
int[ ]  [ ]a[ ];
int[ ][ ]  [ ]a;
int[ ][ ]  a[ ];
int   [ ][ ]a[ ];
int  [ ]a[ ][ ];
All above are Valid Declaration.

Question: Which of the following array declarations are Invalid?
1. int[ ] a = new int[ ];
2. int[ ] a = new int[3];
3. int[ ][ ] a = new int[ ][ ];
4. int[ ][ ] a = new int[3][ ];
5. int[ ][ ] a = new int[ ][4];
6. int[ ][ ] a = new int[3][4];
7. int[ ][ ][ ] a = new int[3][4][5];
8. int[ ][ ][ ] a = new int[3][4][ ];
9. int[ ][ ][ ] a = new int[3][ ][5];
10. int[ ][ ][ ] a = new int[ ][4][5];
Answer: 1, 3, 5, 9, 10 are Invalid
At least base size we must specify.

Array Initialization
Once we create an Array every array element by default initialised with default values.
Example 1:
int[ ] x = new int[3];
Sopln(x);             Output: [I@3e25a5
Sopln(x[0]);       Output: 0

Note: Whenever we are trying to print any reference variable, internally 2 string method will be called which is implemented by default to return the string in the following form “classname@hashcode_in_hexadecimalform”.

Example 2:
int[ ][ ] x = new int[2][3];
Sopln(x);             Output: [[I@3e25a5
Sopln(x[0]);       Output: [I@19821f
Sopln(x[0][0]); Output: 0

Example 3:
int[ ][ ] x = new int[2][ ];

Sopln(x);             Output: [[I@3e25a5
Sopln(x[0]);       Output: null
Sopln(x[0][0]); Output: Runtime Exception: NullPointerException

Note: If we are trying to perform any operation on null then we will get runtime exception saying “NullPointerException”.

Once we create an Array, every array element by default initialised with default values. If we are not satisfied with default values then we can override these values with our customised values.
int[ ] x = new int[6];
x[0] = 10;
x[1] = 20;
x[2] = 30;
x[3] = 40;
x[4] = 50;
x[5] = 60;

Invalid scenarios for above example:

x[6] = 70;  {Runtime Exception: ArrayIndexOutofBoundsException;}
x[-6] = 80;  {Runtime Exception: ArrayIndexOutofBoundsException;}
x[2.5] = 90;  {CE: Possible Loss of Precision, Found: Double; Required: int}

Note: If we are trying to access Array element with out of range index (Either positive value or Negative int value) then we will get runtime exception saying “ArrayIndexOutofBoundsException”.


Array Declaration, Creation and Initialisation in a single line
We can Declare, Create and Initialise an Array in a single line (Shortcut representation).
int[ ] x;
x = new int[3];
x[0] = 10;
x[1] = 20;
x[2] = 30;
OR
int[ ] x = {10, 20, 30};

other data types as well can be represented in a single like
char[ ] ch = {‘a’, ‘e’, ‘i’};
string[ ] s = {“nick”, “nav”, “java”};

We can use this shortcut representation for multidimensional arrays also.
int[ ][ ] x = {{10, 20}, {30, 40, 50}};

Example:
int[ ][ ][ ] x = {{{10, 20, 30}, {40, 50, 60}},{{70, 80}, {90, 100, 110}}};
1. Sopln(x[0][1][2]);                                 Output: 60
2. Sopln(x[1][0][1]);                                 Output: 80
3. Sopln(x[2][0][0]);                                 Output: RE: ArrayIndexOutOfBoundException
4. Sopln(x[1][2][0]);                                 Output: RE: ArrayIndexOutOfBoundException
5. Sopln(x[1][1][1]);                                 Output: 100
6. Sopln(x[2][1][0]);                                 Output: RE: ArrayIndexOutOfBoundException

Conclusion:
If we want to use this shortcut, compulsory we should perform all activities in a single line.
If we are trying to divide into multiple lines then we will get Compile Time Error.
int[ ] x = {10, 20, 30};

int[ ] x;
x= {10, 20, 30};                                            Compile Time Error: Illegal start of expression.


length vs length()
length:
length is a final variable applicable for arrays.
length variable represents the size of the array.
Example:
int[ ] x = new int[6];
Sopln(x.length());    Output: CE: Can’t find symbol, symbol: method length(), location: class int[ ]
Sopln(x.length);       Output: 6
Length():
length() method is a final method applicable for string objects.
length() method returns number of characters present in the string.
string s = “noddy”
Sopln(s.length);   Output: CE: Can’t find symbol, symbol: variable length, location: class j.l.String
Sopln(s.length());  Output: 5

NOTE:
Ø  length variable applicable for arrays but not for string objects.
Ø  length() method applicable for string objects but not for arrays.

Question: String[ ] s = {“A”, “XX”, “ABC”};
1. Sopln(s.length);     Output: 3
2. Sopln(s.length());   Output: CE: Can’t find symbol, Symbol: method length(), location: class String[ ]
3. Sopln(s[0].length);   Output: CE: Can’t find symbol, Symbol: variable length, location: class j.l.String
4. Sopln(s[0].length());  Output: 1


Note: In multidimensional array length variable represents only base size but not total size.
int[ ][ ] x = new int[6][3];
Sopln(x.length); Output: 6                     //row
Sopln(x[0].length); Output: 3               //column

Note: There is no direct way to find total length of multidimensional array but indirectly we can find as follows
X[0].length + X[1].length + X[2].length + X[3].length …….

Anonymous Array
Sometimes we can declare an array without a name, such type of nameless arrays is called anonymous arrays.
The main purpose of anonymous arrays is just for instant use. (One-time usage)
We can create anonymous array as follows:
new int[ ] {10, 20, 30, 40}
While creating anonymous array we can’t specify the size. Otherwise we will get compile time error
new int[3] {10, 20, 30, 40}                     //Invalid
new int[ ] {10, 20, 30, 40}                      //Valid

We can create multidimensional anonymous array also.
new int[ ][ ] {{10, 20}, {30, 40, 50}}

Based on our requirement we can give the name for anonymous array then it is no longer anonymous.
int[ ] x = new int[ ] {10, 20, 30};


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