How do I declare and initialize an array in Java

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Type   variableName   new Type capacity    Type   variableName    comma-delimited values      Type variableName     new Type capacity     Type variableName      comma-delimited values     is also valid  but I prefer the brackets after the type  because it s easier to see that the variable s type is actually an array

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If you want to create arrays using reflections then you can do like this    int size   3   int   intArray    int    Array newInstance int class  size

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There are two types of array    One Dimensional Array  Syntax for default values   int   num   new int 5     Or  less preferred   int num     new int 5     Syntax with values given  variable field initialization    int   num    1 2 3 4 5     Or  less preferred   int num      1  2  3  4  5     Note  For convenience  int   num is preferable because it clearly tells that you are talking here about array  Otherwise no difference  Not at all   Multidimensional array  Declaration  int     num   new int 5  2     Or  int num       new int 5  2     Or  int   num     new int 5  2     Initialization   num 0  0  1   num 0  1  2   num 1  0  1   num 1  1  2   num 2  0  1   num 2  1  2   num 3  0  1   num 3  1  2   num 4  0  1   num 4  1  2    Or   int     num    1 2    1 2    1 2    1 2    1 2       Ragged Array  or Non-rectangular Array    int     num   new int 5      num 0    new int 1    num 1    new int 5    num 2    new int 2    num 3    new int 3     So here we are defining columns explicitly  Another Way     int     num    1    1 2    1 2 3 4 5    1 2    1 2 3       For Accessing   for  int i 0  i lt  num length   i           for  int j 0 j lt num i  length j            System out println num i  j        Alternatively   for  int   a   num      for  int i   a        System out println i           Ragged arrays are multidimensional arrays  For explanation see multidimensional array detail at the official java tutorials

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Declare Array  int   arr   Initialize Array  int   arr   new int 10   10 represents the number of elements allowed in the array  Declare Multidimensional Array  int     arr   Initialize Multidimensional Array  int     arr   new int 10  17   10 rows and 17 columns and 170 elements because 10 times 17 is 170   Initializing an array means specifying the size of it

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One another full example with a movies class  public class A        public static void main String   args             class Movie                String movieName              String genre              String movieType              String year              String ageRating              String rating               public Movie String    str                                this movieName   str 0                   this genre   str 1                   this movieType   str 2                   this year   str 3                   this ageRating   str 4                   this rating   str 5                                    String    movieDetailArr     quot Inception quot    quot Thriller quot    quot MovieType quot    quot 2010 quot    quot 13  quot    quot 10 10 quot             Movie mv   new Movie movieDetailArr            System out println  quot Movie Name   quot   mv movieName           System out println  quot Movie genre   quot   mv genre           System out println  quot Movie type   quot   mv movieType           System out println  quot Movie year   quot   mv year           System out println  quot Movie age    quot   mv ageRating           System out println  quot Movie  rating   quot   mv rating

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Array is a sequential list of items  int item   value   int    one dimensional array     value  value  value      value     int      two dimensional array         value  value  value     value        value  value  value     value                                       value  value  value     value        If it s an object  then it s the same concept  Object item   new Object     Object    one dimensional array     new Object    new Object       new Object       Object      two dimensional array         new Object    new Object       new Object          new Object    new Object       new Object                                                new Object    new Object       new Object          In case of objects  you need to either assign it to null to initialize them using new Type      classes like String and Integer are special cases that will be handled as following  String    a      hello    world        is equivalent to String    a     new String   h   e   l   l   o     new String   w   o   r   l   d        Integer    b     1234  5678       is equivalent to Integer    b     new Integer 1234   new Integer 5678       In general you can create arrays that s M dimensional  int          array         M times    brackets                  M times   bracket                this is array 0  0    0                               M times  0                   M times   bracket     It s worthy to note that creating an M dimensional array is expensive in terms of Space  Since when you create an M dimensional array with N on all the dimensions  The total size of the array is bigger than N M  since each array has a reference  and at the M-dimension there is an  M-1 -dimensional array of references  The total size is as following  Space   N M   N  M-1    N  M-2         N 0                                          array reference           actual data

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Alternatively      Either method works String arrayName     new String 10   String   arrayName   new String 10     That declares an array called arrayName of size 10  you have elements 0 through 9 to use

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An array can contain primitives data types as well as objects of a class depending on the definition of the array  In case of primitives data types  the actual values are stored in contiguous memory locations  In case of objects of a class  the actual objects are stored in the heap segment   One-Dimensional Arrays  The general form of a one-dimensional array declaration is type var-name    OR type   var-name   Instantiating an Array in Java var-name   new type  size    For example  int intArray        Declaring an array intArray   new int 20       Allocating memory to the array     The below line is equal to line1   line2 int   intArray   new int 20      Combining both statements in one int   intArray   new int    1  2  3  4  5  6  7  8  9  10        Accessing the elements of the specified array for  int i   0  i  lt  intArray length  i        System out println  quot Element at index  quot    i    quot    quot   intArray i     Ref  Arrays in Java

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I find it is helpful if you understand each part   Type   name   new Type 5     Type   is the type of the variable called name   name  is called the identifier    The literal  Type  is the base type  and the brackets mean this is the array type of that base  Array types are in turn types of their own  which allows you to make multidimensional arrays like Type      the array type of Type     The keyword new says to allocate memory for the new array   The number between the bracket says how large the new array will be and how much memory to allocate  For instance  if Java knows that the base type Type takes 32 bytes  and you want an array of size 5  it needs to internally allocate 32   5   160 bytes   You can also create arrays with the values already there  such as  int   name    1  2  3  4  5     which not only creates the empty space but fills it with those values  Java can tell that the primitives are integers and that there are 5 of them  so the size of the array can be determined implicitly

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There are various ways in which you can declare an array in Java   float floatArray       Initialize later int   integerArray   new int 10   String   array   new String     a    b      You can find more information in the Sun tutorial site and the JavaDoc

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You can either use array declaration or array literal  but only when you declare and affect the variable right away  array literals cannot be used for re-assigning an array     For primitive types    int   myIntArray   new int 3   int   myIntArray    1  2  3   int   myIntArray   new int   1  2  3       Since Java 8  Doc of IntStream  https   docs oracle com javase 8 docs api java util stream IntStream html  int    myIntArray   IntStream range 0  100  toArray       From 0 to 99 int    myIntArray   IntStream rangeClosed 0  100  toArray       From 0 to 100 int    myIntArray   IntStream of 12 25 36 85 28 96 47  toArray       The order is preserved  int    myIntArray   IntStream of 12 25 36 85 28 96 47  sorted   toArray       Sort    For classes  for example String  it s the same   String   myStringArray   new String 3   String   myStringArray     a    b    c    String   myStringArray   new String    a    b    c      The third way of initializing is useful when you declare the array first and then initialize it  The cast is necessary here   String   myStringArray  myStringArray   new String    a    b    c

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Also  in case you want something more dynamic there is the List interface   This will not perform as well  but is more flexible   List lt String gt  listOfString   new ArrayList lt String gt      listOfString add  foo    listOfString add  bar     String value   listOfString get 0   assertEquals  value   foo

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For creating arrays of class Objects you can use the java util ArrayList  to define an array   public ArrayList lt ClassName gt  arrayName  arrayName   new ArrayList lt ClassName gt       Assign values to the array   arrayName add new ClassName class parameters go here     Read from the array   ClassName variableName   arrayName get index     Note   variableName is a reference to the array meaning that manipulating variableName will manipulate arrayName  for loops     repeats for every value in the array for  ClassName variableName   arrayName       Note that using this for loop prevents you from editing arrayName   for loop that allows you to edit arrayName  conventional for loop    for  int i   0  i  lt  arrayName size    i           manipulate array here

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An array has two basic types   Static Array  Fixed size array  its size should be declared at the start and can not be changed later    Dynamic Array  No size limit is considered for this   Pure dynamic arrays do not exist in Java  Instead  List is most encouraged    To declare a static array of Integer  string  float  etc   use the below declaration and initialization statements   x000D   x000D  int   intArray   new int 10   String   intArray   new int 10   float   intArray   new int 10       Here you have 10 index starting from 0 to 9 x000D   x000D   x000D   To use dynamic features  you have to use List    List is pure dynamic Array and there is no need to declare size at beginning  Below is the proper way to declare a list in Java -  x000D   x000D  ArrayList lt String gt  myArray   new ArrayList lt String gt     myArray add  Value 1  something    myArray add  Value 2  something more    x000D   x000D   x000D

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package com examplehub basics   import java util Arrays   public class Array        public static void main String   args            int   numbers    1  2  3  4  5                          numbers 0    1            numbers 1    2            numbers 2    3            numbers 3    4            numbers 4    5                     System out println  quot numbers 0     quot    numbers 0            System out println  quot numbers 1     quot    numbers 1            System out println  quot numbers 2     quot    numbers 2            System out println  quot numbers 3     quot    numbers 3            System out println  quot numbers 4     quot    numbers 4                           Array index is out of bounds                       System out println numbers -1              System out println numbers 5                            numbers 0    1            numbers 1    2            numbers 2    3            numbers 3    4            numbers 4    5                     for  int i   0  i  lt  5  i                  System out println  quot numbers  quot    i    quot      quot    numbers i                                     Length of numbers   5                     System out println  quot length of numbers    quot    numbers length                          numbers 0    1            numbers 1    2            numbers 2    3            numbers 3    4            numbers 4    5                     for  int i   0  i  lt  numbers length  i                  System out println  quot numbers  quot    i    quot      quot    numbers i                                     numbers 4    5            numbers 3    4            numbers 2    3            numbers 1    2            numbers 0    1                     for  int i   numbers length - 1  i  gt   0  i--                System out println  quot numbers  quot    i    quot      quot    numbers i                                     12345                     for  int number   numbers                System out print number                     System out println                            1  2  3  4  5                      System out println Arrays toString numbers               String   company     quot Google quot    quot Facebook quot    quot Amazon quot    quot Microsoft quot                           company 0    Google            company 1    Facebook            company 2    Amazon            company 3    Microsoft                     for  int i   0  i  lt  company length  i                  System out println  quot company  quot    i    quot      quot    company i                                     Google            Facebook            Amazon            Microsoft                     for  String c   company                System out println c                                     Google  Facebook  Amazon  Microsoft                      System out println Arrays toString company             int     twoDimensionalNumbers                      1  2  3                    4  5  6  7                    8  9                    10  11  12  13  14  15                                    total rows    4                     System out println  quot total rows     quot    twoDimensionalNumbers length                          row 0 length   3            row 1 length   4            row 2 length   2            row 3 length   6                     for  int i   0  i  lt  twoDimensionalNumbers length  i                  System out println  quot row  quot    i    quot  length    quot    twoDimensionalNumbers i  length                                    row 0   1 2 3            row 1   4 5 6 7            row 2   8 9            row 3   10 11 12 13 14 15                     for  int i   0  i  lt  twoDimensionalNumbers length  i                  System out print  quot row  quot    i    quot     quot                for  int j   0  j  lt  twoDimensionalNumbers i  length  j                      System out print twoDimensionalNumbers i  j     quot   quot                              System out println                                     row 0    1  2  3             row 1    4  5  6  7             row 2    8  9             row 3    10  11  12  13  14  15                      for  int i   0  i  lt  twoDimensionalNumbers length  i                  System out println  quot row  quot    i    quot     quot    Arrays toString twoDimensionalNumbers i                                      1 2 3            4 5 6 7            8 9            10 11 12 13 14 15                     for  int   ints   twoDimensionalNumbers                for  int num   ints                    System out print num    quot   quot                              System out println                                      1  2  3              4  5  6  7              8  9              10  11  12  13  14  15                      for  int   ints   twoDimensionalNumbers                System out println Arrays toString ints                        int length   5          int   array   new int length           for  int i   0  i  lt  5  i                  array i    i   1                                    1  2  3  4  5                      System out println Arrays toString array              Source from examplehub java

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It s very easy to declare and initialize an array  For example  you want to save five integer elements which are 1  2  3  4  and 5 in an array  You can do it in the following way   a   int   a   new int 5     or  b   int   a    1  2  3  4  5     so the basic pattern is for initialization and declaration by method a  is   datatype   arrayname   new datatype requiredarraysize     datatype should be in lower case   So the basic pattern is for initialization and declaration by method a is   If it s a string array   String   a     as    asd    ssd      If it s a character array   char   a     a    s    w      For float double  the format of array will be same as integer   For example   double   a    1 2  1 3  12 3     but when you declare and initialize the array by  method a  you will have to enter the values manually or by loop or something   But when you do it by  method b  you will not have to enter the values manually

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Another way to declare and initialize ArrayList   private List lt String gt  list   new ArrayList lt String gt          add  e1        add  e2

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Declaring an array of object references   class Animal     class Horse extends Animal       public static void main String   args                           Array of Animal can hold Animal and Horse  all subtypes of Animal allowed                      Animal   a1   new Animal 10           a1 0    new Animal            a1 1    new Horse                           Array of Animal can hold Animal and Horse and all subtype of Horse                     Animal   a2   new Horse 10           a2 0    new Animal            a2 1    new Horse                           Array of Horse can hold only Horse and its subtype  if any  and not            allowed supertype of Horse nor other subtype of Animal                      Horse   h1   new Horse 10           h1 0    new Animal       Not allowed         h1 1    new Horse                           This can not be declared                      Horse   h2   new Animal 10      Not allowed

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There are two main ways to make an array   This one  for an empty array   int   array   new int n       n  being the number of spaces to allocate in the array   And this one  for an initialized array   int   array    1 2 3 4         You can also make multidimensional arrays  like this   int     array2d   new int x  y       x  and  y  specify the dimensions int     array2d      1 2 3        4 5 6

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In Java 8 you can use something like this  String   strs   IntStream range 0  15      15 is the size      mapToObj i - gt  Integer toString i        toArray String    new

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If by  quot array quot  you meant using java util Arrays  you can do it like that   List lt String gt  number   Arrays asList  quot 1 quot    quot 2 quot    quot 3 quot     Out    quot 1 quot    quot 2 quot    quot 3 quot    This one is pretty simple and straightforward

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With local variable type inference you only have to specify the type once   var values   new int     1  2  3      Or  int   values     1  2  3

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In Java 9  Using different IntStream iterate and IntStream takeWhile methods   int   a   IntStream iterate 10  x - gt  x  lt   100  x - gt  x   10  toArray     Out   10  20  30  40  50  60  70  80  90  100    int   b   IntStream iterate 0  x - gt  x   1  takeWhile x - gt  x  lt  10  toArray     Out   0  1  2  3  4  5  6  7  8  9    In Java 10  Using the Local Variable Type Inference   var letters   new String    A    B    C

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There are a lot of answers here  I am adding a few tricky ways to create arrays  from an exam point of view it s good to know this    Declare and define an array  int intArray     new int 3     This will create an array of length 3  As it holds a primitive type  int  all values are set to 0 by default  For example   intArray 2      Will return 0  Using box brackets    before the variable name  int   intArray   new int 3   intArray 0    1      Array content is now  1  0  0   Initialise and provide data to the array  int   intArray   new int   1  2  3     This time there isn t any need to mention the size in the box bracket  Even a simple variant of this is   int   intArray    1  2  3  4    An array of length 0  int   intArray   new int 0   int length   intArray length     Will return length 0   Similar for multi-dimensional arrays  int intArray       new int 2  3      This will create an array of length 2 and   each element contains another array of length 3        0 0 0   0 0 0    int lenght1   intArray length     Will return 2 int length2   intArray 0  length     Will return 3    Using box brackets before the variable       int     intArray   new int 2  3     It s absolutely fine if you put one box bracket at the end       int   intArray      new int 2  4       int   intArray       new int 2  3  4    Some examples      int    intArray      new int       1 2 3   4 5 6        int    intArray1      new int      new int    1 2 3   new int     4 5 6        int    intArray2      new int      new int    1 2 3   4 5 6          All the 3 arrays assignments are valid        Array looks like   1 2 3   4 5 6     It s not mandatory that each inner element is of the same size       int      intArray   new int 2         intArray 0     1 2 3       intArray 1     4 5         array looks like   1 2 3   4 5        int     intArray   new int   2       This won t compile  Keep this in mind    You have to make sure if you are using the above syntax  that the forward direction you have to specify the values in box brackets  Else it won t compile  Some examples       int        intArray   new int 1           int        intArray   new int 1  2         int        intArray   new int 1  2  3     Another important feature is covariant      Number   numArray    1 2 3 4        java lang Number     numArray 0    new Float 1 5f        java lang Float     numArray 1    new Integer 1         java lang Integer       You can store a subclass object in an array that is declared       to be of the type of its superclass        Here  Number  is the superclass for both Float and Integer      Number num     new Float 5      This is also valid   IMPORTANT  For referenced types  the default value stored in the array is null

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Declare and initialize for Java 8 and later  Create a simple integer array   int    a1   IntStream range 1  20  toArray    System out println Arrays toString a1       Output   1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19    Create a random array for integers between  -50  50  and for doubles  0  1E17    int    a2   new Random   ints 15  -50  50  toArray    double    a3   new Random   doubles 5  0  1e17  toArray      Power-of-two sequence   double    a4   LongStream range 0  7  mapToDouble i - gt  Math pow 2  i   toArray    System out println Arrays toString a4       Output   1 0  2 0  4 0  8 0  16 0  32 0  64 0    For String   you must specify a constructor   String    a5   Stream generate   - gt  I will not squeak chalk   limit 5  toArray String    new   System out println Arrays toString a5      Multidimensional arrays   String      a6   List of new String    a    b    c     new String    d    e    f    g         toArray new String 0      System out println Arrays deepToString a6       Output    a  b  c    d  e  f  g

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int   SingleDimensionalArray   new int 2   int     MultiDimensionalArray   new int 3  4

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Take the primitive type int for example  There are several ways to declare and int array   int   i   new int capacity   int   i   new int    value1  value2  value3  etc   int   i    value1  value2  value3  etc     where in all of these  you can use int i   instead of int   i   With reflection  you can use  Type    Array newInstance Type class  capacity    Note that in method parameters      indicates variable arguments  Essentially  any number of parameters is fine  It s easier to explain with code   public static void varargs int fixed1  String fixed2  int    varargs            varargs 0      100      fixed1   0  fixed2       varargs    100  varargs 0      100  200      fixed1   0  fixed2       varargs    100  200     Inside the method  varargs is treated as a normal int    Type    can only be used in method parameters  so int    i   new int      will not compile   Note that when passing an int   to a method  or any other Type     you cannot use the third way  In the statement int   i     a  b  c  d  etc    the compiler assumes that the       means an int    But that is because you are declaring a variable  When passing an array to a method  the declaration must either be new Type capacity  or new Type           Multidimensional Arrays  Multidimensional arrays are much harder to deal with  Essentially  a 2D array is an array of arrays  int     means an array of int  s  The key is that if an int     is declared as int x  y   the maximum index is i x-1  y-1   Essentially  a rectangular int 3  5  is    0  0   1  0   2  0   0  1   1  1   2  1   0  2   1  2   2  2   0  3   1  3   2  3   0  4   1  4   2  4

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The following shows the declaration of an array  but the array is not initialized    int   myIntArray   new int 3     The following shows the declaration as well as initialization of the array   int   myIntArray    1 2 3     Now  the following also shows the declaration as well as initialization of the array   int   myIntArray   new int   1 2 3     But this third one shows the property of anonymous array-object creation which is pointed by a reference variable  myIntArray   so if we write just  new int   1 2 3    then this is how anonymous array-object can be created   If we just write   int   myIntArray    this is not declaration of array  but the following statement makes the above declaration complete   myIntArray new int 3

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int   x   new int enter the size of array here    Example  int   x   new int 10                   Or int   x    enter the elements of array here    Example  int   x    10  65  40  5  48  31

[java] How do I declare and initialize an array in Java?

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