Language Fundamentals - Arrays

copy from http://www.janeg.ca/scjp/lang/arrays.html

 

Array declarations

• arrays are Java objects
• all Java arrays are technically one-dimensional. Two-dimensional arrays are arrays of arrays.
• declaring an array does not create an array object or allocate space in memory; it creates a variable with a reference to an array
• array variable declarations must indicate a dimension by using []

  Examples of valid array declarations: (JJ pg84)
  
  
      String[]s;
      String []s;
      String [] s;
      String [ ] s;       // extra white space ignored
      String[] s;
      String[   ] s;      // extra white space ignored
      String s[];
      String s [];
      String s [   ];     // extra white space ignored
          
      String[] s[];
      String[][]s;
      String s [] [  ];   // extra white space ignored
  

• declaring the size of the array with the following notation is illegal

      String[5] s;        // illegal declaration
  
  
  

• the standard convention for declaring arrays is:

      String[] s;         // one-dimensional array
      String[][] s;       // two-dimensional array
  

Initializing arrays

• all arrays are zero-based
• arrays must be indexed by int values or byte, short or char values (as these can be promoted to int) (JLS §10.4)
• using a long index value to access an array causes a compile error
• attempting to access an array with an index less than 0 or greater than the length of the array causes an ArrayIndexOutOfBoundsException to be thrown at runtime (JLS §10.4)
• since arrays are Objects they can be initialized using the new operator
• when created, arrays are automatically initialized with the default value of their type

  String[] s = new String[100];   // default values: null
  boolean[] b = new boolean[4];   // default values: false
  int[] i = new int[10][10];      // default values: 0
  

• array references declared as members are initialized to null BUT array references declared in methods are not initialized

class TestArray  {
    int[] arr;          // member declaration, initialized to 'null'

    public static void main(String[] args) {
        int[] arr1;     // reference variable 'arr1' not initialized
        
        // compiles ok
        System.out.println("arr:" + new TestArray().arr); 
        // compile error

        System.out.println("arr1: " + arr1);
    }
}

• as arrays are allocated at runtime, you can use a variable to set their dimension

      int arrSize = 100;
      String[] myArray = new String[arrSize];
  

• you can use curly braces {} as part of an array declaration to initialize the array
  

      String[] oneDimArray = { "abc","def","xyz" };
  

 

Note

Curly braces {} can only be used in array declaration statements. String[] s; // illegal initialization s = { "abc", "def", "hij"); int[] arr = new int[] {1,2,3}; // legal

• you can assign an array a null value but you can't create an empty array by using a blank index

    
      int[] array = null;             // legal
      // illegal initialization
  
  
      int[] array = new int[];        
  

Initializing two-dimensional arrays

• the first dimension represents the rows, the second dimension, the columns
• curly braces {} may also be used to initialize two dimensional arrays. Again they are only valid in array declaration statements.

      int[][] twoDimArray = { {1,2,3}, {4,5,6}, {7,8,9} };
  

• you can initialize the row dimension without initializing the columns but not vice versa

      int[][] myArray = new int[5][];
      // illegal
  
      int[][] myArray = new int[][5];
   
  

• the length of the columns can vary

  
   class TestTwoDimArrays {
      // initialize # of rows
      static int [][] myArray = new int[3][]; 
  
      public static void main(String[] args) {
  
      myArray[0] = new int[3];   // initialize # of cols
      myArray[1] = new int[4];   // in each row
      myArray[2] = new int[5];
  
      for(int i=0; i<3; i++)    // fill and print the array
          fillArray(i, i+3);
  
      System.out.println();
      } // end main()
  
      private static void fillArray(int row, int col) {
  
          for( int i=0; i<col; i++)
              myArray[row][i] = i;
  
          for( int i=0; i<col; i++)
              System.out.print(myArray[row][i]);
  
          System.out.println();
      }
  
  }
  
  Output of TestTwoDimArrays:
  
      012
      0123
      01234    
  
      
  

Tips

• array index operator [] has highest level of precedence
• integer variables can be used as array dimension values

Traps

• incorrect array declaration statements, particularly:
     arrayType [#] varName;
• incorrect array initialization statements, particularly:
    arrayType[] varName = new arrayType[2];
    varName = { value, value, value };
• negative values for array index
• long value for array index
• array declaration used as an array creation statement