12. Array Basic Facts

The fundamentals:

• Create (make space for) an array:

  int[] data = new int[50];
  String[] names = { "Justyn", "Connor", "Leo", "Ellie" };

• Arrays in Java do not grow or shrink. There is no += for arrays.

• The length of the data array is data.length.

• Arrays without new have no space created for them.

  int[] nums;
  int len = nums.length; // error, array is not made at all!!
  nums[0] = -1; // error, certainly no item 0

• Getting values from an array:

  int first = data[0];
  int last = data[data.length - 1];

• Setting values in an array:

  data[0] = 10;
  data[1] = 50;

• Printing out an array.

  • At start of program:

    import java.util.*;

  • When you want to print an array, use Arrays.toString:

    int[] e = {1,2,3};
    String ee = Arrays.toString(e);
    System.out.println(ee);

• Miscellaneous

  • Arrays are handled like Java Objects in memory. The name of the array is a reference to a block of memory where the array data is stored.

  • You cannot use the {10,30,90} form of the initializer anywhere except the declaration of the object.

    int[] nums = {10,30,90}; // OK
    int[] bad;
    bad = {10,30,90}; // ERROR

  • In that case, you need to make a new (unnamed) array before you assign it:

    int[] decent;
    decent = new int[]{10,30,90};