Arrays in Java

Objectives

• use arrays in Java
• use multidimensional arrays

Arrays

• declare array: int[] num;
• create array: num = new int[15];
• combine declaration and creation: int[] num = new int[15];
• declare, create, and initialize: int[] num = { 5, 7, 1, 3, 1 };
• can make array of objects: String[] names = new String[8];
• getting the length of an array: num.length
• pass to methods by just using name: Arrays.sort(num);
• receive in a method using "datatype[] arrayname"
• arrays passed by reference, so methods can change contents of arrays passed to them
• you can create and initialize anonymous arrays (good for passing as arguments to methods): new String[]{"abc", "def"}
• typical loop through array: "for (int i=0; i<arrayname.length; i++)"
• can use new for loop to iterate through array: "for ( var : arrayname )"
• see numerous array examples on the CIS 160 demonstration programs page
• System.arraycopy(Object src, int srcPos, Object dest, int destPos, int length)
• Arrays class
  • binarySearch(arrayname, key)
  • equals(arrayname1, arrayname2)
  • fill(arrayname, value)
  • fill(arrayname, from, to, value)
  • sort(arrayname)
  • sort(arrayname, from, to)
  • toString(arrayname)
  • see http://www.leepoint.net/data/arrays/70sorting.html for nice notes on array sorting
  • see http://www.leepoint.net/data/collections/comparators.html for nice notes on Comparators used in array sorting
  • SortArray1.java: use Array class to sort an array of Integer objects
  • SortArray2.java: use Array class to sort an array of ints
  • SortArray3.java: use Array class and a custom Comparator to do a customized sort of an array of objects

Multidimensional arrays

• a two dimensional array in Java is basically an array of one dimensional arrays
• creating a variable that can reference a two dimensional array:
  int[][] arr;
• creating a two dimensional array:
  arr = new int[2][3];
• you can combine creating the variable with creating the array:
  int[][] arr = new int[2][3];
• you can combine initialization with creation:
  int[][] arr = { {1, 2, 3}, {4, 5, 6} };
  where arr[0][0] is 1, arr[0][1] is 2, ..., and arr[1][2] is 6
• since each row of a multidimensional array is an array, the arrays in different rows can be different sizes:
  int[][] arr = { {1, 2, 3}, {4, 5} };
• you can use each row of a multidimensional as a regular array (the following code treats the 4th row of arr[][] as a regular array:
  for (i=0; i<arr[3].length; i++)
• individual rows of an array can be created independently:
  int[][] c = new int[2];
  c[0] = new int[3];
  c[1] = new int[2];
• see MultiDim1.java: multidimensional array demonstration