array

Array

Introduction

• A fixed-length data structure that is used to hold more than one value of the same data type,
• All elements of an array are stored contiguously in memory,
  • In case of a reference type array, the array elements store the references of the objects,
  • Those references in the elements are stored contiguously, not the objects they are referring to.
  • The objects are stored on heap and their locations are, typically, not contiguous,
• All arrays in Java are objects but abstract i.e. can't create object directly,
• Since array are objects, so it can be assigned to other object. Rule is same,
• Arrays are created dynamically at runtime,
• The length of an array cannot be modified after it has been created,
• Ex:

  int[] arr = new int[5];

• Index is zero(0) based just like C or C++,
• Make sure to assign values before using,
• Ex:

  private static void printArray(int[] arr){
      for (int t : arr) {
          System.out.print(t + " ");
      }
      System.out.println("");
  }
  private static <T> void printArray(T[] arr){
      for (T t : arr) {
          System.out.print(t + " ");
      }
      System.out.println("");
  }
  private static void arrayTest(){
      int[] arr = new int[5];
      for(int i=0;i<arr.length;i++) arr[i] = i+5;
  
      Object obj = arr; // array are object. So assignment is possible
      printArray(arr); // 5 6 7 8 9
  
      int[] brr = (int[])obj;
      printArray(brr); // 5 6 7 8 9
  
      // default
      String[] names = new String[4];
      printArray(names); // null null null null
  
      names[0] = "Pantho";
      names[2] = "Aslam";
      printArray(names); // Pantho null Aslam null
  
      names[1] = "Maruf";
      names[3] = "Anik";
  
      printArray(names); // Pantho Maruf Aslam Anik
      for (int i=0;i<names.length; i++){
          names[i] = names[i].substring(0,1); // -------(a)
      }
      printArray(names); // P M A A
  }

  • If you don't assign Maruf and Anik, then program will crash at (a),

Initializing Array Elements

• Array elements are initialized to a default value no matter it is global or local,
• Array elements of primitive data type are initialized to the default value for their data types,
  • Numeric array elements are initialized to zero(0),
  • boolean elements to false,
• Reference type elements to null,
• Ex:

  String[] names = new String[4];
  printArray(names); // null null null null

• Explicit initializing:

  int[] rolls = new int[]{1,2,3,4,5}; // or
  int[] rolls2 = {1,2,3,4,5};

• You cannot specify the length of an array if you specify the array initialization list,
• The length of the array is the same as the number of values specified in the initialization list, i.e. 5 in above example,

Limitations of Arrays

• Cannot be expanded or shrunk after it is created,

Variable-Length Arrays

• ArrayList and Vector are two classes in the java.util package that can be used whenever variable-length arrays are needed,
• ArrayList and Vector classes work the same way except,
  • Methods in the Vector class are synchronized(Thread safe),
  • Whereas methods in ArrayList are not synchronized,
  • Vector is slower, ArrayList is faster,
• Use ArrayList,
• Ex:

  private static void arrayListTest(){
      ArrayList<Integer> arr = new ArrayList<>();
      arr.add(1);
      arr.add(2000);
      arr.add(3);
  
      for(int i=0; i<arr.size(); i++) System.out.print(arr.get(i)+" ");
      System.out.println("");
      // 1 2000 3
  
      System.out.println(arr.size()); // 3
  
      arr.add(20);
      arr.add(22);
      System.out.println(arr.size()); // 5
  }

Passing array as parameter

• Can be passed to method just like other variable,
• Alert when modifying array inside called method. Remember, array are object, so modifying inside method will also modify original array,
• Ex:

  private static void resizeValues(int[] arr){
      for(int i=0; i<arr.length; i++) {
          if(arr[i] > 20) arr[i] = 20;
      }
  }
  private static void passArray(){
      int[] arr = {1,2,23,24,25,3};
  
      printArray(arr); // 1 2 23 24 25 3
      resizeValues(arr);
      printArray(arr); // 1 2 20 20 20 3 
  }

• Make a copy then pass to method if you are going to modify inside method,
• Remember, main(String[]) method takes an array as parameter.

Multi-Dimensional Arrays

• Just like C and C++,

• Ex:

  private static void multiDimenArray(){
      int[][] table = new int[3][2];
  
      int[][] tbl = {
          {1,2},
          {3,4},
          {5,6}
      };
  
      for(int i=0;i<tbl.length; i++){
          for(int j=0; j<tbl[i].length; j++){
              System.out.print(tbl[i][j]+" ");
          }
          System.out.println("");
      }
  
      System.out.println("---");
      // better approach
      for(int[] row : tbl){
          for(int num : row){
              System.out.print(num+" ");
          }
          System.out.println("");
      }
  }

  Output

  1 2
  3 4
  5 6
  ---
  1 2
  3 4 
  5 6
  

• All operations are like normal array. Just don't be confused,

Converting an ArrayList to an Array

• Accessing array elements is faster than ArrayList,
• You may convert when you know size won't be changed anymore.
• Ex:

  private static void arrayListToArray(){
      System.out.println("------------------------------ arrayListToArray ---------------------");
      ArrayList<String> names = new ArrayList<>();
  
      names.add("John");
      names.add("Jane");
      names.add("Mary");
  
      Object[] arr = names.toArray(); // <--------- way-1
      printArray(arr); // John Jane Mary
  
      String[] brr = new String[names.size()];
      names.toArray(brr); // <------------ way-2
      printArray(brr); // John Jane Mary
  
  }