Review:    helper/support methods for array

  • Previously, we have studied the array data structure in Chapter 7 of the text book

  • When we discussed the array, we also learned about some support methods in the java.util.arrays class (), such as: 

      Arrays.sort( arrayName ): 
         sort an array

  Arrays.equals( array1, array2 ):
         check if 2 arrays contains the same elements

  Arrays.copyOf(arrayName, arrayName.length): 
         returns a copy of an array

  Arrays.toString(arrayName): 
         returns a string of the elements in the array

  and so on

  • Java's library also contains support methods for ArrayList -- discussed next

  Support methods for ArrayList  

  • The support methods for ArrayList are stored in the:

    • java.util.Collections class (online doc: )

  • We will discuss the following support methods for:

    • Convert between an array and an ArrayList:

      • Converting an array to an ArrayList

      • Converting an ArrayList to an array

    • Sort an ArrayList

    • Find the maximum and minimum value stored in an ArrayList

    • Shuffle the elements stored in an ArrayList

Converting an Array into an ArrayList     Array ⇒ ArrayList

  • To convert an array to an array list, you instantiate an array list using an array as follows: 

     String[] myArray = {"red", "green", "blue"};  // Array to be converted

 ArrayList<String> myList = new ArrayList<>( Arrays.asList(myArray) );

    Arrays.asList(myArray) returns a list of elements in myArray.

  • Alternately, we can use a loop and add the elements one at a time to the ArrayList: 

     String[] myArray = {"red", "green", "blue"};

 ArrayList<String> myList = new ArrayList<>(); // Create ArrayList
        
 // Add elements in myArray to myList
 for (int i = 0; i < myArray.length; i++) 
     myList.add( myArray[i] );

DEMO: demo/11-arrayList/05-support/ArrayToArrList.java

Converting an ArrayList into an Array     ArrayList ⇒ Array

  • To convert an array list to an array, you use the instance method toArray(myArray) that copies the elements in an ArrayList to the array myArray: 

      ArrayList<String> myList = ... ;    // ArrayList to be converted

  String[] myArray = new String[ myList.size() ]; // Create array

  myList.toArray( myArray ); // Copies elems in myList to myArray

  • Alternately, we can use a loop and copy the elements one at a time to the array: 

      ArrayList<String> myList = ... ;    // ArrayList to be converted

  String[] myArray = new String[ myList.size() ]; // Create array
        
  for ( int i = 0; i < myArray.length; i++ )  // Loop to copy elements
     myArray[i] = myList.get(i);

DEMO: demo/11-arrayList/05-support/ArrListToArray.java

The sort support method for ArrayList objects

  • The java.util.Collections class contains a (static) sort(list) method that sorts elements stored in an ArrayList

  • Example of the use of sort( ): 

     public static void main(String[] args)
 {
     ArrayList<String> myList = new ArrayList<>();
     myList.add("red"); myList.add("green"); myList.add("blue");

     System.out.println( myList );  // Unsorted

     Collections.sort( myList );    // Sorts a list

     System.out.println( myList );  // Sorted
 }

DEMO: demo/11-arrayList/05-support/Sort.java

Find the maximum or the minimum in an ArrayList

  • The java.util.Collections class contains a (static) max(list) and min(list) method that find the max/min value stored in an ArrayList object: 

      Collections.max( ArrayList ): returns the maximum element
                                in the ArrayList

  Collections.min( arrayList ): returns the minimum element
                                in the ArrayList

    Example: 

     public static void main(String[] args)
 {
     ArrayList<String> myList = new ArrayList<>();
     myList.add("red"); myList.add("green"); myList.add("blue");

    System.out.println("Max = " + Collections.max( myList ) );
    System.out.println("Min = " + Collections.min( myList ) );
 }

DEMO: demo/11-arrayList/05-support/MaxMin.java

Shuffle the elements in an ArrayList

  • The java.util.Collections class contains a (static) shuffle(list) method that randomly shuffles the elements stored in an ArrayList object: 

      Collections.shuffle( ArrayList )

    Example: 

     public static void main(String[] args)
 {
     ArrayList<String> myList = new ArrayList<>();
     myList.add("aaa"); myList.add("bbb"); myList.add("ccc");
     myList.add("ddd"); myList.add("eee"); myList.add("fff");
 
     System.out.println( myList );   // Original order

     Collections.shuffle( myList );  // Shuffles a list

     System.out.println( myList );   // Show new order
    }

DEMO: demo/11-arrayList/05-support/Shuffle.java