sorting collections

■ To sort a Set, make it a TreeSet. ie.

Set hs = new HashSet( );

hs.add("5");

hs.add("3");

hs.add("1");

hs.add("2");

hs.add("4");

System.out.println( hs ); // HashSet is unordered

Set ts = new TreeSet(hs);

System.out.println(ts); // TreeSet is ordered

■ To sort a Map, make it a TreeMap. i.e

HashMap hm = new HashMap();

Integer one = new Integer( 1 );

Integer two = new Integer( 2 );

Integer three = new Integer( 3 );

Integer four = new Integer( 4 );

Integer five = new Integer( 5 );

hm.put( five, "five" );

hm.put( two, "two" );

hm.put( three, "three" );

hm.put( one, "one" );

hm.put( four, "four" );

System.out.println( hm ); // HashMap is unordered

Map tm = new TreeMap(hm);

System.out.println( tm ); //TreeMap is ordered

■ To sort a List, use Collections.sort(list); . It sorts the list directly – doesn’t return anything.

Note that it throws a ClassCastException if it encounters unlike elements (of different types). i.e

String[ ] array = {"5", "2", "1", "4", "3"};

ArrayList myList = new ArrayList (Arrays.asList(array) );

System.out.println("List contains: " + myList);

Collections.sort(myList);

System.out.println("List now contains: " + myList);

■ To reverse their order, there’s also a Collections.sort(list, Comparator) which lets you specify a reversing Comparator to be used. See reversing their order. i.e.

Collections.sort(myList, Collections.reverseOrder( ));