Computer Science 15-100 (Sections T & U), Spring 2008
Class Notes:  Java Collections

Logistics

1. Schedule
   1. Due tomorrow:  hw9b
   2. Due Friday:  hw9b
   3. Bonus Lecture #4 (Tetris!) makeup:  Monday at 7pm
2. Reading:
   1. Ch 12 (sort of):  Collections

Topic Outline:

1. Simple examples:

   // List, Set, and Map examples
   import java.util.*;
   @SuppressWarnings("unchecked")
   public class Foo {
         
     public static void listExample() {
       ArrayList list = new ArrayList();
       list.add("Let's");
       list.add("Go");
       list.add("Pens!");
       verify(list.toString().equals("[Let's, Go, Pens!]"));
     } 
     
     public static void setExample() {
       HashSet set = new HashSet();
       set.add("fred");
       set.add("wilma");
       set.add("barney");
       set.add("fred"); // second "add" of "fred" has no effect
       verify(set.contains("fred") == true);
       verify(set.contains("betty") == false);
   
       ArrayList list = new ArrayList(set);
       // notice two things about the toString call, which
       // demonstrate the key differences between Sets and Lists:
       // 1. "fred" is there only once
       // 2. the order is not necessarily as given
       //    (we added "fred" first, yet "wilma" is first in the toString!)
       verify(list.toString().equals("[wilma, fred, barney]"));
     }
     
     public static void mapExample() {
       HashMap map = new HashMap();
       map.put("fred","flintstone");
       map.put("wilma","flintstone");
       map.put("barney","rubble");
       verify(map.get("fred").equals("flintstone"));
       verify(map.get("betty") == null);
       // change the map for fred
       map.put("fred","astaire");
       verify(map.get("fred").equals("astaire"));
     }
   
     public static void main(String[] args) throws Exception {
       System.out.println("Running examples for list, set, and map:");
       listExample();
       setExample();
       mapExample();
       System.out.println("Passed all tests!");
     }
     
     public static void verify(boolean b) {
       if (!b) throw new RuntimeException("Ack!");
     }
   }

2. List, Set, and Map iteration examples:

   // List, Set, and Map iteration examples
   import java.util.*;
   @SuppressWarnings("unchecked")
   public class Foo {
         
     public static void listIterationExample1() {
       ArrayList list = new ArrayList();
       list.add("abc");
       list.add("def");
       list.add("ghi");
       String check = "";
       for (int i=0; i<list.size(); i++) {
         String s = (String)list.get(i);
         check += s;
       }
       verify(check.equals("abcdefghi"));
     } 
   
     public static void listIterationExample2() {
       ArrayList list = new ArrayList();
       list.add("abc");
       list.add("def");
       list.add("ghi");
       String check = "";
       Iterator iterator = list.iterator();
       while (iterator.hasNext()) {
         String s = (String)iterator.next();
         check += s;
       }
       verify(check.equals("abcdefghi"));
     }
    
     public static void setIterationExample() {
       HashSet set = new HashSet();
       set.add("abc");
       set.add("def");
       set.add("ghi");
       set.add("abc"); // second "add" of "abc" has no effect
       String check = "";
       Iterator iterator = set.iterator();
       while (iterator.hasNext()) {
         String s = (String)iterator.next();
         check += s;
       }
       verify(check.equals("ghidefabc")); // note the unpredictable order!
     }
     
     public static void mapIterationExample() {
       HashMap map = new HashMap();
       map.put("abc","one");
       map.put("def","one");
       map.put("ghi","two");
   
       // Here is how you iterate over the keys:
       String keysCheck = "";
       Iterator keysIterator = map.keySet().iterator();
       while (keysIterator.hasNext()) {
         String s = (String)keysIterator.next();
         keysCheck += s;
       }
       verify(keysCheck.equals("ghidefabc")); // <- unpredictable order!
   
       // And here is how you iterate over the values
       String valuesCheck = "";
       Iterator valuesIterator = map.values().iterator();
       while (valuesIterator.hasNext()) {
         String s = (String)valuesIterator.next();
         valuesCheck += s;
       }
       verify(valuesCheck.equals("twooneone")); // <- unpredictable order!
     }
   
     public static void main(String[] args) throws Exception {
       System.out.println("Running examples for list, set, and map:");
       listIterationExample1();
       listIterationExample2();
       setIterationExample();
       mapIterationExample();
       System.out.println("Passed all tests!");
     }
     
     public static void verify(boolean b) {
       if (!b) throw new RuntimeException("Ack!");
     }
   }

3. Enhanced "for" loop ("foreach" loop)

   // Enhanced "for" loop ("foreach" loop) example
   import java.util.*;
   @SuppressWarnings("unchecked")
   public class Foo {
   
     public static void foreachExampleWithArray() {
       String[] array = { "abc", "def", "ghi" };
       String check = "";
       for (String s : array)
         check += s;
       verify(check.equals("abcdefghi"));
     } 
   
     public static void foreachExampleWithoutGenerics() {
       ArrayList list = new ArrayList();
       list.add("abc");
       list.add("def");
       list.add("ghi");
       String check = "";
       for (Object obj : list) {
         String s = (String) obj; // must cast from "Object" to "String"
         check += s;
       }
       verify(check.equals("abcdefghi"));
     } 
   
     public static void foreachExampleWithGenerics() {
       ArrayList<String> list = new ArrayList<String>();
       list.add("abc");
       list.add("def");
       list.add("ghi");
       String check = "";
       for (String s : list) {  // no need to cast!
         check += s;
       }
       verify(check.equals("abcdefghi"));
     } 
   
     public static void main(String[] args) throws Exception {
       System.out.println("Running examples for 'foreach' loop:");
       foreachExampleWithArray();
       foreachExampleWithoutGenerics();
       foreachExampleWithGenerics();
       System.out.println("Passed all tests!");
     }
     
     public static void verify(boolean b) {
       if (!b) throw new RuntimeException("Ack!");
     }
   }

4. Some List (ArrayLists) methods
   • add(index, element)
   • add(element)
   • addAll(collection)
   • clear()
   • contains(element)
   • get(index)
   • indexOf(element)
   • isEmpty()
   • lastIndexOf(element)
   • remove(index)
   • removeRange(fromIndex, toIndex)
   • set(index, element)
   • size()
   • toArray()  <-- complicated
      
5. Set and HashSet
   • add(element)
   • clear()
   • contains(element)
   • isEmpty()
   • remove(element)
   • size()
      
6. Map and HashMap
    
7. Queue (List), Stack, and PriorityQueue
    
8. Arrays class
    
9. Collections class
    
10. Generics

carpe diem   -   carpe diem   -   carpe diem   -   carpe diem   -   carpe diem   -   carpe diem   -   carpe diem   -   carpe diem   -   carpe diem