Java Data Structure - Collections.

Collections

• Autoboxing
• Array List vs Linked List vs Queue
• Hash map vs Tree map
• untyped collections and wrapper class with untyped collections
• Wrapper classes for primitive types
• Byte (byte)
• Short (short)
• Integer (int)
• Long (long)
• Float (float)
• Double (double)
• Character (char)
• Boolean (boolean)
• Java Collection Framework (Collections is the basic methods):
• Lists (ordered) - ArrayList and LinkedList
• Sets  (no dupicate) - HashSet
• Mapes (key value pair) - HashMap and TreeMap
• How it's different from Arrays
• Collections are classes in Java API, array is a Java Language feature.
• Collection classes have methods.
• Collections are varied in size.
• Collections are containers for objects, not for primitive types.
• Collections can process without indices while indices are usually required to process arrays.
• Generic collections
• ex. ArrayList<String> al = new ArrayList<String>();

Example

// This is an untyped array list
ArrayList al = new ArrayList();
al.add("item1");
al.add("item2");
for (Object o : al)
    { ... }

ArrayList p = new ArrayList();
p.add (new className (...));

for (int i = 0; i < p.size(); i++) {
    className c = (className)p.get(i);
    ...
}

// untyped array list will result in compiler warning
// Note: file.java uses unchecked or unsafe operations.
// Note: Recompile with -Xlint:unchecked for details.


ArrayList Numbers = new ArrayList();
numbers.add(new Interger(1));
numbers.add("Mary");

//
// and gives run time errors:
// Exception in thread "main" java.lang.ClassCastException: java.lang.String 
// cannot be cast to java.lang.Integer at Demo.main(file.java:37)

        ArrayList numbers = new ArrayList();
           numbers.add(new Integer(1));
           numbers.add(new Integer(2));
           numbers.add("Mary");
           numbers.add("Helen");

        for (int i = 0; i < numbers.size(); i++)
           {
               int number = (Integer)numbers.get(i);
               System.out.println(number);
           }



// Use collection in a generic array

ArrayList<String> codes = new ArrayList<String>();
codes.add("Mary");
codes.add("Helen");
codes.add("Raymond");
codes.add(100); //compiler error, wrong type, has to be String

System.out.println(codes);


// Using wrappers for primitives
ArrayList Numbers = new ArrayList();
numbers.add(new Integer(1));

Classes and Packages

• java.util.Arrays
• java.util.ArrayList
• Constructors
• ArrayList<E>()
• ArrayList<E>(intCapacity)
• ArrayList<E>(Collection)
• Methods:
• add(object)
• add(index, object)
• clear()
• contains(object)
• get(index)
• indexOf(object)
• isEmpty()
• remove(index)
• remove(object)
• set(index, object)
• size()
• toArray()