Essential Java.util Classes

Discussion in 'Java' started by shabbir, Jul 6, 2014.

  1. shabbir

    shabbir Administrator Staff Member

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    Java.util package contains the collections framework, legacy collection classes, event model, date and time facilities, and miscellaneous utility classes.

    Java.util.ArrayList Class



    The java.util.ArrayList class provides resizable-array and implements the List interface.
    • It implements all optional list operations and it also permits all elements, including null.
    • It provides methods to manipulate the size of the array that is used internally to store the list.
    Class declaration
    Code:
    public class ArrayList<E>
       extends AbstractList<E>
          implements List<E>, RandomAccess, Cloneable, Serializable
    
    Here <E> represents an Element. For example, if you're building an array list of Integers then you'd initialize it as
    ArrayList<Integer> list = new ArrayList<Integer>();

    Class constructors
    1. ArrayList() - Empty list is created having an initial capacity sufficient to hold 10 elements.
    2. ArrayList(Collection<? extends E> c) - List containing the elements of the specified collection is created.
    3. ArrayList(int initialCapacity) - Empty list is created with an initial capacity.
    Class methods
    1. boolean add(E e) - Specified element is appended to the end of this list.
    2. void add(int index, E element) - Specified element is inserted at the specified position in this list.
    3. boolean addAll(int index, Collection<? extends E> c) - Elements are inserted in the specified collection into this list, starting at the specified position.
    4. void clear() - This method removes elements from this list.
    5. boolean contains(Object o) - This method returns true if this list contains the specified element.
    6. E get(int index) - Element is returned at the specified position in this list.
    7. int indexOf(Object o) - This method returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contains the element.
    8. boolean isEmpty() - This method returns true if this list contains no elements.
    9. int lastIndexOf(Object o) - Index of the last occurrence of the specified element is returned in this list, or -1 if this list does not contain the element.
    10. E remove(int index) - Element is removed at the specified position in this list using this method.
    11. boolean remove(Object o) - First occurrence of the specified element is removed from this list, if it is present.
    12. protected void removeRange(int fromIndex, int toIndex) - All elements of list are removed whose index is between fromIndex(inclusive) and toIndex(exclusive).
    13. E set(int index, E element) - Element is replaced at the specified position in this list with the specified element.
    14. int size() - Returns the number of elements in this list.
    Example
    Code:
    import java.util.ArrayList;
    class ArrayListDemo {
    	public static void main (String dt[]) {
    		//  creates a new ArrayList
    		ArrayList al=new ArrayList();
    		//   add elements to the list
    		al.add(14);
    		al.add(27);
    		al.add(27.25);
    		al.add('M');
    		al.add("Sanju");
    		//   traverse the elements of the list
    		for (int i=0;i<al.size();i++) {
    			System.out.println(al.get(i));
    		}
    	}
    }
    
    Output

    [​IMG]

    Java.util.Calendar Class



    The java.util.calendar class is an abstract class that provides methods for converting a specific instant in time and a set of calendar fields such as YEAR, MONTH, DAY_OF_MONTH, HOUR, and so on, and for manipulating the calendar fields, such as getting the date of the next week.
    • This class also provides additional fields and methods for implementing a concrete calendar system outside the package.
    • Calendar defines the range of values returned by certain calendar fields.
    Class declaration
    Code:
    public abstract class Calendar
       extends Object
          implements Serializable, Cloneable, Comparable<Calendar>
    
    Class constructors
    1. protected Calendar() - Calendar object is constructed with the default time zone and locale.
    2. protected Calendar(TimeZone zone, Locale aLocale) - Calendar is constructed with the specified time zone and locale.
    Class methods
    1. abstract void add(int field, int amount) - Specified amount of time is added to the given calendar field, based on the calendar's rules.
    2. int compareTo(Calendar anotherCalendar) - Time values represented by two Calendar objects are compared by this method.
    3. boolean equals(Object obj) - This Calendar is compared to the specified Object.
    4. int getFirstDayOfWeek() - This method is used to get the first day of the week.
    5. static Calendar getInstance() - This method is used to get calendar using the default time zone and locale.
    6. static Calendar getInstance(Locale aLocale) - This method is used to get calendar using the default time zone and specified locale.
    7. static Calendar getInstance(TimeZone zone) - This method is used to get a calendar using the specified time zone and default locale.
    8. static Calendar getInstance(TimeZone zone, Locale aLocale) - This method is used to get calendar with the specified time zone.
    9. Date getTime() - Date object is returned representing this Calendar's time value.
    10. long getTimeInMillis() - Calendar's time value is returned in milliseconds.
    11. TimeZone getTimeZone() - This method is used to get the time zone.
    12. void set(int field, int value) - Given calendar field is set to the given value.
    13. void set(int year, int month, int date) - Calendar fields YEAR, MONTH, and DAY_OF_MONTH are set using this method.
    14. void set(int year, int month, int date, int hourOfDay, int minute) - Calendar fields YEAR, MONTH, DAY_OF_MONTH, HOUR_OF_DAY, and MINUTE are set using this method.
    15. void setTime(Date date) - Calendar's time with the given Date is being set by this method.
    16. void setTimeInMillis(long millis) - Calendar's current time from the given long value is being set by this method.
    17. String toString() - String representation of this calendar is returned.
    Example
    Code:
    import java.util.Calendar;
    class CalenderDemo
    {
    	public static void main(String arr[])
    	{
    		Calendar cal=Calendar.getInstance();
    		int y=cal.get(Calendar.YEAR);
    		int x=cal.get(Calendar.MONTH);		
    		int z=cal.get(Calendar.DAY_OF_MONTH);
    		int r=cal.get(Calendar.DAY_OF_WEEK);
    		int m=cal.get(Calendar.DAY_OF_YEAR);
    		
    		System.out.println("Current Year="+y);
    		System.out.println("Current Month="+(x+1));
    		System.out.println("Current Day of Month="+z);
    		System.out.println("Current Day of Week="+r);
    		System.out.println("Current Day of Year="+m);
    	}	
    }
    
    Output

    [​IMG]

    Java.util.HashMap Class



    The java.util.HashMap class is the Hash table based implementation of the Map interface.
    • Hash map is used to store collection in the form of key and value pair.
    • This class permits null values and the null key.
    Class declaration
    Code:
    public class HashMap<K,V>
       extends AbstractMap<K,V>
           implements Map<K,V>, Cloneable, Serializable
    
    Parameters
    java.util.HashMap class have following parameters:
    • K -- This is the type of keys maintained by this map.
    • V -- This is the type of mapped values.
    Class constructors
    • HashMap() - Empty HashMap is constructed with the default initial capacity (16) and the default load factor (0.75).
    • HashMap(Collection<? extends E> c)
      Empty HashMap is constructed with the specified initial capacity and the default load factor (0.75).
    • HashMap(Map<? extends K,? extends V> m)
      New HashMap is created with the same mappings as the specified Map.
    Class methods
    1. void clear() - All of the mappings from this map are removed by this method.
    2. boolean containsKey(Object key) - If this map contains a mapping for the specified key this method returns true.
    3. boolean containsValue(Object value) - If this map maps one or more keys to the specified value this method will return true.
    4. V get(Object key) - Value to which the specified key is mapped is returned, or null if this map contains no mapping for the key.
    5. boolean isEmpty() - This method returns true if this map contains no key-value mapping.
    6. Set<K> keySet() - A Set view of the keys is returned contained in this map.
    7. V put(K key, V value) - Specified value is associated with the specified key in this map.
    8. void putAll(Map<? extends K,? extends V> m) - Mappings from the specified map are copied to this map.
    9. V remove(Object key) - Mapping for the specified key is removed from this map if present.
    10. int size() - Number of key-value is returned mappings in this map.
    11. Collection<V> values() - Collection view of the values is returned contained in this map.
    Example
    Code:
    import java.util.HashMap;
    import java.util.Iterator;
    import java.util.Map;
    import java.util.Set;
    class HashMapDemo
    {
    	public static void main (String dt[])
    	{
    		HashMap<Integer,String> hm=new HashMap<Integer,String>();
    		hm.put(101,"Aman");
    		hm.put(103,"Ajay");
    		hm.put(102,"Vishal");
    		hm.put(103,"Mohit");
    		hm.put(104,"Sanju");
    		hm.put(105,"Nikhil");
    		System.out.println(hm);
    		Set s=hm.keySet();
    		Iterator itr=s.iterator();
    		while(itr.hasNext())
    		{
    			int k=(Integer)itr.next();
    			String v=hm.get(k);
    			System.out.println(k+"="+v);
    		}
    	}
    }
    
    Output

    [​IMG]
     
    Last edited: Jan 21, 2017

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