Basic Programming Session 4

7/25/2019 Basic Programming Session 4

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endava.com

QUALITY. PRODUCTIVITY. INNOVATION.

Basic Programming

Session 4


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Basic Programming

Garbage collection

String & StringBuilder

toString

Collections ArrayList

HashSet

HashMap

Q & A



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3

Memory management and Garbage Collection


Memory management

Garbage collection is a way in which Java recollects the space occupied

by loitering objects.

In general, a garbage collector is responsible with three tasks:

Allocating memory for new objects

Ensuring that any reference objects (live objects) remain in

memory

Recovering memory used by objects that are no longer reachable(dead objects)

WhenDoes the Garbage Collector Run?

How Does the Garbage Collector Work?


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Reassigning a Reference Variableset the referencevariable to refer to another object.

class GarbageTruck {

public static void main(String [] args) {

StringBuffer s1 = new StringBuffer("hello");

StringBuffer s2 = new StringBuffer("goodbye");

System.out.println(s1);

// At this point the StringBuffer "hello" isnot eligible

s1 = s2; // Redirects s1 to refer to the"goodbye" object

// Now the StringBuffer "hello" is eligible for

collection

}

}

Nulling a Reference - set the reference variable

that refers to the object to null.

public class GarbageTruck {

public static void main(String [] args) {

StringBuffer sb = new

StringBuffer("hello");

System.out.println(sb);

// The StringBuffer object is not eligiblefor collection

sb = null;

// Now the StringBuffer object is eligiblefor collection

}

}

Writing Code That Explicitly MakesObjects Eligible for Collection



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String

The String Class

KEY concept: once a String object is created, it can never

be changed !

Immutable Objects


Step 1

String myString = Hello;

Step 2

String secondString =

myString;

Step 3

myString =

myString.concat( World)

The heap

HellomyString

(reference

variable)

The heap

Hello

myString(reference variable)

secondString

(reference variable)

The heap

Hello

myString(reference variable)

secondString(reference variable)

Hello World


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Important Methods in the String class



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toString()

Implementing toStringmethod in java is done by overriding the Objects toString method.

The java toString() method is used when we need a string representation of an object. It is defined inObject class. This method can be overridden to customize the String representation of the Object.

Demo

Conclusion

Since our example has no toString method, the default one in Object is used. The format of the defaulttoString method of the Object is as shown below.

Class Name, @, and the hex version of the objects hashcode concatenated into a string.The default hashCode method in Object is typically implemented by converting the memory address of theobject into an integer.



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The StringBuffer and StringBuilderClasses

The java.lang.StringBuffer andjava.lang.StringBuilderclasses should be usedwhen you have to make a lot of modifications to strings of characters.

The StringBuilderclass was added in Java 5. It has exactly the same API as the

StringBufferclass, except StringBuilderis not thread safe.

Sunrecommends that you use StringBuilderinstead of StringBufferwheneverpossible because StringBuilder will run faster.



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Important Methods in the StringBufferand StringBuilder Classes


This method returns a StringBuffer object and updates the value of the StringBuffer

object that invoked the method call. In both cases, the characters in the StringBufferare reversed, the first character becoming the last, the second becoming the second

to the last, and so on:

public synchronized

StringBuffer

reverse()

This method returns the value of the StringBuffer object that invoked the method call

as a String

public String

toString()

update the value of the object that invoked the method, whether or not the return is

assigned to a variable. This method will take many different arguments, including

boolean, char, double, float, int, long, and others

public synchronized

StringBuffer

append(String s)

This method returns a StringBuilder object and updates the value of the StringBuilder

object that invoke the method call. In both cases, a substring is removed from the

original object.

public StringBuilder

delete(int start, intend)

This method returns a StringBuilder object and updates the value of the StringBuilder

object that invoked the method call. In both cases, the String passed in to the second

argument is inserted into the original StringBuilder starting at the offset location

represented by the first argument (the offset is zero-based).

public StringBuilder

insert(int offset,String s)


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Java Collections

A collection represents a group of objects

java.util package.

They are basically used to group multiple elements into a single unit. Some collectionsallow duplicate elements while others do not. Some collections are ordered and others arenot.

What can we do with a Collection?

1. Addobjects to the collection.

2. Removeobjects from the collection.

3. Find out if an object(or group of objects) is in the collection

4. Retrievean object from the collection (without removing it).5. Iteratethrough the collection, looking at each element (object) one after another.



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11 QUALITY. PRODUCTIVITY. INNOVATION.


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Classification

Lists - Listsof things (classes that implement List)

Sets - Uniquethings (classes that implement Set)

Maps - Things with an uniqueID (classes that implement Map)

QUALITY. PRODUCTIVITY.

INNOVATION.


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Collection Interface concrete implementationclasses


Class Map Set List Ordered Sorted

HashMap x No No

Hashtable x No No

TreeMap x Sorted By natural order or

custom comparison rules

LinkedHashMap x By insertion orderor last access order

No

HashSet x No No

TreeSet x Sorted By natural order or

custom comparison rules

LinkedHashSet x By insertion order No

ArrayList x By index No

Vector x By index No

LinkedList x By index No


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ArrayList

ArrayList supports dynamic arrays that cangrow as needed.

In Java, standard arrays are of a fixed length.After arrays are created, they cannot grow orshrink, which means that you must know in

advance how many elements an array will

hold. But, sometimes, you may not know untilrun time precisely how large of an array youneed.

Solution: ArrayLista variable-length arrayof object references. That is, an ArrayList candynamically increase or decrease in size.Array lists are created with an initial size.When this size is exceeded, the collection isautomatically enlarged. When objects areremoved, the array may be shrunk.

A java ArrayList is used to store an ordered

group of elements where duplicatesareallowed.

Most important methods


add (Object o) - adds an object to the ArrayList

add(int index, Object o)adds the object o the the array list at

the given index

get(int index) - retrieves object reference from ArrayList index

position

size() - returns ArrayList size

remove(int index) - removes element at specified position in the

list. Shifts any subsequent elements to the left

remove (Object o) - removes the object o from the list

set(int index, Object o)- used for updating an element; it

replaces the element present at the specified index with the

object o

int indexOf(Object o)- finds the index in the list of the first

occurrence of the specified element. If the element is not

present in the list returns -1

Object get(int index)returns the object of list which is present

at the specified index

clear() - removes all the elements

boolean contains(Object o)- checks whether the given object o

is present in the array list


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HashSet

implements the Set interface

doesnt maintain any order, the elements

would be returned in any random order. doesnt allow duplicates. If you try to add a

duplicate element in HashSet, the old valuewould be overwritten.

allows null values however if you insert morethan one nulls it would still return only onenull value.

is non-synchronized



boolean add(Element e) - adds the element e to the Set

void clear()removes all the elements from the Set

boolean contains(Object o)checks whether the specified

Object o is present in the list or not. If the object has been

found it returns true, false otherwise.

boolean isEmpty()returns true if there is no element in the

Set

int size()- returns the number of elements of a Set

Boolean remove(Object o)removes the specified object from

the Set


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HashMap

HashMap implementation provides constant-

time performance for the basic operations(get and put), assuming the hash functiondisperses the elements properly among thebuckets.

It is not an ordered collection which means itdoes not return the keys and values in thesame order in which they have been insertedinto the HashMap.

It does not any kind of sorting to the storedkeys and Values



void clear()- removes all the keys and values form the specified

map

boolean containsKey(Object key)returns true or false based

on whether the specified key is found in the map

boolean containsValue(Object Value)- similar to containsKeymethod, however it looks for the specified value instead of key

Value get(Object key)returns the value of the specified key

boolean isEmpty()checks whether the map is empty

Set keySet()- returns the Set of the keys fetched from the map

value put(Key k, Value v)-inserts key value mapping into the

map.

int size()returns size of the map

Collection values()returns a Collection of values of map

Value remove(Object key)- removes the key-value pair for the

specified key

void putAll(Map m)copies all the elements of a map into the

specified map m


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Q & A


Documents

Basic Programming Session 4