Statics in java | Constructors | Exceptions in Java | String in java| class 3

Java Class 4
“Code is like humor. When you have to explain it, it’s
bad.”
•Statics in java
•Constructors
•Exceptions in Java
•String in java

Java Values 2
What is Static Keyword ?
The static keyword in Java is used for
memory management mainly. We can apply
static keyword with variables, methods,
blocks and nested classes. The static
keyword belongs to the class than an
instance of the class.
Variable (also known as a class variable)
Method (also known as a class method)
Block
Nested class

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Static Variable
Fields that have the static modifier in their
declaration are called static fields or class
variables.
They are associated with the class, rather
than with any object.
Every instance of the class shares a class
variable, which is in one fixed location in
memory.
Any object can change the value of a class
variable, but class variables can also be
manipulated without creating an instance of
the class.
Syntax : <class-name>.<variable-name>

Static methods
The Java programming language supports
static methods as well as static variables.
main method is static , since it must be
accessible for an application to run , before any
instantiation takes place.
Static methods, which have the static modifier
in their declarations, should be invoked with the
class name, without the need for creating an
instance of the class, as in
ex. ClassName.methodName(args)
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Static methods
A common use for static methods is to
access static fields.
For example, we could add a static method
to the Bicycle class to access the static field
numberOfBicycles :
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public static int getNumberOfBicycles() {
return numberOfBicycles;
}

Static methods
Not all combinations of instance and class
variables and methods are allowed:
• Instance methods can access instance
variables and instance methods directly.
• Instance methods can access class variables
and class methods directly.
• Class methods can access class variables and
class methods directly.
• Class methods cannot access instance
variables or instance methods directly—they
must use an object reference. Also, class
methods cannot use the this or super keywords,
as there is no instance to refer to.
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Constants
Constant fields are often declared as static.
For example NUM_OF_WHEELS which is a
characteristic of any Bicycle, and not of a
certain instance.
If there’s a sense to expose a constant field
to the outside world, it is common to declare
the field as public, rather than through a
getter.
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Singleton pattern
Restricting the instantiation of a certain class
to exactly one object.
This is useful when exactly one object is
needed to coordinate actions across the
system.
A singleton object is accessible globally
using a static method.
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Singleton
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public class Controller {
private static Controller instance = null;
private Controller () { ... }
public static Controller getInstance() {
if (instance == null) {
instance = new Controller ();
}
return instance;
}
...
}
* Not thread-safe
Notice the private
constructor

A static block
The static block, is a block of statement
inside a Java class that will be executed
when a class is first loaded in to the JVM.
A static block helps to initialize the static
data members, just like constructors help to
initialize instance members.
 public class Controller {
private static Controller instance;
static {
instance = new Controller ();
}
private Controller () { }
public static Controller getInstance() {
return instance;
}
 }
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Example of static
EX: 1
class A2{
static{
System.out.println("static block is
invoked");
}
public static void main(String args
[]){
System.out.println("Hello main");
}
}
output : ?
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EX: 2
class TestOuter1{
static int data=30;
static class Inner{
void msg(){
System.out.println("data is : "+data);
}
}
public static void main(String args[]){
TestOuter1.Inobj=new TestOuter1.Inner();
obj.msg();
}
}

Java static nested class
A static class i.e. created inside a class is called
static nested class in java. It cannot access non-
static data members and methods. It can be
accessed by outer class name.
It can access static data members of outer class
including private.
Static nested class cannot access non-static
(instance) data member or method.
For Example Refer previous slide EX:2 output?
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What is Constructors ?
In Java, a constructor is a block of codes
similar to the method. It is called when an
instance of the class is created. At the time
of calling constructor, memory for the object
is allocated in the memory.
It is a special type of method which is used
to initialize the object.
Every time an object is created using the
new() keyword, at least one constructor is
called.
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Rules for creating constructor
There are two rules defined for the constructor.
Constructor name must be the same as its class
name
A Constructor must have no explicit return type.
A Java constructor cannot be abstract, static, final,
and synchronized
class Bike1{
Bike1(){ //creating a default constructor
System.out.println("Bike is created");
}
Bike1 b=new Bike1(); //calling a default constructor
}
}
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Type of Constructors
There are two types of constructors in Java:
1. Default constructor (no-arg constructor)
2. Parameterized constructor
If there is no constructor in a class, compiler automatically creates a
default constructor.
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class Student4{
int id; // default value 0
String name; //default value null
// if we not pass the int I and string n value it will give default value
Student4(int i,String n){ //creating a parameterized constructor
id = i;
name = n;
}
//method to display the values
void display(){System.out.println(id+" "+name);}
//creating objects and passing values
Student4 s1 = new Student4(111,"Karan");
Student4 s2 = new Student4(222,"Aryan");
//calling method to display the values of object
s1.display();
s2.display();
}
}
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Output:?

Constructor Chaining
Calling a constructor from the another
constructor of same class is known as
Constructor chaining..
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This and Super constructors
this() and super() are used to call
constructors explicitly.
1. Using this() you can call the current class’s constructor
2. Using super() you can call the constructor of the super
class.
Java Values 18

class A
{
public int x, y;
public A(int x, int y) {
this.x = x; this.y = y;
}
}
class B extends A {
public int x, y;
public B() {
this(0, 0);
}
public B(int x, int y) {
super(x + 1, y + 1); // calls parent
class constructor
this.x = x;
this.y = y;
}
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public void print() {
System.out.println("Base class : {" + x + ", "
+ y + "}");
System.out.println("Super class : {" +
super.x + ", " + super.y + "}");
}
}
class Point
{
public static void main(String[] args) {
B obj = new B();
obj.print();
obj = new B(1, 2);
obj.print();
}
}
? Answer

Constructors for Enumerated Data Types
enum Car {
lamborghini(900),tata(2),audi(50),fiat(15),honda(12); //Enum datatype
private int price;
Car(int p) {
price = p; }
int getPrice() {
return price;
}
}
public class Main {
System.out.println("All car prices:");
for (Car c : Car.values())
System.out.println( c + " costs " + c.getPrice() + " thousand dollars.");
}
} Java Values 20

Exception ??
Java Values 22Exception is an abnormal condition.

What ? Why ? How?
In Java, an exception is an event that disrupts the
normal flow of the program. It is an object which is
thrown at runtime.
Exception Handling is a mechanism to handle
runtime errors such as ClassNotFoundException,
IOException, SQLException, RemoteException,
etc.
The core advantage of exception handling is to
maintain the normal flow of the application. An
exception normally disrupts the normal flow of the
application that is why we use exception handling.
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API hierarchy for Exceptions
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Parent class

Types of Exceptions
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Java Values.com

Keywords in Exception API
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Catching Exceptions
 The try-catch Statements
● Syntax:
try {
<code to be monitored for exceptions>
} catch (<ExceptionType1> <ObjName>) {
<handler if ExceptionType1 occurs>
}
...
} catch (<ExceptionTypeN> <ObjName>) {
<handler if ExceptionTypeN occurs>
 } finally {
 <code to be executed before the try block ends>
 }
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Throwing Exceptions
The throw Keyword
● Java allows you to throw exceptions (generate exceptions)
ex. throw <exception object>;
● An exception you throw is an object
– You have to create an exception object in the same way you create
any other object
● Example:
throw new ArithmeticException(“testing...”);
class TestHateString {
public static void main(String args[]) {
String input = "invalid input";
try {
if (input.equals("invalid input")) {
throw new HateStringExp();
}
System.out.println("Accept string.");
Java Values 29
} catch (HateStringExp e) {
System.out.println("Hate string!”);
}
}
}

Example 2
class InvalidAgeException extends Exception{
InvalidAgeException(String s){
super(s);
}
}
class TestCustomException1{
static void validate(int age)throws InvalidAgeException{
if(age<18)
throw new InvalidAgeException("not valid");
else
System.out.println("welcome to vote");
}
try{
validate(13);
}catch(Exception m){System.out.println("Exception occured: "+m);}
System.out.println("rest of the code...");
}
}
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Output:?

String in Java
Java Values 31
Java Values

Strings
String is a sequence of characters placed in
double quote(“ ”). //(“JAVA”)
Strings are constant , their values cannot be
changed in the same memory after they are
created.
There are two ways to create String
object:
1)By string literal.
2)By new keyword.
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String Creation
By string literal:
For Example: String s1=“welcome";
String s2=“welcome”;
//no new object will be created
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By new keyword:-
For Example:
String s=new String(“Sachin");
String s=newString(“SachinTendulkar");
//create two objects and one reference variable
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Immutability
In java, string objects are immutable.
Immutable simply means unmodifiable or
unchangeable.
Once string object is created its data or state
can't be changed but a new string object is
created.
Advantage of immutability is Use of less
memory
Disadvantages of Immutability: Less efficient —
you need to create a new string and throw away
the old one even for small changes
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String word1 = "Java";
String word2 = word1;
Word1
Java
word2
OK
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String word1 = “Java";
String word2 =new String(word1);
Word1 Java
Word2 Java
Less efficient: wastes memory

String Methods
substring(int begin):
Returns substring from begin index to end of the String.
Example: String s=“nacre”;
System.out.println(s.substring(2));//cre
equals():
To perform content comparision where case is important.
Example: String s=“java”;
System.out.println(s.equals(“java”));//true
concat(): Adding two strings we use this method
Example: String s=“nacre”;
s= s.concat(“software”);
System.out.println(s);// nacresoftware
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length() , charAt()
int length(); Returns the number of characters in the string
char charAt(i); Returns the char at position i.
Character positions in strings are numbered starting from
0 – just like arrays.
Returns:
“Problem".length(); 7
“Window”. charAt (2); ’n'
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StringBuffer, StringBuilder
StringTokenizer
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Java values
All These classes are final and subclass of Serializable.

Limitation of String in Java ?
What is mutable string?
A string that can be modified or changed is known
as mutable string. StringBuffer and StringBuilder
classes are used for creating mutable string.
Differences between String and StringBuffer in java?
Main difference between String and StringBuffer is
String is immutable while StringBuffer is mutable
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StringBuffer
StringBuffer is a synchronized and allows us to
mutate the string.
StringBuffer has many utility methods to manipulate
the string.
This is more useful when using in multithreaded
environment.
Always modified in same memory location.
Methods:
1. Append 2. Insert 3.Delete 4.Reverse
5.Replacing Character at given index
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StringBuilder
StringBuilder is the same as the StringBuffer class.
The StringBuilder class is not synchronized and
hence in a single threaded environment, the
overhead is less than using a StringBuffer.
public class Test
{
public static void main(String[] args)
{
String str = “Java";
StringBuffer sbr = new StringBuffer(str);
sbr.reverse();
System.out.println(sbr);
// conversion from String object to StringBuilder
StringBuilder sbl = new StringBuilder(str);
sbl.append(“Values");
System.out.println(sbl);
}
}
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Output?

StringTokenizer
A token is a portion of a string that is separated from another
portion of that string by one or more chosen characters (called
delimiters).
The StringTokenizer class contained in the java.util package
can be used to break a string into separate tokens. This is
particularly useful in those situations in which we want to read
and process one token at a time; the BufferedReader class
does not have a method to read one token at a time.
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Java Values 44
Java Values
“Hello world welcome to Java Values”
String Tokenizer
Token
Hello
world
welcome
to
Java
Values

 Statics
variable, Method, block, nested class
 Constructors
default ,no-arg, parameterize
 Exceptions in Java
Checked –unchecked, exception, error , try-catch-finally-throw-throws,
customize exception
 String
String –Stringbuffer-StringBuilder-String Tokenizer
Q & A
???
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