CHAINED EXCEPTION

Chained Exception

Chained Exception was added to Java in JDK 1.4. This feature allow you to relate one exception with another exception, i.e one exception describes cause of another exception. For example, consider a situation in which a method throws an ArithmeticException because of an attempt to divide by zero but the actual cause of exception was an I/O error which caused the divisor to be zero. The method will throw onlyArithmeticException to the caller. So the caller would not come to know about the actual cause of exception. Chained Exception is used in such type of situations.

Two new constructors and two methods were added to Throwable class to support chained exception.

1. Throwable( Throwable cause )
2. Throwable( String str, Throwable cause )

In the first form, the paramter cause specifies the actual cause of exception. In the second form, it allows us to add an exception description in string form with the actual cause of exception.

getCause() and initCause() are the two methods added to Throwable class.

• getCause() method returns the actual cause associated with current exception.
• initCause() set an underlying cause(exception) with invoking exception.

---

Example

import java.io.IOException;
public class ChainedException 
 {
  public static void divide(int a, int b)
  {
   if(b==0)
   {
    ArithmeticException ae = new ArithmeticException("top layer");
    ae.initCause( new IOException("cause") );
    throw ae;
   }
   else
   {
    System.out.println(a/b);
   }
  }

 public static void main(String[] args)
 {
  try {
   divide(5, 0);
  } 
  catch(ArithmeticException ae) {
   System.out.println( "caught : " +ae);
   System.out.println("actual cause: "+ae.getCause());
  }
 }
}

output
caught:java.lang.ArithmeticException: top layer
actual cause: java.io.IOException: cause

METHOD OVERRIDING WITH EXCEPTIONS

Method Overriding with Exception Handling

There are few things to remember when overriding a method with exception handling. If super class method does not declare any exception, then sub class overriden method cannot declare checked exception but it can declare unchecked exceptions.

---

Example of Subclass overriden Method declaring Checked Exception

import java.io.*;
class Super
{
 void show() { System.out.println("parent class"); }
}

public class Sub extends Super 
{
 void show() throws IOException  //Compile time error   
  { System.out.println("parent class"); } 

 public static void main( String[] args )
 {
  Super s=new Sub();
  s.show();
 }  
}

As the method show() doesn't throws any exception while in Super class, hence its overriden version can also not throw any checked exception.

---

Example of Subclass overriden Method declaring Unchecked Exception

import java.io.*;
class Super
{
 void show(){ System.out.println("parent class"); }
}

public class Sub extends Super 
{
 void show() throws ArrayIndexOutOfBoundsException      //Correct
   { System.out.println("child class"); }

 public static void main(String[] args)
 {
  Super s=new Sub();
  s.show();
 }  
}

Output : child class

Because ArrayIndexOutOfBoundsException is an unchecked exception hence, overrided show() method can throw it.

---

More about Overriden Methods and Exceptions

If Super class method throws an exception, then Subclass overriden method can throw the same exception or no exception, but must not throw parent exception of the exception thrown by Super class method.

It means, if Super class method throws object of NullPointerException class, then Subclass method can either throw same exception, or can throw no exception, but it can never throw object of Exception class (parent of NullPointerException class).

Example of Subclass overriden method with same Exception

import java.io.*;
class Super
{
 void show() throws Exception 
  {  System.out.println("parent class");  }
}

public class Sub extends Super {
 void show() throws Exception  //Correct     
   { System.out.println("child class"); } 

 public static void main(String[] args)
 {
  try {
   Super s=new Sub();
   s.show();
   }
  catch(Exception e){}
 }  
}

Output : child class

---

Example of Subclass overriden method with no Exception

import java.io.*;
class Super
{
 void show() throws Exception 
  {  System.out.println("parent class");  }
}

public class Sub extends Super {
 void show()               //Correct     
   { System.out.println("child class"); } 

 public static void main(String[] args)
 {
  try {
   Super s=new Sub();
   s.show();
   }
  catch(Exception e){}
 }  
}

Output : child class

---

Example of Subclass overriden method with parent Exception

import java.io.*;
class Super
{
 void show() throws ArithmeticException 
  {  System.out.println("parent class");  }
}

public class Sub extends Super {
 void show() throws Exception           //Cmpile time Error     
   { System.out.println("child class"); } 

 public static void main(String[] args)
 {
  try {
   Super s=new Sub();
   s.show();
   }
  catch(Exception e){}
 }  
}

USER MADE EXCEPTION SUBCLASS

User defined Exception subclass

You can also create your own exception sub class simply by extending java Exception class. You can define a constructor for your Exception sub class (not compulsory) and you can override the toString() function to display your customized message on catch.

class MyException extends Exception
{
 private int ex;
 MyException(int a)
 {
  ex=a;
 }
 public String toString()
 {
  return "MyException[" + ex +"] is less than zero";
 }
}

class Test
{
 static void sum(int a,int b) throws MyException
 {
  if(a<0)
  {
   throw new MyException(a);
  }
  else 
  { 
   System.out.println(a+b); 
  }
 }

 public static void main(String[] args)
 {
  try
  {
   sum(-10, 10);
  }
  catch(MyException me)
  {
   System.out.println(me);
  }
 }
}

---

Points to Remember

1. Extend the Exception class to create your own ecxeption class.
2. You don't have to implement anything inside it, no methods are required.
3. You can have a Constructor if you want.
4. You can override the toString() function, to display customized message.

THROW , THROWS AND FINALLY

throw Keyword

throw keyword is used to throw an exception explicitly. Only object of Throwable class or its sub classes can be thrown. Program execution stops on encountering throw statement, and the closest catch statement is checked for matching type of exception.

Syntax :

throw ThrowableInstance

---

Creating Instance of Throwable class

There are two possible ways to get an instance of class Throwable,

1. Using a parameter in catch block.
2. Creating instance with new operator.

   new NullPointerException("test");
   

   This constructs an instance of NullPointerException with name test.

---

Example demonstrating throw Keyword

class Test
{
 static void avg() 
 {
  try
  {
   throw new ArithmeticException("demo");
  }
  catch(ArithmeticException e)
  {
   System.out.println("Exception caught");
  } 
 }

 public static void main(String args[])
 {
  avg(); 
 }
}

In the above example the avg() method throw an instance of ArithmeticException, which is successfully handled using the catch statement.

---

throws Keyword

Any method capable of causing exceptions must list all the exceptions possible during its execution, so that anyone calling that method gets a prior knowledge about which exceptions to handle. A method can do so by using the throws keyword.

Syntax :

type method_name(parameter_list) throws exception_list
{
 //definition of method
}

NOTE : It is necessary for all exceptions, except the exceptions of type Error and RuntimeException, or any of their subclass.

---

Example demonstrating throws Keyword

class Test
{
 static void check() throws ArithmeticException
 {
  System.out.println("Inside check function");
  throw new ArithmeticException("demo");
 }

 public static void main(String args[])
 {
  try
  {
   check();
  }
  catch(ArithmeticException e)
  {
   System.out.println("caught" + e);
  }
 }
}

---

finally clause

A finally keyword is used to create a block of code that follows a try block. A finally block of code always executes whether or not exception has occurred. Using a finally block, lets you run any cleanup type statements that you want to execute, no matter what happens in the protected code. A finally block appears at the end of catch block.

Example demonstrating finally Clause

Class ExceptionTest
{
 public static void main(String[] args)
 {
  int a[]= new int[2];
  System.out.println("out of try");
  try 
  {
   System.out.println("Access invalid element"+ a[3]);
   /* the above statement will throw ArrayIndexOutOfBoundException */
  }
  finally 
  {
   System.out.println("finally is always executed.");
  }
 }
}

Output:
Out of try
finally is always executed. 
Exception in thread main java. Lang. exception array Index out of bound exception.  

You can see in above example even if exception is thrown by the program, which is not handled by catch block, still finally block will get executed.

TRY WITH RESOURCE STATEMENT

Try with Resource Statement

JDK 7 introduces a new version of try statement known as try-with-resources statement. This feature add another way to exception handling with resources management,it is also referred to as automatic resource management.

Syntax

try(resource-specification)
{
//use the resource
}catch()
{...}

This try statement contains a paranthesis in which one or more resources is declare. Any object that implements java.lang.AutoCloseable or java.io.Closeable, can be passed as a parameter to try statement. A resource is an object that is used in program and must be closed after the program is finished. The try-with-resources statement ensures that each resource is closed at the end of the statement, you do not have to explicitly close the resources.

---

Example without using try with Resource Statement

import java.io.*;
class Test
{
public static void main(String[] args)
{
try{
String str;
//opening file in read mode using BufferedReader stream
BufferedReader br=new BufferedReader(new FileReader("d:\\myfile.txt"));   
while((str=br.readLine())!=null)
 {
System.out.println(str);
 }      
br.close(); //closing BufferedReader stream
}catch(IOException ie)
{  System.out.println("exception");  }
 }
}

---

Example using try with Resource Statement

import java.io.*;
class Test
{
public static void main(String[] args)
{
try(BufferedReader br=new BufferedReader(new FileReader("d:\\myfile.txt")))
{
String str;
while((str=br.readLine())!=null)
{
System.out.println(str);
}
}catch(IOException ie)
{  System.out.println("exception");  }
}
}

NOTE: In the above example, we do not need to explicitly call close() method to close BufferedReader stream.

TRY AND CATCH BLOCK

Exception Handling Mechanism

In java, exception handling is done using five keywords,

1. try
2. catch
3. throw
4. throws
5. finally

Exception handling is done by transferring the execution of a program to an appropriate exception handler when exception occurs.

---

Using try and catch

Try is used to guard a block of code in which exception may occur. This block of code is called guarded region. A catch statement involves declaring the type of exception you are trying to catch. If an exception occurs in guarded code, the catch block that follows the try is checked, if the type of exception that occured is listed in the catch block then the exception is handed over to the catch block which then handles it.

---

Example using Try and catch

class Excp
{
 public static void main(String args[])
 {
  int a,b,c;
  try
  {
   a=0;
   b=10;
   c=b/a;
   System.out.println("This line will not be executed");
  }
  catch(ArithmeticException e)
  {
   System.out.println("Divided by zero"); 
  }
  System.out.println("After exception is handled");
 }
}
output:
Divided by zero
After exception is handled

An exception will thrown by this program as we are trying to divide a number by zero inside try block. The program control is transfered outside try block. Thus the line "This line will not be executed" is never parsed by the compiler. The exception thrown is handle in catch block. Once the exception is handled the program controls continue with the next line in the program. Thus the line "After exception is handled" is printed.

---

Multiple catch blocks:

A try block can be followed by multiple catch blocks. You can have any number of catch blocks after a single try block.If an exception occurs in the guarded code the exception is passed to the first catch block in the list. If the exception type of exception, matches with the first catch block it gets caught, if not the exception is passed down to the next catch block. This continue until the exception is caught or falls through all catches.

---

Example for Multiple Catch blocks

class Excep
{
 public static void main(String[] args)
 {
  try
  {
   int arr[]={1,2};
   arr[2]=3/0;
  } 
  catch(ArithmeticException ae)
  {
   System.out.println("divide by zero");
  }
  catch(ArrayIndexOutOfBoundsException e)
  {
   System.out.println("array index out of bound exception");
  }
 }
}
Output:
divide by zero

---

Example for Unreachable Catch block

While using multiple catch statements, it is important to remember that exception sub classes inside catchmust come before any of their super classes otherwise it will lead to compile time error.

class Excep
{
 public static void main(String[] args)
 {
  try
  {
   int arr[]={1,2};
   arr[2]=3/0;
  }
  catch(Exception e)    //This block handles all Exception
  {
   System.out.println("Generic exception");
  }
  catch(ArrayIndexOutOfBoundsException e)    //This block is unreachable
  {
   System.out.println("array index out of bound exception");
  }
 }
}

---

Nested try statement

try statement can be nested inside another block of try. Nested try block is used when a part of a block may cause one error while entire block may cause another error. In case if inner try block does not have a catchhandler for a particular exception then the outer try is checked for match.

class Excep
{
 public static void main(String[] args)
 {
  try
  {
   int arr[]={5,0,1,2};
   try
   {
    int x=arr[3]/arr[1];
   }
   catch(ArithmeticException ae)
   {
    System.out.println("divide by zero");
   }
   arr[4]=3;
  }
  catch(ArrayIndexOutOfBoundsException e)
  {
   System.out.println("array index out of bound exception");
  }
 }
}

---

Important points to Remember

1. If you do not explicitly use the try catch blocks in your program, java will provide a default exception handler, which will print the exception details on the terminal, whenever exception occurs.
2. Super class Throwable overrides toString() function, to display error message in form of string.
3. While using multiple catch block, always make sure that exception subclasses comes before any of their super classes. Else you will get compile time error.
4. In nested try catch, the inner try block, uses its own catch block as well as catch block of the outer try, if required.
5. Only the object of Throwable class or its subclasses can be thrown.

INTRODUCTION TO EXCEPTIONS

Exception Handling

Exception Handling is the mechanism to handle runtime malfunctions. We need to handle such exceptions to prevent abrupt termination of program. The term exception means exceptional condition, it is a problem that may arise during the execution of program. A bunch of things can lead to exceptions, including programmer error, hardware failures, files that need to be opened cannot be found, resource exhaustion etc.

---

Exception

A Java Exception is an object that describes the exception that occurs in a program. When an exceptional events occurs in java, an exception is said to be thrown. The code that's responsible for doing something about the exception is called an exception handler.

---

Exception class Hierarchy

All exception types are subclasses of class Throwable, which is at the top of exception class hierarchy.

• Exception class is for exceptional conditions that program should catch. This class is extended to create user specific exception classes.
• RuntimeException is a subclass of Exception. Exceptions under this class are automatically defined for programs.

---

Exception are categorized into 3 category.

• Checked Exception
The exception that can be predicted by the programmer.Example : File that need to be opened is not found. These type of exceptions must be checked at compile time.
• Unchecked Exception
Unchecked exceptions are the class that extends RuntimeException. Unchecked exception are ignored at compile time. Example : ArithmeticException, NullPointerException, Array Index out of Bound exception. Unchecked exceptions are checked at runtime.
• Error
Errors are typically ignored in code because you can rarely do anything about an error. Example : if stack overflow occurs, an error will arise. This type of error is not possible handle in code.

---

Uncaught Exceptions

When we don't handle the exceptions, they lead to unexpected program termination. Lets take an example for better understanding.

class UncaughtException
{
 public static void main(String args[])
 {
  int a = 0;
  int b = 7/a;     // Divide by zero, will lead to exception
 }
}

This will lead to an exception at runtime, hence the Java run-time system will construct an exception and then throw it. As we don't have any mechanism for handling exception in the above program, hence the default handler will handle the exception and will print the details of the exception on the terminal.

JAVA IMMUTABLE

1.1 Introduction to Java Immutable 

Immutable means that content of the object cannot be changed once it is created. In java, String is immutable and its content cannot be changed.

By the statement “content cannot be changed” we mean that whenever you try to update the content of a string, a new object of string is created and returned.

We know there are several methods available like concat which is used to update the String but such methods returns the new String.

1.2 Example

Let’s use concat method and we can see that initial string content will remain same.

public class Test {
    public static void main(String[] args) {    
        
        String str = "This is the intital String ";        
        System.out.println("INITIAL STRINg="+str);        
        String str1= str.concat("APPEND");        
        System.out.println("INITAL STRING PSOT CONCAT="+str);        
        System.out.println("RETURNED STRING =" + str1);        
    }    
}

Output

STRING BUILDERCLASS

StringBuilder class

StringBuilder is identical to StringBuffer except for one important difference it is not synchronized, which means it is not thread safe. Its because StringBuilder methods are not synchronised.

---

StringBuilder Constructors

1. StringBuilder ( ), creates an empty StringBuilder and reserves room for 16 characters.
2. StringBuilder ( int size ), create an empty string and takes an integer argument to set capacity of the buffer.
3. StringBuilder ( String str ), create a StringBuilder object and initialize it with string str.

---

Difference between StringBuffer and StringBuilder class

StringBuffer class	StringBuilder class
StringBuffer is synchronized.	StringBuilder is not synchronized.
Because of synchronisation, StringBuffer operation is slower than StringBuilder.	StringBuilder operates faster.

---

Example of StringBuilder

class Test {
 public static void main(String args[])
 {
  StringBuilder str = new StringBuilder("study");
  str.append( "tonight" );
  System.out.println(str);
  str.replace( 6, 13, "today");
  System.out.println(str);
  str.reverse();
  System.out.println(str);
  str.replace( 6, 13, "today");
 }
}

Output :studytonight 
        studyttoday
        yadottyduts

STRING BUFFERCLASS

StringBuffer class

StringBuffer class is used to create a mutable string object. It represents growable and writable character sequence. As we know that String objects are immutable, so if we do a lot of changes with String objects, we will end up with a lot of memory leak.

So StringBuffer class is used when we have to make lot of modifications to our string. It is also thread safe i.e multiple threads cannot access it simultaneously. StringBuffer defines 4 constructors. They are,

1. StringBuffer ( )
2. StringBuffer ( int size )
3. StringBuffer ( String str )
4. StringBuffer ( charSequence [ ]ch )

• StringBuffer() creates an empty string buffer and reserves room for 16 characters.
• stringBuffer(int size) creates an empty string and takes an integer argument to set capacity of the buffer.

---

Example showing difference between String and StringBuffer

class Test {
 public static void main(String args[])
 {
  String str = "study";
  str.concat("tonight");
  System.out.println(str);      // Output: study

  StringBuffer strB = new StringBuffer("study");
  strB.append("tonight");
  System.out.println(strB);    // Output: studytonight
 }
}

---

Important methods of StringBuffer class

The following methods are some most commonly used methods of StringBuffer class.

append()

This method will concatenate the string representation of any type of data to the end of the invokingStringBuffer object. append() method has several overloaded forms.

StringBuffer append(String str)

StringBuffer append(int n)

StringBuffer append(Object obj)

The string representation of each parameter is appended to StringBuffer object.

StringBuffer str = new StringBuffer("test");
str.append(123);
System.out.println(str);

Output : test123

---

insert()

This method inserts one string into another. Here are few forms of insert() method.

StringBuffer insert(int index, String str)

StringBuffer insert(int index, int num)

StringBuffer insert(int index, Object obj)

Here the first parameter gives the index at which position the string will be inserted and string representation of second parameter is inserted into StringBuffer object.

StringBuffer str = new StringBuffer("test");
str.insert(4, 123);
System.out.println(str);

Output : test123

---

reverse()

This method reverses the characters within a StringBuffer object.

StringBuffer str = new StringBuffer("Hello");
str.reverse();
System.out.println(str);

Output : olleH

---

replace()

This method replaces the string from specified start index to the end index.

StringBuffer str = new StringBuffer("Hello World");
str.replace( 6, 11, "java");
System.out.println(str);

Output : Hello java

---

capacity()

This method returns the current capacity of StringBuffer object.

StringBuffer str = new StringBuffer();
System.out.println( str.capacity() );

Output : 16

---

ensureCapacity()

This method is used to ensure minimum capacity of StringBuffer object.

StringBuffer str = new StringBuffer("hello");
str.ensureCapacity(10);

STRING CLASS FUNCTIONS

String class function

The following methods are some of the most commonly used methods of String class.

charAt()

charAt() function returns the character located at the specified index.

String str = "studytonight";
System.out.println(str.charAt(2));

Output : u

---

equalsIgnoreCase()

equalsIgnoreCase() determines the equality of two Strings, ignoring thier case (upper or lower case doesn't matters with this fuction ).

String str = "java";
System.out.println(str.equalsIgnoreCase("JAVA"));

Output : true

---

length()

length() function returns the number of characters in a String.

String str = "Count me";
System.out.println(str.length());

Output : 8

---

replace()

replace() method replaces occurances of character with a specified new character.

String str = "Change me";
System.out.println(str.replace('m','M'));

Output : Change Me

---

substring()

substring() method returns a part of the string. substring() method has two forms,

public String substring(int begin);

public String substring(int begin, int end); 

The first argument represents the starting point of the subtring. If the substring() method is called with only one argument, the subtring returned, will contain characters from specified starting point to the end of original string.

But, if the call to substring() method has two arguments, the second argument specify the end point of substring.

String str = "0123456789";
System.out.println(str.substring(4));

Output : 456789

System.out.println(str.substring(4,7));

Output : 456

---

toLowerCase()

toLowerCase() method returns string with all uppercase characters converted to lowercase.

String str = "ABCDEF";
System.out.println(str.toLowerCase());

Output : abcdef

---

valueOf()

Overloaded version of valueOf() method is present in String class for all primitive data types and for type Object.

NOTE : valueOf() function is used to convert primitive data types into Strings.

But for objects, valueOf() method calls toString() function.

---

toString()

toString() method returns the string representation of the object used to invoke this method. toString() is used to represent any Java Object into a meaningful string representation. It is declared in the Object class, hence can be overrided by any java class. (Object class is super class of all java classes.)

public class Car {
 public static void main(String args[])
 {
  Car c=new Car();  
  System.out.println(c);
 }
 public String toString()
 {
  return "This is my car object";
 }
}

Output : This is my car object

Whenever we will try to print any object of class Car, its toString() function will be called. toString() can also be used with normal string objects.

String str = "Hello World";
System.out.println(str.toString());

Output : Hello World

---

toString() with Concatenation

Whenever we concatenate any other primitive data type, or object of other classes with a String object,toString() function or valueOf() function is called automatically to change the other object or primitive type into string, for successful concatenation.

int age = 10;
String str = "He is" + age + "years old.";

In above case 10 will be automatically converted into string for concatenation using valueOf() function.

---

toUpperCase()

This method returns string with all lowercase character changed to uppercase.

String str = "abcdef";
System.out.println(str.toUpperCase());

Output : ABCDEF

---

trim()

This method returns a string from which any leading and trailing whitespaces has been removed.

String str = "   hello  ";
System.out.println(str.trim());

Output : hello

INTRODUCTION TO STRINGS

String is probably the most commonly used class in java library. String class is encapsulated underjava.lang package. In java, every string that you create is actually an object of type String. One important thing to notice about string object is that string objects are immutable that means once a string object is created it cannot be altered.

---

What is an Immutable object?

An object whose state cannot be changed after it is created is known as an Immutable object. String, Integer, Byte, Short, Float, Double and all other wrapper class's objects are immutable.

---

Creating an Immutable class

public final class MyString
{
 final String str;
 MyString(String s)
 {
  this.str = s;
 }
 public String get()
 {
  return str;
 }
}

In this example MyString is an immutable class. MyString's state cannot be changed once it is created.

---

Creating a String object

String can be created in number of ways, here are a few ways of creating string object.

1) Using a String literal

String literal is a simple string enclosed in double quotes " ". A string literal is treated as a String object.

String str1 = "Hello";

2) Using another String object

String str2 = new String(str1);

3) Using new Keyword

String str3 = new String("Java");

4) Using + operator (Concatenation)

String str4 = str1 + str2;
or,
String str5 = "hello"+"Java";

Each time you create a String literal, the JVM checks the string pool first. If the string literal already exists in the pool, a reference to the pool instance is returned. If string does not exist in the pool, a new string object is created, and is placed in the pool. String objects are stored in a special memory area known as string constant pool inside the heap memory.

---

String object and How they are stored

When we create a new string object using string literal, that string literal is added to the string pool, if it is not present there already.

String str= "Hello";

---

And, when we create another object with same string, then a reference of the string literal already present in string pool is returned.

String str2=str;

---

But if we change the new string, its reference gets modified.

str2=str2.concat("world");

---

Concatenating String

There are 2 methods to concatenate two or more string.

1. Using concat() method
2. Using + operator

1) Using concat() method

string s = "Hello";
string str = "Java";
string str2 = s.concat(str);
String str1 = "Hello".concat("Java");    //works with string literals too.

2) Using + operator

string str = "Rahul"; 
string str1 = "Dravid";
string str2 = str + str1;
string st = "Rahul"+"Dravid";

---

String Comparison

String comparison can be done in 3 ways.

1. Using equals() method
2. Using == operator
3. By CompareTo() method

Using equals() method

equals() method compares two strings for equality. Its general syntax is,

boolean equals (Object str)

It compares the content of the strings. It will return true if string matches, else returns false.

String s = "Hell";
String s1 = "Hello";
String s2 = "Hello";
s1.equals(s2);    //true
s.equals(s1) ;   //false

Using == operator

== operator compares two object references to check whether they refer to same instance. This also, will return true on successful match.

String s1 = "Java";
String s2 = "Java";
String s3 = new string ("Java");
test(Sl == s2)     //true
test(s1 == s3)      //false

By compareTo() method

compareTo() method compares values and returns an int which tells if the string compared is less than, equal to or greater than th other string. Its general syntax is,

int compareTo(String str)

To use this function you must implement the Comparable Interface. compareTo() is the only function in Comparable Interface.

String s1 = "Abhi";
String s2 = "Viraaj";
String s3 = "Abhi";
s1.compareTo(S2);     //return -1 because s1 < s2 
s1.compareTo(S3);     //return 0 because s1 == s3 
s2.compareTo(s1);     //return 1 because s2 > s1