Collections

A Collection allows a group of objects to be treated as a single unit. Collections define a set of core interfaces. These are -

• Collection
• Set
• List
• SortedSet
• Map
• SortedMap

Collections also provide implementation for these interfaces.

Core Interfaces

The Object hierarchy of Core Interfaces defined in Collections is given below.Figure: Core Interfaces of Collections

Collection Interface

The Collection interface is the root of Collection hierarchy, and is used for common functionality across all collections. There is no direct implementation of Collection interface.

Set Interface

The Set interface is used to represent a group of unique elements. It extends the Collection interface. The class HashSet implements the Set interface.

SortedSet Interface

The SortedSet interface extends the Set interface. It provides extra functionality of keeping the elements sorted. So SortedSet interface is used to represent collections consisting of unique, sorted elements. The class TreeSet is an implementation of interface SortedSet.

List Interface

The list interface extends the Collection interface to represent sequence of numbers in a fixed order. Classes ArrayList, Vector and LinkedList are implementation of List interface.

Map Interface

The Map Interface is a basic interface that is used to represent mapping of keys to values. Classes HashMap and Hashtable are implementations of Map interface.

SortedMap Interface

The SortedMap Interface extends Map interface and maintains their mappings in key order. The class TreeMap implements SortedMap interface.

The table below gives the list of Collection interfaces and the classes that implement them.

Threads

A thread is in process in execution within a program. Within a program each thread defines a separate path of execution.

Creation of a thread

A thread can be created in two ways a) By implementing the Runnable interface. The Runnable interface consists of only one method – the run method. The run method has a prototype of public void run(); b) By extending the class Thread.

Execution of a thread

To execute a thread, the thread is first created and then the start() method is invoked on the thread. Eventually the thread would execute and the run method would be invoked. The example below illustrates the two methods of thread creation. You should note that the run method should not be invoked directly.

 public class ThreadExample extends Thread { public void run() { System.out.println("Thread started"); } public static void main(String args[]) { ThreadExample t = new ThreadExample(); t.start(); } } Example - Creation of Thread by extending the Thread class. 

When the run method ends, the thread is supposed to “die”. The next example shows the creation of thread by implementing the Runnable interface.

 public class ThreadExample2 implements Runnable { public void run() { .../* Code which gets executed when thread gets executed. */ } public static void main(String args[]) { ThreadExample2 Tt = new ThreadExample2(); Thread t = new Thread(Tt); t.start(); } } Example - Creating thread by implementing Runnable 

States of thread

A thread can be in one of the following states – ready, waiting for some action, running, and dead. These states are explained below. Running State A thread is said to be in running state when it is being executed. This thread has access to CPU. Ready State A thread in this state is ready for execution, but is not being currently executed. Once a thread in the ready state gets access to the CPU, it gets converted to running state. Dead State A thread reaches “dead” state when the run method has finished execution. This thread cannot be executed now. Waiting State In this state the thread is waiting for some action to happen. Once that action happens, the thread gets into the ready state. A waiting thread can be in one of the following states – sleeping, suspended, blocked, waiting for monitor. These are explained below.

Yielding to other processes

A CPU intensive operation being executed may not allow other threads to be executed for a “large” period of time. To prevent this it can allow other threads to execute by invoking the yield() method. The thread on which yield() is invoked would move from running state to ready state.

Sleep state of a thread

A thread being executed can invoke the sleep() method to cease executing, and free up the CPU. This thread would go to the “sleep” state for the specified amount of time, after which it would move to the “ready” state. The sleep method has the following prototypes.

public static void sleep (long millisec)
            throws InterruptedException;
public static void sleep (long millisec, int nanosec)
            throws InterruptedException;

Synchronized state

A code within the synchronized block is “atomic”. This means only one thread can execute that block of code for a given object at a time. If a thread has started executing this block of code for an object, no other thread can execute this block of the code (or any other block of synchronized code) for the same object.

public synchronized void synchExample() {
   /* A set of synchronized statements. Assume
      here that x is a data member of this class. */
   if(x == 0)
      x = 1;
}

File Handling and Input/Output

java.io package
Classes related to input and output are present in the JavaTM language package java.io . Java technology uses “streams” as a general mechanism of handling data. Input streams act as a source of data. Output streams act as a destination of data.

File class
The file class is used to store the path and name of a directory or file. The file object can be used to create, rename, or delete the file or directory it represents. The File class has the following constructors -
File(String pathname); // pathname could be file or a directory name
File(String dirPathname, String filename);
File(File directory, String filename);
<!–more–>
The File class provides the getName() method which returns the name of the file excluding the directory name.
String getName();

Byte Streams
The package java.io provides two set of class hierarchies – one for handling reading and writing of bytes, and another for handling reading and writing of characters. The abstract classes InputStream and OutputStream are the root of inheritance hierarchies handling reading and writing of bytes respectively.

read and write methods
InputStream class defines the following methods for reading bytes -
int read() throws IOException
int read(byte b[]) throws IOException
int read(byte b[], int offset, int length) throws IOException
Subclasses of InputStream implement the above mentioned methods.

OutputStream class defines the following methods for writing bytes -
void write(int b) throws IOException
void write(byte b[]) throws IOException
void write(byte b[], int offset, int length) throws IOException
Subclasses of OutputStream implement the above mentioned methods.

The example below illustrates code to read a character.
//First create an object of type FileInputStream type using the name of the file.
FileInputStream inp = new FileInputStream(“filename.ext”);
//Create an object of type DataInputStream using inp.
DataInputStream dataInp = new DataInputStream(inp);
int i = dataInp.readInt();

Reader and Writer classes
Similar to the InputStream and OutputStream class hierarchies for reading and writing bytes, Java technology provides class hierarchies rooted at Reader and Writer classes for reading and writing characters.

A character encoding is a scheme for internal representation of characters. Java programs use 16 bit Unicode character encoding to represent characters internally. Other platforms may use a different character set (for example ASCII) to represent characters. The reader classes support conversions of Unicode characters to internal character shortage. Every platform has a default character encoding. Besides using default encoding, Reader and Writer classes can also specify which encoding scheme to use.
The Reader class hierarchy is illustrated below.
The Writer class hierarchy is illustrated below.

The table below gives a brief overview of key Reader classes.
CharArrayReader The class supports reading of characters from a character array.
InputStreamReader The class supports reading of characters from a byte input stream. A character encoding may also be specified.
FileReader The class supports reading of characters from a file using default character encoding.

The table below gives a brief overview of key Writer classes.
CharArrayWriter The class supports writing of characters from a character array.
OutputStreamReader The class supports writing of characters from a byte output stream. A character encoding may also be specified.
FileWriter The class supports writing of characters from a file using default character encoding.

The example below illustrates reading of characters using the FileReader class.
//Create a FileReader class from the file name.
FileReader fr = new FileReader(“filename.txt”);
int i = fr.read(); //Read a character

Array Fundamentals
Arrays are used to represent fixed number of elements of the same type. The following are legal syntax for declaring one-dimensional arrays.

int anArray[];
int[] anArray;
int []anArray;

It is important to note that the size of the array is not included in the declaration. Memory is allocated for an array using the new operator as shown below.
anArray = new int[10];
The declaration and memory allocation may be combined together as shown below.
int anArray[] = new int[10];
The elements of the array are implicitly initialized to default values based on array types (0 for integral types, null for objects etc.). This is true for both local arrays as well as arrays which are data members. In this respect arrays are different from normal variables. Variable defined inside a method are not implicitly initialized, where as array elements are implicitly initialized.

Array Initializations
Arrays are initialized using the syntax below
int intArray[] = {1,2,3,4};
The length operator can be used to access the number of elements in an array (for example – intArray.length).

Multidimensional Arrays
The following are legal examples of declaration of a two dimensional array.
int[] arr[];
int[][] arr;
int arr[][];
int []arr[];

When creating multi-dimensional arrays the initial index must be created before a later index. The following examples are legal.

int arr[][] = new int[5][5];
int arr[][] = new int[5][];

The following example will not compile;
int arr[][] = new int[][5];

Class Fundamentals
A class defines a new type and contains methods and variables. The example below illustrates a simple class.

class City {
String name; // member variable
String getName() // member method
{
return name;
}
public static void main(String arg[]) {
}
}

Method overloading
JavaTM technology allows two methods to have the same name as long as they have different signatures. The signature of a method consists of name of the method, and count and type of arguments of the method. Thus as long as the argument types of two methods are different, they may be over-loaded (have the same name).

Class constructors
Constructors are member methods that have same name as the class name. The constructor is invoked using the new operator when a class is created. If a class does not have any constructors then Java language compiler provides an implicit default constructor. The implicit default constructor does not have any arguments and is of the type -
class_name() { }

If a class defines one or more constructors, an implicit constructor is not provided. The example below gives a compilation error.

class Test {
int temp;
Test(int x) {
temp = x;
}
public static void main() {
Test t = new Test(); /* This would generate a
compilation error, as there is no constructor
without any arguments. */
}
}

Operators and Assignments

Commonly used operators
Following are some of the commonly used JavaTM technology operators – Multiplication (*), Addition (+), Subtraction (-), logical and (&&) Conditional Operator ?:, Assignment (=), left shift (<<), right shift (>> and >>>), Equality comparison (==), Non-equality comparison (!=).

Conversion rules in Assignments
In the description below, I am giving basic conversion rules for assignment when source and destination are of different types.

If source and destination are of the same type, assignment happens without any issues.
If source is of smaller size than destination but source and destination are of compatible types, then no casting is required. Implicit widening takes place in this case. An example is assigning an int to a long.
If source and destination are of compatible types, but source is of larger size than destination, explicit casting is required. In this case, if no casting is provided then the program does not compile.

Floating point numbers
Decimal numbers (for example 1.3) are of type double by default. To make them of type float they must be followed by F (say, 1.3F).

The equality operator
The equality operator (==) when applied to objects return true if two objects have same reference value, false otherwise. The example below illustrates this –

String str1 = “first string”;
String str2 = new String(“first string”);
String str3 = “first string”;
boolean test1 = (str1 == str2);
boolean test2 = (str1 == str3);

In the example above, test1 is set to false because str1 and str2 point to different references. As str1 and str3 point to the same reference, test2 gets set to true. When a string is initialized without using the new operator, and with an existing string, then the new string also points to the first string’s location. So in the example above, str1 and str3 point to the same pool of memory and hence test2 gets set to true. The string str2 on the other hand is created using the new operator and hence points to a different block of memory. Hence test1 gets set to false.

The conditional operators && and ||
Operator && returns true if both operands are true, false otherwise. Operator || returns false if both operands are false, true otherwise. The important thing to note about these operators is that they are short-circuited. This means that the left operand is evaluated before the right operator. If the result of the operation can be evaluated after computing the left operand, then the right side is not computed. In this respect these operators are different from their bit-wise counterparts – bit-wise and (&), and bit-wise or (|). The bit-wise operators are not short-circuited. This means both the operands of bit-wise operator are always evaluated independent of result of evaluations.

Storing integral types
All the integer types in Java technology are internally stored in two’s complement. In two’s complement, positive numbers have their corresponding binary representation. Two’s complement representation of negative numbers is generated using the following three step process -

First get the binary representation of the number.
Then interchange zeros and ones in the binary representation.
Finally add one to the result. So for example two’s complement of -18 would be (assuming one byte representation) -
Converting 18 to binary — 0001 0010
Interchanging 0s and 1s — 1110 1101
Adding 1 — 1110 1110

So 1110 1110 would be binary representation of -18 using two bytes and using two’s complement representation.

The shift operators
The shift left operator in Java technology is “<<”. There are two operators for doing the right shift – signed right shift (>>) and zero fill right shift (>>>).

The left shift operator fills the right bits by zero. The effect of each left shift is multiplying the number by two. The example below illustrates this -

int i = 13; // i is 00000000 00000000 00000000 0000 1101
i = i << 2; // i is 00000000 00000000 00000000 0011 0100 After this left shift, i becomes 52 which is same as multiplying i by 4 Zero fill shift right is represented by the symbol >>>. This operator fills the leftmost bits by zeros. So the result of applying the operator >>> is always positive. (In two’s complement representation the leftmost bit is the sign bit. If sign bit is zero, the number is positive, negative otherwise.) The example below illustrates applying the operator >>> on a number.

int b = 13; // 00000000 00000000 00000000 0000 1101
b = b >>> 2; // b is now 00000000 00000000 00000000 0000 0011

So the result of doing a zero fill right shift by 2 on 13 is 3. The next example explains the effect of applying the operator >>> on a negative number.

int b = -11; //11111111 11111111 11111111 1111 0101
b = b >>> 2; // b now becomes 00111111 11111111 11111111 1111 1101

So the result of applying zero fill right shift operator with operand two on -11 is 1073741821.

Signed right shift operator (>>) fills the left most bit by the sign bit. The result of applying the signed shift bit has the same sign as the left operand. For positive numbers the signed right shift operator and the zero fill right shift operator both give the same results. For negative numbers, their results are different. The example below illustrates the signed right shift.

int b = -11; // 11111111 11111111 11111111 1111 0101
b = b >> 2; // 11111111 11111111 11111111 1111 1101 (2′s complement of -3)
// Here the sign bit 1 gets filled in the two most significant bits.

The new value of b becomes -3.

1. Identifiers are names of variables, functions, classes etc. The name used as an identifier must follow the following rules in Java^TMtechnology.
   • Each character is either a digit, letter, underscore(_) or currency symbol ($,¢, £ or ¥)
   • First character cannot be a digit.
   • The identifier name must not be a reserved word.
2. A keyword or reserved word in Java technology has special meaning and cannot be used as a user defined identifier. The list of keywords in Java technology is given below. It is important to completely remember this list as you can expect a question in Java Certification exam related to this.
   

   abstract	boolean	break	byte	case	catch
   char	class	const	continue	default	do
   double	else	extends	final	finally	float
   for	goto	if	implements	import	instanceof
   int	interface	long	native	new	null
   package	private	protected	public	return	short
   static	strictfp	super	switch	synchronized	this
   throw	throws	transient	try	void	volatile
   while	assert	enum

   
   It is important to note the following

   1. const and goto are not currently in use.
   2. null, true, and false are reserved literals but can be considered as reserved words for the purpose of exam.
   3. It is important to understand that Java language is case-sensitive. So even though super is a keyword, Super is not.
   4. All the Java technology keywords are in lower case.
   5. strictfp is a new keyword added in Java 1.2. assert is added in Java 1.4 and enum in Java 5.0
   6. The list of keywords as defined by Sun is present here.
3. A literal in Java technology denotes a constant value. So for example 0 is an integer literal, and ‘c’ is a character literal. The reserved literals true and false are used to represent boolean literals. “This is a string” is a string literal.
4. Integer literals can also be specified as octal (base 8), or hexadecimal (base 16). Octal and hexadecimal have 0 and 0x prefix respectively. So 03 and 0×3 are representation of integer three in octal and hexa-decimal respectively.
5. Java technology supports three type of comments
   1. A single line comment starting with //
   2. A multi-line comment enclosed between /* and */
   3. A documentation or javadoc comment is enclosed between /** and */. These comments can be used to generate HTML documents using the javadoc utility, which is part of Java language.
6. Java technology supports the following primitive types – boolean (for representing true or false), a character type called char, four integer types (byte, short, int and long) and two floating point types (float and double). The details of these types are given below -
   

   Data types	Width (in bytes)	Minimum value	Maximum Value
   byte	1	-27	27 – 1
   short	2	-215	215-1
   int	4	-231	231 – 1
   long	8	-263	263 – 1
   char	2	0×0	0xffff
   float	4	1.401298e-45	3.402823e+38
   double	8	4.940656e-324	1.797693e+308

7. Corresponding to all the primitive type there is a wrapper class defined. These classes provide useful methods for manipulating primitive data values and objects.
   

   Data types	Wrapper class
   int	Integer
   short	Short
   long	Long
   byte	Byte
   char	Character
   float	Float
   double	Double

8. Instance variables (data members of a class) and static variables are initialized to default values. Local variables (i.e. variables defined in blocks or inside member functions) are not initialized to default values. Local variables must be explicitly initialized before they are used. If local variables are used before initialization, compilation error gets generated. The defaults for static and instance variables are given in the table below.
   

   Data types	Default Values
   boolean	false
   char	‘\u0000′
   Integer types (byte, short, int, long)	0
   Floating types (float, double)	0.0F or 0.0 D
   Object References	null

    public static void main(String args[]) { int i; System.out.println(i); } 

   In this example printing of i generates a compilation error because local variable i is used before being initialized. The initialization of instance and static variables is an important concept both for understanding of Java language, and for Java Certification exam.

9. A Java source file has the following elements in this specific order.
   • An optional package statement. All classes and interfaces defined in the file belong to this package. If the package statement is not specified, the classes defined in the file belong to a default package. An example of a package statement is -
     package testpackage;
   • Zero or more import statements. The import statement makes any classes defined in the specified package directly available. For example if a Java source file has a statement importing the class “java.class.Button”, then a class in the file may use Button class directly without providing the names of the package which defines the Button class. Some examples of import statement are -
     import java.awt.*; // All classes in the awt package are imported.
import java.applet.Applet;
   • Any number of class and interface definitions may follow the optional package and import statements.

   If a file has all three of the above constructs, they must come in the specific order of package statement, one or more import statements, followed by any number of class or interface definitions. Also all the above three constructs are optional. So an empty file is a legal Java file.

10. The Java interpreter executes a method called main, defined in the class specified in command line arguments. The main method is the entry point for execution of a class. In Java technology the main method must have the following signature -
    public static void main(String args[])
    The java interpreter is invoked with the name of the class as an argument. The class name is followed by possible set of arguments for the main function of the class. When a Java program is invoked then the Java interpreter name “java” and the class name are not passed to the main() method of the class. The rest of the command line arguments are passed as an array of String. For example invoking java Sky blue gray
    would invoke main method of Sky class with an array of two elements – blue and gray