An introduction to packages

1. An introduction to packages Session 9

2. Objectives • Explore the java.lang package • Discuss various Wrapper classes • Discuss the String and StringBuffer classes • Discuss the concept of immutability • Identify the methods of the following classes and interfaces • Math • System • Object • Class • ThreadGroup • Runtime An introduction to packages / 2 of 31

3. Code libraries • The basic idea behind using a code library is to sort out files or functions based on their functionality • Using these codes saves a lot of coding time • In C they are known as header files, in C++ as class libraries and in Java as packages • A library comprises a group of related files • For eg: a math library will contain functions or subroutines that are used in mathematical calculations An introduction to packages / 3 of 31

4. Creating packages in Java (1) • A package in Java can be created by including a package statement as the first statement in a Java program • Syntax to define a package is – • package <pkgname>; • In Java, a package is a combination of classes, interfaces and sub-packages • For eg: java.awt package has a sub-package called event An introduction to packages / 4 of 31

5. Example An introduction to packages / 5 of 31

6. Creating packages in Java (2) • Uses one of two elements to find a class – • The package name • The directories listed in the CLASSPATH environment variable • If no CLASSPATH is defined, then JVM looks for the default java\lib directory and the current working directory • When a Java program is executed, the JVM searches for the classes used within the program on the file system An introduction to packages / 6 of 31

7. Packages and access control(1) • A public member of a class can be accessed from anywhere; within the package, outside the package, within a subclass, as well as within a non-subclass • A member of a class that is declared private can be accessed only within the class but nowhere outside the class An introduction to packages / 7 of 31

8. Packages and access control(2) • A protected member of a class can be accessed from any class in the same package and from a subclass that is outside the package • If no access specifier is given, the member would be accessible within any class in the same package but not outside the package An introduction to packages / 8 of 31

9. Wrapper classes (1) • They encapsulate simple primitive types in the form of classes • Useful whenever we need object representations of primitive types • All numeric Wrapper classes extend the abstract superclass ‘Number’ • Wrapper classes are a part of java.lang package An introduction to packages / 9 of 31

10. Wrapper classes (2) • The six numeric Wrapper classes are Double, Float, Byte, Short, Integer and Long • Character is a wrapper class for the primitive char data type • Boolean is a wrapper class for boolean values • Constructor of Integer Wrapper class : • Integer(int value) • Integer(String a) throws NumberFormatException An introduction to packages / 10 of 31

11. Example (1) Output An introduction to packages / 11 of 31

12. Example (2) Output An introduction to packages / 12 of 31

13. String class (1) • Hence manipulation of strings will be done through the use of the methods provided by the String class • In Java, a string literal is an object of type String class An introduction to packages / 13 of 31

14. String class (2) • public String() – creates an empty string • public String(String value) – creates a new string that is a copy of the given string • public String(char[] value, int begin, int count) – this constructs a new string based on the character array starting from the position begin, which is count characters long • public String(StringBuffer buffer) – creates a new string based on a StringBuffer value • Some String class constructors: An introduction to packages / 14 of 31

15. String class (3) • The == operator and equals() method can be used for string comparison • The == operator checks if the two operands being used are one and the same object • The equals() method checks if the contents of the two operands are the same • String length() – this method determines the length of a string An introduction to packages / 15 of 31

16. Example Output An introduction to packages / 16 of 31

17. String class (4) • indexOf() method searches within a string for a given character or String • Syntax : • public int indexOf (int ch) • public int indexOf(Stringvalue) • Searching Strings An introduction to packages / 17 of 31

18. String class (5) • Extracting portions of a String • public char charAt(int index) – used to extract a single character from given position • public char[] toCharArray() – used to obtain an entire string in the form of character array • public String substring(int index) : used to extract portions of a string starting from position index • public String substring(int beginindex, int endindex) – used to extract portions of a string starting from beginindex position to endindex position An introduction to packages / 18 of 31

19. Example Output An introduction to packages / 19 of 31

20. Immutability • This is known as immutability in Strings • To overcome this, Java provides StringBuffer class • Strings in Java once created cannot be changed directly An introduction to packages / 20 of 31

21. Example An introduction to packages / 21 of 31

22. Math class • All methods of this class are static • The class is final and hence cannot be subclassed • This class defines methods for basic numeric operations as well as geometric functions + * / % - An introduction to packages / 22 of 31

23. Example Output An introduction to packages / 23 of 31

24. Runtime class • Used for memory management and executing additional processes • Every Java program has a single instance of this class • We can determine memory allocation details by using totalMemory() and freeMemory() methods • Encapsulates the runtime environment An introduction to packages / 24 of 31

25. System class • Provides means to access properties associated with the Java runtime system • Fields of this class are in, out and err that represent the standard input, output and error respectively • The System class cannot be instantiated to create objects • Provides facilities such as standard input, output and error streams An introduction to packages / 25 of 31

26. Example Output An introduction to packages / 26 of 31

27. Class • This allows us to retrieve information during runtime • There is no public constructor for Class • Instance of this class encapsulates the run time state of an object in a running Java application An introduction to packages / 27 of 31

28. Example Object An introduction to packages / 28 of 31

29. Object class • In the code above, we have not imported any class or package. Yet we can make use of the equals() method • Object class is the superclass of all classes • Even is a user-defined class does not extend any other class, it extends Object by default An introduction to packages / 29 of 31

30. Thread, ThreadGroup and Runnable • ThreadGroup is used to create a group of threads • Whenever it is necessary to manipulate a group of threads as a whole, it is handy to use the ThreadGroup class • Multithreading support in Java is provided by means of the Thread and ThreadGroup classes and the Runnable interface An introduction to packages / 30 of 31

31. Example Output An introduction to packages / 31 of 31