Exercise on Java Basics

1. Exercise on Java Basics

2. Complete and Correct this Code public _________ Coin { private int value; public Coin( int value ) { ____________________ ; } public void getValue( ) { return value; } public String toString( ) { System.out.println( value . "-Baht"); }

3. Packaging and Commenting Code package coinpurse; /** * Coin represents money with an integer value. * @author Bill Gates */ public class Coin { private int value; /** * Initialize a new coin object. * @param value is the value of the coin */ public Coin( int value ) { this.value = value; }

4. Packages • Java uses packages to organize classes. • Packages reduce size of name space and avoid name collisions Example: The Java 8 API has... 2 classes named "Date" 1 class and 4 interfaces named "Element" 1 class and 1 interface named "List" (java.util.List & java.awt.List)

5. Name these Packages

6. Package you never need to import

7. Package Name using Domain Name The domain for the JUnit testing framework is: http://junit.org The Java package for JUnit is org.junit What should be the Java package for each of these?

8. Resolving Ambiguity: Date class There is a Date class in java.utilandjava.sql In this code, whichDate class will Java use? • java.util.Date (because it is imported first) • java.sql.Date (because it is imported last) • depends on Java implementation • compile error import java.util.*; // has java.util.Date import java.sql.*; // has java.sql.Date class Ambiguous { Date today = new Date( ); ... }

9. Resolving Ambiguity (2) • How can you fix this problem (2 solutions)? Solution 1: Solution 2: import java.util.*; import java.sql.*; // (1) class Ambiguous { // (2) __________ today = new _____________; }

10. Resolving Ambiguity (3) • How can you resolve this problem (2 solutions)? Solution 1: import a specific class Solution 2: use the full path in Java code import java.util.*; import java.sql.*; import java.util.Date; // Solution (1) class Ambiguous { java.util.Date today = new java.util.Date( ); // (2) ... }

11. How to convert a String to a number? How to get the "int" value from String s? String s = "1234"; // At least 3 possible solutions: int n1 = int n2 = Scanner scanner = int n3 = Methods in the Integer class can also be used to evaluate strings with different base (radix) such as binary or hexadecimal. Example: Integer.parseInt( "4AE7", 16)

12. How to convert number to String? How to create a String for a number n? int n = 100; String s = n; // error: must convert to string // At least 4 possible solutions: String s1 = String s2 = String s3 = String s4 =

13. How to convert number to String? How to convert a number n to a String? int n = 100; String s = n; // error: must convert to string // At least 4 possible solutions: String s1 = Integer.toString( n ); String s2 = "" + n; String s3 = String.valueOf( n ); String s4 = String.format( "%d", n );

14. String Concatenation What is output by each of these? Why? int m = 11; int n = 22; System.out.println( m + n ); System.out.println( "The sum is " + m + n ); System.out.println( m + n + " is the sum" ); System.out.println( "The sum is " +(m + n) );

15. Java Primitive Data Types • Java has 8 primitive data types • These are not classes or objects. No properties. NameValuesExamples true falsetrue, false character'a', 'A', '1', 'ก', 'ค', '\t' 8-bit integer-127, ..., -1, 0, 1, ..., 127 16-bit integer-32768 ... 0 ... 32767 32-bit integer-400 47 20000000 64-bit integer-1234567890L 0L 888L 32-bit decimal3.14159F 0.0F -2.5E-8F 64-bit decimal3.14159265358979E234

16. Write an equals( ) method for Coin equals is true only if: 1) parameter is a Coin 2) parameter (as Coin) has same value as this Coin public class Coin { private int value; public boolean equals( Object arg ) { //1. test for null //2. test if arg is really a Coin //3. cast arg to type Coin //4. compare the values }

17. How to Convert Primitive to Object? A List (like ArrayList) can only contain objects. How can we convert a primitive type to an object? List mylist = new ArrayList( ); int id = 51651111; // this works in Java 5+, because java converts it mylist.add( id ); // How to convert id to an object type?

18. Wrapper Classes Primitive Wrapper boolean Boolean char Character byte Byte short Short int Integer long Long float Float double Double double root = Math.sqrt( 2.0 ); Double d1 = new Double( root ); // same thing: automatic boxing Double d2 = root; // print as a string out.println( d2.toString( ) ); // static method to make a string out.println( Integer.toString( 2 ) );

19. Wrapper to convert to/from String int n = 2*3*5*7*11*13*17*19*23*29*31; // how to convert n to a String? String product =_______________________; String s = "2.5"; // how to convert s to a double? double d = __________________________;

20. Range limits for numeric types • What is the largest "int" value? • What is the smallest "long" value? • What is the range (smallest, biggest) of double? int biggest = long smallest = double minsize = double maxsize =

21. What happens beyond the limit? int n = Integer.MAX_VALUE; n = n + 1; System.out.println( n ); double d = Double.MAX_VALUE; d = d + 1; System.out.println( d ); d = d * 1.000001; System.out.println( d );

22. C# is different • "int", "float", "double" are struct types. // This is C# int n = int.MaxValue; String s = "The biggest integer is " + n.ToString( ) ; // range checking is enforced n = n + 1; System.OverflowException: Arithmetic operation resulted in an overflow.

23. How to parse? How can you read 2 integers from this String? // String containing some integers String s = "12 45"; // How to get the integers from s?

24. Careful parsing A String contains some words, but we don't know how many. How would you parse the words? // String containing some words String s = "Java 8 is complicated"; // How to get each word from s?

25. 3 ways to parse a String 1. String.split( regex ) String [] args = s.split("\\s+"); // splits the string at whitespace chars 2. java.util.Scanner(powerful but slow) Scanner in = new Scanner( s ); while ( in.hasNext( ) ) String arg = in.next( ); 3. StringTokenizer (old but faster) StringTokenizer tokens = new StringTokenizer(s); while ( tokens.hasMoreElements( ) ) String arg = tokens.nextToken( );