CSC Java Programming, Spring, 2010 Week 2: Java Data Types, Control Constructs, and their C++ counterparts

Summary of Java built-in types Primitive type variables are not class objects byte is 8-bit signed, -128 through 127 (page 32 in text) short is 16-bit signed, through int is 32-bit signed, long is 64-bit signed float is 32-bit type and double is 64-bit type boolean is 1-bit true or false, char is UNICODE Some class types are built into the language java.lang.String a class built into the language Wrapper classes in java.lang wrap native types as classes. Examples are Integer, Float, Long, Double.

Summary of Java control constructs boolean (true or false) is the basis for control flow /export/home/faculty/parson/JavaLang/week1/bool for, while and do-while loops, just like C++ if.. else if.. else selection, just like C++ switch statement (text p. 86) also like C++ break exits the innermost loop or switch continue goes to the top of innermost loop There are no stand-alone functions! A method always exists inside a class. A non-static method uses a class object. A static method uses only class-static data.

Java Methods A method is equivalent to a C++ member function. In Java all methods resides in classes. A static method does not require an object. int intvar = java.lang.Integer.parseInt(“2”); System.out.println(“integer: “ + intvar); Statics method use static class variables. A regular method needs an object reference. Integer intObject = new Integer(“3”); System.out.println(“integer: “ + intObject.intValue());

Java storage of class and object data Only one copy of a static data element exists for its class. Both static methods and non- static methods (object methods) can use it. static java.lang.String oneStringPerClass = new String(); There is one copy of each non-static data element in each object. Only non-static methods can use it. String oneStringPerObject = new String(“my value”);

Java access to class and object methods and data Public methods and data can be used by any code that imports the class. Protected methods and data can be used only by the defining class and derived classes. Private methods and data can be used only by the defining class. If there is no explicit access restriction, then package methods and data can be used by any class in the package.

First example Java program CountArgString counts distinct, non- overlapping occurrences of a string in a file /export/home/faculty/parson/JavaLang/countargs Make sure you “cp –pr ~parson/JavaLang ~/JavaLang” to get JavaLang into your home directory Make sure CLASSPATH include your $HOME/JavaLang makefile drives execution of compile and test steps gmake build to compile Java files gmake test to run test cases gmake clean to clean up test output and compiled files

Boolean example code /export/home/faculty/parson/JavaLang/bool Make sure you have a copy of this. cp –pr ~parson/JavaLang/bool ~/JavaLang/bool Program verifies valid command line args. 1. Program reads command line into an array of floats, raising an Exception if any of the command line arguments is not a float. 2. Verify that array is in ascending order using &&. 3. Verify that array is in ascending order using ||.

Booleans Boolean variables and constants boolean isSortedAnd = true, isOutOfOrderOr = false ; Boolean expressions isOutOfOrderOr || ! isLessThanOrEqual(numbers[i], numbers[i+1]) Condition control constructs use booleans if (isSortedAnd != ! isOutOfOrderOr) { Conditional expressions use booleans System.exit(isSortedAnd ? 0 : 1);

Class java.lang.System System.in is equivalent to C++ cin. import java.util.Scanner ; Scanner inputScanner = new Scanner(System.in); System.out and System.err are equivalent to C++ cout and cerr. System.out.println("sort result = " + isSortedAnd); System.exit(0) causes the process to exit with status of 0, meaning there is no error. A non-0 exits status signifies an error.

Creating Java Arrays Arrays are constructed using new (like C++). “float [] numbers ;” creates a null array reference. float [] numbers = new float [ INTEGER_SIZE ] ; Java implicitly initializes an array of numbers with 0. Booleans gets false, chars get ‘\0000’. float [] numbers = {1.9F, 2.9F, 3.4F, 3.5F} ; Myclass [] myobject = new Myclass[ INTEGER_SIZE ] ; Java initializes an array of object references with null. Java objects and arrays created using “new” need not be “deleted.” JAVA HAS GARBAGE COLLECTION!

Using Java Arrays “String [] args” is the parameter for Java main. public static void main(String args[]) { main is needed for running a class from the UNIX command line. Expression “numbers.length” gives the number of elements in the array. if (args.length == 0) { float [] numbers = new float [ args.length ] A subscript access and element of an array numbers[i] = Float.parseFloat(args[i]);

Helper class java.util.Arrays Utility methods for manipulating arrays. public static int[] copyOf(int[] original, int newLength); » First available in Java 1.6, not 1.5 on Mac OSX. public static void fill(int[] a, int val); public static void sort(int[] a); public static int binarySearch(int[] a, int key); public static T[] copyOf(T[] original, int newLength); public static void sort(T[] a, Comparator c) ;Comparator Comparator int compare(T o1, T o2) returns -1 or 0 or 1 for o1 o2 respectively.T

Multidimensional Arrays int [][] matrix ; // an array of arrays, null int [][] matrix = new int[4][3]; // 4 rows of 3 int [][] matrix = { // See Figure 6.11, p. 204 {1, 2, 3}, {4, 5, 6}, {7, 8, 9}, {10, 11, 12} }; matrix[0][0] is the initial element of the initial row matrix[3][2] is the last element of the last sub-array (last column of the last row)

“Ragged Arrays” int [][] triangle = { // p. 205 {1, 2, 3, 4, 5}, {2, 3, 4, 5}, {3, 4, 5}, {4, 5}, {5} }; int [][] triangle = new int [5][]; for (int i = 0 ; i < triangle.length ; i++) { triangle[i] = new int [triangle.length – i]; } triangle[1].length gives the length of the second sub-array triangle[1], which == 4.

Handling an exception from a library method Read on-line doc on java.lang.Float.parseFloat try { numbers[i] = Float.parseFloat(args[i]); } catch (NumberFormatException nfexp) { System.err.println("format error on " + args[i] + ": " + nfexp.getMessage()); isSortedAnd = false ; isOutOfOrderOr = true ; // Initialize array element to a default value. numbers[i] = 0.0F ; }

Notes on the makefile If you run a test that you intend to fail – System.exit(NON-0) – then put a “-” in front of that command invocation in the makefile. -java $(PACKAGE).$(BASENAME) fred Also, since your error messages are sent to System.err, you need to redirect System.err to a file to capture its output to use with diff. >> $(BASENAME).out 2>&1 >> $(BASENAME).out 2>$(BASENAME).err

Programming practices Always handle exceptions! We may handle some by explicitly ignoring them. Always use { curly braces } for control blocks. Use coding standards that we discuss in class. Write Javadoc documentation. Use both healthy and degenerate tests. » Ignoring these rules will cost you points.