1. CS 152: Programming Language Paradigms April 21 Class Meeting Department of Computer Science San Jose State University Spring 2014 Instructor: Ron Mak www.cs.sjsu.edu/~mak

2. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 2 Name Resolution and Overloading  Many languages allow you to redefine a name within the same scope. Example: Redefine the function max, each time with different parameter types or number of parameters. _

3. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 3 Java Name Resolution and Overloading public class OverloadMax { private int max(int x, int y) { System.out.print( "int max(int x, int y): "); return x > y ? x : y; } private double max(double x, double y) { System.out.print( "double max(double x, double y): "); return x > y ? x : y; } private int max(int x, int y, int z) { System.out.print( "int max(int x, int y, int z): "); return x > y ? (x > z ? x : z) : (y > z ? y : z); } } int max(int x, int y)  3 double max(double x, double y)  : 3.0 int max(int x, int y, int z)  4 double max(double x, double y)  3.0 public static void main(String[] args) { OverloadMax om = new OverloadMax(); System.out.println(om.max(2, 3)); System.out.println(om.max(2.0, 3.0)); System.out.println(om.max(2, 3, 4)); System.out.println(om.max(2, 3.0)); } (int) 2  (double) 2.0

4. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 4 C++ Operator Overloading  You can overload certain C++ operators such as +. At least one argument must be a user-defined type. char *concatenate(char *str1, char *str2) { char *cat = (char *) malloc(strlen(str1) + strlen(str2)); strcpy(cat, str1); strcat(cat, str2); return cat; } int main() { char *strA = concatenate("Hello", " "); char *strB = concatenate(strA, "world!"); cout << strB << endl; } Hello world!

5. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 5 C++ Operator Overloading, cont’d class MyString { public: char *data; MyString(char *str) { data = (char *) malloc(strlen(str)); strcpy(data, str); } }; char* operator + (MyString str1, MyString str2) { char *cat = (char *) malloc(strlen(str1.data) + strlen(str2.data)); strcpy(cat, str1.data); strcat(cat, str2.data); return cat; } Happy Easter! Rumor: A future version of Java will support operator overloading. int main() { MyString strX("Happy"); MyString strY(" "); MyString strZ("Easter!"); cout << strX + strY + strZ << endl; } Demo

6. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 6 Data Types  In a programming language, a data type defines: A set of values  Example: integer values, real values, string values A set of legal operations that can be performed on the values.  Example: integer + - * /  Example: string length, substring, concatenation, truncation _

7. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 7 Data Types, cont’d  What can have types? data values literals variables procedure and function parameters function return values _

8. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 8 Strongly Typed Language  In a strongly typed language, each variable has a type. The parser in the front end can perform static type checking. _

9. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 9 Advantages of Static Type Checking  Catch more errors during translation.  Improve program security and reliability.  Improve program readability.  Remove ambiguities.  Verify interface consistency and correctness.  Greater programmer discipline.  Generate more efficient object code. _

10. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 10 Weakly Typed Language  Some attributes of a weakly typed language: A variable does not have a type and therefore it can be bound to a value of any type at run time. Values have types. Type checking occurs at run time to ensure that operations have arguments of the proper types. Implicit type conversions can occur at run time. _

11. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 11 Advantages of Weakly Typed Languages  Programmers write less code.  Less burden on programmers. _

12. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 12 Simple Types  Scalar types Predefined types such as integer, real, character, boolean, etc.  Enumerated types The programmer explicitly lists an enumerated type’s values. Each value is named by an identifier. The values are ordered. Operations include predecessor and successor. Example (C): enum Color {RED, GREEN, BLUE};

13. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 13 Simple Types  Subrange types Values are a contiguous subset of the values of a base type, such as integer or enumeration. Operations are inherited from the base type. Examples (Pascal): TYPE Color = (red, green, blue, yellow, orange, purple); MyColor = blue..orange; SmallInt = 0..9;

14. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 14 Type Constructors  Programmers can create new types by composing other types. Array types Record or structure types  Operations include field selection. Classes  Fields and methods. Set types  Example (Pascal): TYPE Color = (red, green, blue, yellow, orange, purple); ColorSet = SET OF Color; VAR c : Color; warm, cool : ColorSet; BEGIN c := purple; warm := [red, orange]; cool := [yellow, blue]; IF c IN warm + cool {union} OR c IN warm * cool {intersection} OR [yellow, red] <= warm {contains} THEN... END.

15. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 15 Pointer Types  Represents the address of the value of a base type. Example (C): Variable pchar points to values of the base character type.  Some languages such as C allows explicit pointer arithmetic. Dangerous! char *pchar; pchar + 1;

16. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 16 Reference Types  Similar to a pointer type, except that the language does not allow the programmer to manipulate the pointer explicitly. No pointer arithmetic.  A Java variable of an object type has values that are references to objects of that type. _

17. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 17 Type Compatibility  Type A is assignment compatible with type B if: A value of type A can be assigned to a variable of type B.  Example (Java): Type int is assignment compatible with type float. You can assign an int value to a float variable.  Example (Java): Type int is not assignment compatible with type String. You cannot assign an int value to a String variable. A value of type A can be passed by value to a parameter of type B.

18. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 18 Type Compatibility, cont’d  Type A is comparison compatible with type B if a type A value can be compared to a type B value. Example: With some languages, strings of different lengths are comparison compatible. The shorter string is padded at the end with blanks at run time before the comparison is made. _

19. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 19 Polymorphism  An interface and some classes that implement it. public interface Food { public void prepare(); public void cook(); public void serve(); } public class Chicken implements Food { public void prepare() { /* preparing */ } public void cook() { /* cooking */ } public void serve() { /* serving */ } } public class PorkChops implements Food { public void prepare() { /* preparing */ } public void cook() { /* cooking */ } public void serve() { /* serving */ } } public class Broccoli implements Food { public void prepare() { /* preparing */ } public void cook() { /* cooking */ } public void serve() { /* serving */ } } Each class that implements interface Food must implement each of the interface’s declared methods. The interface serves as a contract for all users of the implementing classes.

20. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 20 Polymorphism  Variable food has type Food (an interface). A variable can have an interface type.  Its value is a reference to an object of type Broccoli. A value can have an object type but never an interface type. public interface Food { public void prepare(); public void cook(); public void serve(); } public class Broccoli implements Food { public void prepare() { /* preparing */ } public void cook() { /* cooking */ } public void serve() { /* serving */ } } Food food = new Broccoli(); food.prepare(); food.cook(); food.serve(); Type Food Type Broccoli

21. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 21 Polymorphism, cont’d  Because variable food is of the interface type, its value can be a reference to a Broccoli, Chicken, or PorkChop object.  Due to polymorphism, the correct prepare(), cook(), or serve() method is called at run time depending on the type of the value, not the type of the variable. public interface Food { public void prepare(); public void cook(); public void serve(); } public class Chicken implements Food { public void prepare() { /* preparing */ } public void cook() { /* cooking */ } public void serve() { /* serving */ } } Food food = new Broccoli(); food.prepare(); food.cook(); food.serve(); Type Food Type Broccoli

22. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 22 Array1: Sort an Array of Integers  Main:  Print: public static void main(String[] args) { int numbers[] = new int[] {5, 1, 9, 4, 5, 0, 7, 6}; System.out.print("Before sorting:"); print(numbers); sort(numbers); System.out.print(" After sorting:"); print(numbers); } private static void print(int elements[]) { for (int elmt : elements) { System.out.print(" " + elmt); } System.out.println(); } Primitive int data.

23. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 23 Array1: Sort an Array of Integers, cont’d  Simple exchange sort algorithm: private static void sort(int elements[]) { for (int i = 0; i < elements.length-1; i++) { for (int j = i+1; j < elements.length; j++) { if (elements[j] < elements[i]) { int temp = elements[i]; elements[i] = elements[j]; elements[j] = temp; } } } }

24. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 24 Array2: Use an ArrayList  Main:  An ArrayList can only store objects of a reference type such as Integer, not primitive type data such as int. public static void main(String[] args) { ArrayList numbers = new ArrayList(); numbers.add(new Integer(5)); numbers.add(new Integer(1)); numbers.add(new Integer(9)); numbers.add(new Integer(4)); numbers.add(new Integer(5)); numbers.add(new Integer(0)); numbers.add(new Integer(7)); numbers.add(new Integer(6)); System.out.print("Before sorting:"); print(numbers); sort(numbers); System.out.print(" After sorting:"); print(numbers); } Integer objects.

25. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 25 Array2: Use an ArrayList, cont’d  Print:  A “raw” ArrayList stores Object data. Object is the base of all Java reference types. Therefore, we must coerce each Object element to Integer with a type cast: Class Integer has an intValue() method: private static void print(ArrayList elements) { for (Object elmt : elements) { System.out.print(" " + ((Integer) elmt).intValue()); } System.out.println(); } (Integer) elmt ((Integer) elmt).intValue()

26. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 26 Array2: Use an ArrayList, cont’d  Sort: private static void sort(ArrayList elements) { for (int i = 0; i < elements.size()-1; i++) { for (int j = i+1; j < elements.size(); j++) { if (((Integer) elements.get(j)).intValue() < ((Integer) elements.get(i)).intValue()) { Object temp = elements.get(i); elements.set(i, elements.get(j)); elements.set(j, temp); } } } } Type coercion needed!

27. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 27 Dangers of Using Raw ArrayList  Since a raw ArrayList holds Object data, and Object is the root of all Java reference types, nothing prevents us from doing this:  What happens at run time? ArrayList numbers = new ArrayList(); numbers.add(new Integer(5)); numbers.add(new Integer(1)); numbers.add(new Integer(9)); numbers.add(new Integer(4)); numbers.add(new Date()); numbers.add(new Integer(0)); numbers.add(new Integer(7)); numbers.add(new Integer(6));

28. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 28 Array3: Use ArrayList public static void main(String[] args) { ArrayList numbers = new ArrayList<>(); numbers.add(new Integer(5)); numbers.add(new Integer(1)); numbers.add(new Integer(9)); numbers.add(new Integer(4)); numbers.add(new Integer(5)); numbers.add(new Integer(0)); numbers.add(new Integer(7)); numbers.add(new Integer(6)); System.out.print("Before sorting:"); print(numbers); sort(numbers); System.out.print(" After sorting:"); print(numbers); } Now the compiler will prevent us from adding anything other than Integer data to the array list.

29. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 29 Array3: Use ArrayList, cont’d private static void print(ArrayList elements) { for (Integer elmt : elements) { System.out.print(" " + elmt.intValue()); } System.out.println(); } private static void sort(ArrayList elements) { for (int i = 0; i < elements.size()-1; i++) { for (int j = i+1; j < elements.size(); j++) { if (elements.get(j).intValue() < elements.get(i).intValue()) { Integer temp = elements.get(i); elements.set(i, elements.get(j)); elements.set(j, temp); } } } } We no longer need to coerce element data to Integer because that’s the only allowable data type in ArrayList.

30. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 30 Boxing and Unboxing  Boxing We “wrap” the primitive int value 3 inside an Integer object:  Unboxing We “unwrap” a primitive int value from an Integer object:  Java does autoboxing/unboxing as necessary, so we don’t have to explicitly do it in our code. int i = obj.intValue() Integer obj = new Integer(3);

31. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 31 Array4: Autobox/Unbox public static void main(String[] args) { ArrayList numbers = new ArrayList<>(); numbers.add(5); numbers.add(1); numbers.add(9); numbers.add(4); numbers.add(5); numbers.add(0); numbers.add(7); numbers.add(6); System.out.print("Before sorting:"); print(numbers); sort(numbers); System.out.print(" After sorting:"); print(numbers); } Java will autobox each int value to an Integer object before adding it to the array list.

32. SJSU Dept. of Computer Science Spring 2014: April 21 CS 152: Programming Language Paradigms © R. Mak 32 Array4: Autobox/Unbox, cont’d private static void print(ArrayList elements) { for (Integer elmt : elements) { System.out.print(" " + elmt); } System.out.println(); } private static void sort(ArrayList elements) { for (int i = 0; i < elements.size()-1; i++) { for (int j = i+1; j < elements.size(); j++) { if (elements.get(j) < elements.get(i)) { Integer temp = elements.get(i); elements.set(i, elements.get(j)); elements.set(j, temp); } } } } Auto-unbox the int value from an Integer object. Auto-unbox the int value from an Integer object.