Object-oriented programming in...

Transcription

1 Programming Languages Week 12 Object-oriented programming in... College of Information Science and Engineering Ritsumeikan University

2 plan this week intro to Java advantages and disadvantages language features how it differs from Python basic syntax how it differs from Python exercises: write and run some simple programs next three weeks more language features and syntax exercises: develop some progams that are fun and useful 2

3 Java compared to some other OO languages Python Smalltalk Java development mixed interactive edit-compile-run testing mixed incremental whole program debugging backtrace interactive backtrace typing dynamic dynamic static encapsulation transparent opaque mixed portable sort of not so much yes deployment possible possible easy (desktop, web) popularity second low first great for portable app development (desktop, Android, web apps) getting a job lousy for rapid development casual progamming 3

4 smallest possible non-empty program language features in file MyApplication.java we have the following: class HelloWorld { public static void main(string[] args) { System.out.println("Hello, world"); all code must be in a class classes are only accessible within their own file, unless marked public only one public class allowed per file the name of the file must match the name of the public class methods are only accessible within their own file, unless marked public static methods are class methods (not instance methods) void means a method returns no result there must be one public class method called main where execution begins and it must accept one parameter, an array of Strings 4

5 block structure every statement in Java must end with a semicolon ; a block is a group of statements treated as one Python uses indentation to delimit blocks of statements while (i < 10): print i i += 1 Java uses curly braces around blocks while (i < 10) { System.out.println(i); i += 1; 5

6 Python uses dynamic typing static typing values have a type (integer, list, closure, etc.) types are checked when an operation is applied (e.g., + on integer, strings, etc.) variables, function arguments, function results can be of any type def twice(x): return x + x print twice(1), twice(2.2), twice("abc") Java uses static typing variables, function arguments, function results have a specific type types are checked at compile time attempts to use the wrong type are compile time errors int twice(int x) { return x + x; System.out.println(twice(1)); System.out.println(twice(2.2)); // error System.out.println(twice("abc")); // error 6

7 function overloading polymorphism is the ability of variables and functions to treat data of (m)any types Smalltalk has unlimited polymorphism: recevier type message selector behaviour Python has unlimited polymorphism for methods, limited for built-in operators Java has limited polymorphism a subtype (subclass) can be used where a supertype(class) is expected function overloading simulates polymorphism between unrelated types different implementations of the same function, with different types appropriate implementation chosen at compile time based on the type(s) of the argument(s) static int twice(int i) { return i+i; static double twice(double d) { return d+d; static String twice(string s) { return s+s; System.out.println(twice(1)); System.out.println(twice(2.2)); System.out.println(twice("abc")); 7

8 seven built-in primitive types language features integers boolean char short 16 int 32 long 64 1 bit 8 bits floating point 32 bits float double 64 converting to value of lower precision results in an error double d = ; // ok float f = ; // error (integer constant has type double) d = f; // error float g = f; // ok ( f suffix denotes float, not double) long l = 0; int i = l; // error types can be explicitly cast to a shorter type, avoiding the error long l = 0; int i = (int)l; // ok 8

9 array types arrays are written type[] and have elements of the given type arrays can be modified, but cannot change size array values are created with new type[size] array variables can be initialised with elements written inside {... get and set elements using the usual array syntax int[] array = { 2, 4, 6, 8, 10 ; System.out.println(array[2]); //=> 6 array[2] = 9; most programmers use ArrayLists instead flexible: similar interface to lists (OrderedCollection in Smalltalk) fast: uses an Array to implement the underlying storage import java.util.* ArrayList<String> strings = new ArrayList<String>(); strings.add("hello"); strings.add("world"); while (strings.isempty()) System.out.println(strings.remove(0)); 9

10 object types objects have methods and properties (attributes) just like in Python sometimes inconsistently class Main { public static void main(string[] args) { String s = "hello"; System.out.println(s.length()); // string has length() method System.out.println(args.length); // array has length property string literals have type String strings are immutable they cannot be modified individual characters can be accessed using str.charat(index) 10

11 // single-line comment /* multi-line comment */ if (condition) statement; comments and control structures if (condition) statement1; else statement2; while (condition) statement; do statement; while (condition); for (initialisation; condition; update) statement; // exactly equivalent to: initialisation; // <-- can be a variable declaration while (condition) { statement; update; for (type variable : arrayoftype) statement; // Python style loop 11

12 control structures multi-way branch based on an integer value switch (expression) { // jump to the case whose value == expression case value1: statements; break; case value2: statements; break;... default: statements; break; // or here, if no match break causes the nearest enclosing loop or switch to finish immediately 12

13 terminal input System.out provides output to the terminal System.in provides input from the terminal Scanner class (import from java.util) provides a convenient way to read input data wraps System.in and provides methods to read strings, integers, etc. import java.util.*; Scanner input = new Scanner(System.in); while (input.hasnext()) { String word = input.next();... // do something with your shiny new String 13

14 exercises Java practice: write Hello world program variables and loops nested loops overloaded functions user input using System.in and Scanner a useful program: count unique words in text 14