Java Script cont 2 2 Java Script Java

Java. Script (cont. 2)

2 Java. Script & Java: Similarities • JS (Java. Script) is case-sensitive • Operators – – – arithmetic: unary +, unary -, +, -, *, /, % bit-wise: ~ (unary), &, |, <<, >>> logical: ! (unary), &&, || strings: + (concatenation) compound assignment: • ++, --, • op= where op is one of the binary operators above, e. g. , +=, -=, |=, &&=, – relational: ==, !=, <, >, <=, >= – conditional: op 1 ? op 2 : op 3 (if op 1 then op 2 else op 3)

3 JS & Java: Similarities (cont. ) • Assignment with = • Syntax and semantic of control structures – – – if if-else shift-case while do-while for • except for-in, that's different • Block {…}

4 JS & Java: Similarities (cont. +) • Syntax of identifers – e. g. , index_1, the. Camel. Notation • Syntax of numeric and string constants: – e. g. , -40, 3. 14, 0. 1 E-7, 1. 2 e 2, Na. N, "a string", 0 x. FF 00 FF, 0 x 0 a 1 b 2 c • • Special characters: n, t, f , r, ", ', \, etc. Special values: null, true, false Subprogram calls: sub(p 1, p 2, …) Array initialization – e. g. : var cars = ["Mini", "Miata"]; • Array indices begin with 0, end with length-1 • Array access: cars[0]

5 JS & Java: Similarities (cont. ++) • • break statement continue statement return statement Subprograms can throw exceptions – exceptions are objects, can carry information – difference: subprograms don't declare what they throw • try-catch statement • Object instantiation: new Name(…) • All objects inherit method to. String(…)

6 JS & Java: Differences • • Type-less variables vs. strongly typed Interpreted vs. hybrid Functional features vs. purely imperative Syntax: declarations, subprograms, classes, … Additional statements Different scope rules Only few standard objects vs. rich set of supporting classes (JDK) Very different Object-Oriented Facilities – – object-prototype vs. class concept no encapsulation different inheritance different syntax

7 Variables • Variables can be declared with var – mandatory for locals – optional, but recommended for globals • Global variables are assigned outside of functions • Typing is implicit – values have types, not variables – a variable can hold a value of any type – the expression decides the type • Type conversion errors are detected at run-time!

8 Types • Types: – "primitive": • undefined, null, boolean, number, string – function, object • Operator typeof – returns "undefined", "boolean", "number", "string", "function" or "object" – null or arrays are also "object"s

9 Values • Strings can be enclosed in ' – 'the "other" way' • Special value: undefined – var a = [1, , , 2]; // a[1], a[2] are undefined – undefined can be tested • if (value) {use(value)} else {alert("undefined value"); } • In a condition – 0, "", null and undefined are interpreted as false – everything else is interpreted as true – Watch out: JS doesn't protect you against misuse of values in a condition!

10 Arrays • Array is an explicit object Array – e. g. : var cards = new Array(); – e. g. : var cards = new Array(12); //0. . 11 undefined – e. g. : var nos = new Array(1, 2, 3); //=[1, 2, 3] • Predefined property length • Assigning an element at index that didn't exist is ok – it extends the array • An associative array is not an Array object – indices can be strings – array notation used for accessing an object's properties • see later

11 Array methods • push(item) appends the item at index length • i. e. , increments length • pop() removes and returns the last item – decrements length • shift() removes and returns the first item – decrements length and indices of all other items • slice(from, [to]) returns the subarray from. . to-1 • concat(array) appends all items of array • splice(from, n, [it 1, it 2, …]) – replaces n items starting at from by items it 1, it 2, … • reverse(), sort(), to. String()

12 Statements • Statements don't need to be separated by; – but they can – it's recommended – ; is needed for several statements on one line - don't do that! • for-in statements assigns consecutive indices – e. g. : for (var i in an. Array) {alert(an. Array[i]); } • with statement – allows using properties of an object without object prefix – e. g. : with (document) { write("<h 1>Greetings</h 1>"); //= document. write() write("<p>Hi there!</p>"); }

13 Subprograms • Syntax – function name (…) {…} • Anonymous functions – name = function (…) {…} – called the same way: name(…) • Function body is stored as text – it can be retrieved and modified! • Functions are data – they can be assigned to variables, stored in an array, etc. • A function is an object, can be defined that way – var sum = new Function("a", "b", "return a+b"); • A function can also define a "class" (see later)

14 Function Parameters • It's ok to call a function with fewer parameters – you can check for undefined values – also, arguments property holds an array of all parameter values • Parameter passing – by value • numbers, strings, true, false, null, undefined – by reference • objects • Return values are passed the same way

15 Built-in Functions: Dialogs • alert(message) – displays a modal dialog with message and OK button – often used while debugging • e. g. , alert("variable=" + variable); – used to display multiple lines • e. g. , alert("line 1nline 2nline 3"); • confirm(message) – displays a modal dialog with message and with buttons OK and Cancel – returns true if the user clicked OK – returns false if the she clicked Cancel • prompt(message) – displays a modal dialog with message, input field and OK button – returns the text that the user typed into the input field

16 Built-in Functions (cont. ) • parse. Int(string) – attempts to convert the string into an integer number and return it – returns the integral part of a real number or Na. N if string isn't a number • parse. Float(string) – attempts to convert the string into an floating point number and return it – returns Na. N if string isn't a number • eval(expression) – interprets the expression as Java. Script code

17 Java. Script Pitfalls • Java. Script is "silent" – run-time errors stop script's execution – syntax errors may not matter if the code is not executed don't do that! • JS allows assignments within expressions – like Java – e. g. var x=++y – x=y--; //heck, what? – never use that!

18 Browser Pitfalls • IE just reports "runtime error" • Firefox error line numbers are off – check a line before or after • JS in IE is not the same as JS in Firefox is not the same as JS in Opera is not the same as … – in other words: var browsers = ["Firefox", "IE", "Opera", "Safari"]; var text="You have to test and test becausen"; for (var i = 0; i < browsers. length; i++) { text+="JS in "+browsers[i]+" isn't the same asn"; } alert (text+". . . ");

19 JS & Java • Facit: There a lot of similarities But despite these similarities JS is very different from Java