Java Cheat Sheet to prepare for exams and tests PDF

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Quick reference to go over concepts in Java programming....

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University of Virginia, Department of Computer Science

Java Cheat Sheet

Dr. Mark R. Floryan September 3, 2018

1 Java: General Things to Remember 1. All code must be inside of a class definition (except import and package statements). 2. Every line of code must end with a semi-colon. This excludes lines that formulate "blocks", like if(), while(), or class declarations. 3. The name of the class in a file must match the name of the file. For example, "public class LinkedList" must be in a file called "LinkedList.java" 4. Classes can contain a method "public static void main(String[] args)" as an entry point to the whole program. 5. Whitespace does NOT matter in java. The compiler will completely ignore all whitespace.

2 Primitive Data Types The primitive variable types are:

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int x = 5; // integers d ou bl e d = 3.4; // d ec im al val ue s char c = ’h ’; // c ha rac te rs . U se sin gl e q uotes . bo ol ean b = false ; // true or fa lse

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/* Othe r much less c om mo nl y use d */ byte b = 24 b; sho rt s = -8s ; long l = 2 0 0 0 0 0 0 ; flo at = 4. 56 7; Some examples of using these data types include:

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int x; x ++; x - -;

// au tom a tic al l y set to 0 by d e fa ul t // inc rem en t in te ger by 1 // d ec rem en t in te ger by 1

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int z = 14; 6

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int total = (x + z) * x; int re m a in d e r = x % z;

// e xp re s s io n s // rem a in d e r afte r x / z

/* Boo le an op e ra to rs */ bo ol ean b1 = false ; bo ol ean b2 = true ; bo ol ean res ult ;

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res ul t = b1 && b2 ; res ul t = b1 || b2 ; re su lt = ! b1 ;

// l ogic al AND // l ogic al OR // l ogic al ne ga tion

3 Input Output Input from the keyboard can be done like this: 1

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Sc an ner in = new Scan ne r ( System . in ); int x = in . nex tIn t () ; d ou bl e y = in . ne xtD ou bl e (); flo at f = in . ne xt Fl oa t (); bo ol ean b = in . ne xtB oo lea n (); long l = in . n ex tLo ng (); St ri ng s = in . ne xt ();

// make // re ad // re ad // re ad // re ad // re ad // re ad

a sca nn er objec t int f rom keyboa rd do uble fro m ke ybo ard f loa t f rom keybo ard bo ol f rom keybo a rd lo ng f rom keybo a rd string from keybo ard

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Input from a file can be done like this: 1

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Bu f fe re d R e ad e r in = new B uf fe re d R ea d e r ( new Fil e R e a d e r ( " in p u tf il e . txt" )); St ri ng text = in . re ad L ine (); // re ad s the next line in . clo se (); Output to the console is done like this:

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// p rin ts text co n ca ten a te d with x Sys te m . out . p rin t ( " The a nsw er is " + x ); // p rin ts and mo ves cu rso r to next line Sys te m . out . pri nt ln ( " so me th in g el se " ); Output to a file can be done like this:

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PrintW rite r ou tFil e = new P rin tW rite r ( new Fil e W rite r ( " o u tp u tf il e . txt " ))); ou tFile . print ( " Hel lo " ); ou tFile . p rintln ( " wor ld " ); ou tFile . cl ose ();

4 Strings Strings are reference types in Java (so they are NOT primitives). 1

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Strin g s1 = " Hell o " ; St ri ng s2 ; Strin g s3 = new String ( " Hi " );

// e xam pl e strin g // "" em pty strin g by d e fa ul t // Also make s a s tring

Common operators on strings include: 1

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Strin g re su lt ; res ul t = s1 + s2 ; s1 . le ng th () ; s1 . ch ar At (2); s1 . su bs tr ing (1 ,3); s1 . eq u als ( s2 ); s1 . t oU pp erC as e (); s1 . t oL ow erC as e ();

// " hi " + " th er e " = " hi t he re " // returns len gth of strin g // a c ce ss es the cha r at po s itio n 2 ( in d e xe d f rom 0) // part of s tring from ind e x 1 to 3 ( e xcl us ive ) // c om pa re strings us ing th is stru cture // returns the strin g as all up pe rc as e // returns the strin g as all lo we rc as e

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5 Converting Between Data Types In Java, we often need to convert between different types of variables. Here are some common conversions: 1

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/* int ( or any o th er pr im it iv e ) to st rin g */ int x = 5; Strin g s = "" + x; // "" + va riable c on ca te na tes as a strin g /* String ve rs io n of n umber to int or d ou ble */ int i = In te ge r . pa rs eInt ( " 1 23 " ); // c on ve rts strin g "1 23" into intege d ou bl e d = Dou bl e . p arse Dou ble ( " 3.14 " ) // co nverts string " 3.1 4" in to d o ubl /* d ivid ing integers d oes integer d ivis io n */ int x1 = 3; int x2 = 5; d ou bl e re su lt = x1 / x2 ; // 3 /5= ( int )0.6 = 0 res ul t = ( d ouble ) x1 / ( d ou ble ) x2 ; // 0.6 /* d ou ble to int */ d ou bl e x = 3.46 7; int y = ( int )x; // y is 3 , d ec ima l t run ca te d

6 Arrays Three primary ways to instantiate arrays: 1

d ou bl e [] d Array = new do uble [5]; int [] od d Num be rs = {1 ,3 ,5 ,7 ,9};

// kn ow the size , but not co nten ts // kn ow the con te nts al re ad y

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int x; /* Stuf f he re */ Strin g [] s Array = new S tr ing [ x ];

// use varia bl e to intial ize arra y

int [][] = new int [5 ][4];

// two - d im ens ion al array

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Some common things we do with arrays include: 2

x [3] = 5; x. len gth ;

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/* How to loop th ro ugh an a rra y */

// Acce ss array at p os itio n 3, set to 5. // get the n umber of el em en ts in array

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for ( int i =0; i < x . le ng th ; i ++){ Sys te m . out . pri nt ln ( x[ i ]); }

7 Java Math Library Java contains several functions inside the Math class that are useful. Among them: 1

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Math . abs ( x ); Math . max (a , b ); Math . min (a , b ); Math . sin ( the ta ); Math . cos ( the ta ); Math . tan ( the ta ); Math . toR ad ia ns ( d eg ); Math . toD eg re es ( rad ); Math . exp ( x ); Math . log ( x ); Math . pow (a , b ); Math . sq rt ( a ); Math . E; Math . PI ;

// a bs ol ute va lue of x // l arger of a and b // s ma ll er of a and b // sin trig fu n ctio n // cos trig fu n ctio n // ta ngen t trig fun c tion // co nvert d eg to rad ia ns // co nvert rad to d egre es // ra is es e ^x // na tu ra l lo ga rithm // raise a to pow er of b // sq u are root of a // va lue of c on stan t e // pi

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