Java Cheat Sheet

Java Cheat Sheet

• 1.
  Java Cheat Sheet Java Data Types byte / short / int / long ‑123, 10 float / double 235.13 char 'U' boolean true, false String "Greetings from earth" Java Statements If Statement if ( expression ) { statements } else if ( expression ) { statements } else { statements } While Loop while ( expression ) { statements } Do‑While Loop do { statements } while ( expression ); For Loop for ( int i = 0; i < max ; ++i) { statements } For Each Loop for ( var : collection ) { statements } Switch Statement switch ( expression ) { case value : statements break; case value2 : statements break; default: statements } Exception Handling try { statements; } catch (ExceptionType e1 ) { statements; } catch (Exception e2 ) { catch‑all statements; } finally { statements; } Java Data Conversions String to Number int i = Integer.parseInt(str ); double d = Double.parseDouble(str ); Any Type to String String s = String.valueOf(value ); Numeric Conversions int i = (int) numeric expression ; Java String Methods s .length() length of s s .charAt(i ) extract i th character s .substring(start , end ) substring from start to end ‑1 s .toUpperCase() returns copy of s in ALL CAPS s .toLowerCase() returns copy of s in lowercase s .indexOf(x ) index of first occurence of x s .replace(old , new ) search and replace s .split(regex ) splits string into tokens s .trim() trims surrounding whitespace s .equals(s2 ) true if s equals s2 s .compareTo(s2 ) 0 if equal/+ if s > s2/‑ if s < s2 java.util.ArrayList Methods l .add(itm ) Add itm to list l .get(i ) Return i th item l .size() Return number of items l .remove(i ) Remove i th item l .set(i , val ) Put val at position i ArrayList<String> names = new ArrayList<String>(); java.util.HashMap Methods m .put(key ,value ) Inserts value with key m .get(key ) Retrieves value with key m .containsKey(key ) true if contains key HashMap<String,String> names = new HashMap<String, String>(); Updated by Saeid Zebardast Twitter: @saeid Email: saeid.zebardast@gmail.com Home: http://zebardast.ir About: http://about.me/saeid Java Hello World import java.util.Date; public class Hello { public static void main(String[] args) { System.out.println("Hello, world!"); Date now = new Date(); System.out.println("Time: " + now); } } * Save in Hello.java * Compile: javac Hello.java * Run: java Hello Java Arithmetic Operators x + y add x ‑ y subtract x * y multiply x / y divide x % y modulus ++x / x++ increment ‑‑x / x‑‑ decrement Assignment shortcuts: x op = y Example: x += 1 increments x Java Comparison Operators x < y Less x <= y Less or eq x > y Greater x >= y Greater or eq x == Equal x != y Not equal y Java Boolean Operators ! x (not) x && y (and) x || y (or) Java Text Formatting printf style formatting System.out.printf("Count is %dn", count); s = String.format("Count is %d", count); MessageFormat style formatting s = MessageFormat.format( "At {1,time}, {0} eggs hatched.", 25, new Date()); Individual Numbers and Dates s = NumberFormat.getCurrencyInstance() .format(x); s = new SimpleDateFormat(""h:mm a"") .format(new Date()); s = new DecimalFormat("#,##0.00") .format(125.32); Source: http://cheatography.com/sschaub