Java Fundamentals Cheat Sheet

A Programming › Java cheat sheet in English by sschaub

Java Fundamentals

197,967 views 56,571 downloads 17 Jul 2012 History

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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 ith 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

See http://docs.oracle.com/javase/6/docs/api/java/lang/String.html for more.

java.util.ArrayList Methods

l.add(itm)	Add itm to list
l.get(i)	Return ith item
l.size()	Return number of items
l.remove(i)	Remove ith item
l.set(i, val)	Put val at position i

ArrayList<String> names =
new ArrayList<String>();

See http://docs.oracle.com/javase/6/docs/api/java/util/ArrayList.html for more.

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<StÂrinÂg,String> names =
new HashMap<StÂrinÂg, String>();

See http://docs.oracle.com/javase/6/docs/api/java/util/HashMap.html for more.

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 == y	Equal	x != y	Not equal

Java Boolean Operators

! x (not)

x && y (and)

x || y (or)

Java Text Formatting

printf style formatting
System.out.printf("Count is %d\n", 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);

See http://docs.oracle.com/javase/6/docs/api/java/text/package-frame.html for MessageFormat and related classes

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Comments (5)

[deleted] 12:43 19 Sep 2013

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[deleted] 12:41 19 Sep 2013

try { Â ÂstaÂtemÂents; } catch (ExceÂptiÂonType e1) { Â ÂstaÂtemÂents; } catch (Exception e2) { Â ÂcatÂch-all statemÂents; } finally { Â ÂstaÂtemÂents; } We can write the above code like this also try { Â ÂstaÂtemÂents; } catch (ExceÂptiÂonType | Exception e1) { Â ÂstaÂtemÂents; } finally { Â ÂstaÂtemÂents; }

starius 05:37 30 Oct 2012

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starius 05:36 30 Oct 2012

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Matt 15:21 6 Aug 2012

This is exactly what I need, thank you