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Refcard #024

Core Java

A Power-User's Guide to Java

Written by

Cay Horstmann Professor, SJSU
Ivan Ivanov Senior Developer, SAP Labs Bulgaria

Gives you an overview of key aspects of the Java language and references on the core library, commonly used tools, and new Java 8 features.

Free PDF
Section 1

About Core Java

This Refcard gives you an overview of key aspects of the Java language and cheat sheets on the core library (formatted output, collections, regular expressions, logging, properties) as well as the most commonly used tools (javac, java, jar).

Section 2

Java Keywords

Keyword Description Example
abstract an abstract class or method abstract class Writable { public abstract void write(Writer out); public void save(String filename) { ... } }
assert with assertions enabled, throws an error if condition not fulfilled assert param != null; Note: Run with -ea to enable assertions
boolean the Boolean type with values true and false boolean more = false;
break breaks out of a switch or loop while ((ch = in.next()) != -1) { if (ch == '\n') break; process(ch); } Note: Also see switch
byte the 8-bit integer type byte b = -1; // Not the same as 0xFF Note: Be careful with bytes < 0
case a case of a switch see switch
catch the clause of a try block catching an exception see try
char the Unicode character type char input = 'Q';
class defines a class type class Person { private String name; public Person(String aName) { name = aName; } public void print() { System.out.println(name); } }
continue continues at the end of a loop while ((ch = in.next()) != -1) { if (ch == ' ') continue; process(ch); }

1) the default clause of a switch

2) denotes default implementation of an interface method

1) see switch

public interface Collection<E> {
       default Spliterator<E> spliterator() {
          return Spliterators.spliterator(this, 0);

do the top of a do/while loop do { ch = in.next(); } while (ch == ' ');
double the double-precision floating-number type double oneHalf = 0.5;
else the else clause of an if statement see if
enum an enumerated type enum Mood { SAD, HAPPY };
extends defines the parent class of a class class Student extends Person { private int id; public Student(String name, int anId) { ... } public void print() { ... } }
final a constant, or a class or method that cannot be overridden public static final int DEFAULT_ID = 0;
finally the part of a try block that is always executed see try
float the single-precision floating-point type float oneHalf = 0.5F;
for a loop type for (int i = 10; i >= 0; i--) System.out.println(i); for (String s : line.split("\\s+")) System.out.println(s); Note: In the "generalized" for loop, the expression after the : must be an array or an Iterable
if a conditional statement if (input == 'Q') System.exit(0); else more = true;
implements defines the interface(s) that a class implements class Student implements Printable { ... }
import imports a package import java.util.ArrayList; import com.dzone.refcardz.*;
instanceof tests if an object is an instance of a class if (fred instanceof Student) value = ((Student) fred).getId(); Note: null instanceof T is always false
int the 32-bit integer type int value = 0;
interface an abstract type with methods that a class can implement interface Printable { void print(); }
long the 64-bit long integer type long worldPopulation = 6710044745L;
native a method implemented by the host system
new allocates a new object or array Person fred = new Person("Fred");
null a null reference Person optional = null;
package a package of classes package com.dzone.refcardz;
private a feature that is accessible only by methods of this class see class
protected a feature that is accessible only by methods of this class, its children, and other classes in the same package class Student { protected int id; ... }
public a feature that is accessible by methods of all classes see class
return returns from a method int getId() { return id; }
short the 16-bit integer type short skirtLength = 24;
static a feature that is unique to its class, not to objects of its class public class WriteUtil { public static void write(Writable[] ws, String filename); public static final String DEFAULT_EXT = ".dat"; }
strictfp Use strict rules for floating-point computations
super invoke a superclass constructor or method public Student(String name, int anId) { super(name); id = anId; } public void print() { super.print(); System.out.println(id); }
switch a selection statement switch (ch) { case 'Q': case 'q': more = false; break; case ' '; break; default: process(ch); break; } Note: If you omit a break, processing continues with the next case.
synchronized a method or code block that is atomic to a thread public synchronized void addGrade(String gr) { grades.add(gr); }
this the implicit argument of a method, or a constructor of this class public Student(String id) {this.id = id;} public Student() { this(""); }
throw throws an exception if (param == null) throw new IllegalArgumentException();
throws the exceptions that a method can throw public void print() throws PrinterException, IOException
transient marks data that should not be persistent class Student { private transient Data cachedData; ... }
try a block of code that traps exceptions try { try { fred.print(out); } catch (PrinterException ex) { ex.printStackTrace(); } } finally { out.close(); }
void denotes a method that returns no value public void print() { ... }
volatile ensures that a field is coherently accessed by multiple threads class Student { private volatile int nextId; ... }
while a loop while (in.hasNext()) process(in.next());
Section 3

Standard Java Packages

java.applet Applets (Java programs that run inside a web page)
java.awt Graphics and graphical user interfaces
java.beans Support for JavaBeans components (classes with properties and event listeners)
java.io Input and output
java.lang Language support
java.math Arbitrary-precision numbers
java.net Networking
java.nio "New" (memory-mapped) I/O
java.rmi Remote method invocations
java.security Security support
java.sql Database support
java.text Internationalized formatting of text and numbers
java.time Dates, time, duration, time zones, etc.
java.util Utilities (including data structures, concurrency, regular expressions, and logging)
Section 4

Operator Precedence

Operators with the same precedence Notes
[] . () (method call) Left to right
! ~ ++ -- + (unary) - (unary) () (cast) new Right to left ~ flips each bit of a number
* / % Left to right Be careful when using % with negative numbers. -a % b == -(a % b), but a % -b == a % b. For example, -7 % 4 == -3, 7 % -4 == 3
+ - Left to right
<< >> >>> Left to right >> is arithmetic shift (n >> 1 == n / 2 for positive and negative numbers), >>> is logical shift (adding 0 to the highest bits). The right hand side is reduced modulo 32 if the left hand side is an int or modulo 64 if the left hand side is a long. For example, 1 << 35 == 1 << 3
< <= > >= instanceof Left to right null instanceof T is always false
== != Left to right Checks for identity. Use equals to check for structural equality
& Left to right Bitwise AND; no lazy evaluation with bool arguments
^ Left to right Bitwise XOR
| Left to right Bitwise OR; no lazy evaluation with bool arguments
&& Left to right
|| Left to right
?: Right to left
= += -= *= /= %= &= |= ^= <<= >>= >>>= Right to left
Section 5

Primitive Types

Type Size Range Notes
int 4 bytes -2,147,483,648 to 2,147,483, 647 (just over 2 billion) The wrapper type is Integer. Use BigInteger for arbitrary precision integers
short 2 bytes -32,768 to 32,767
long 8 bytes -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807 Literals end with L (e.g. 1L)
byte 1 byte -128 to 127 Note that the range is not 0 ... 255
float 4 bytes approximately -3.40282347E+38F (6-7 significant decimal digits) Literals end with F (e.g. 0.5F)
double 8 bytes approximately -1.79769313486231570E+308 (15 significant decimal digits) Use BigDecimal for arbitrary precision floating-point numbers
char 2 bytes \u0000 to \uFFFF The wrapper type is Character. Unicode characters > U+FFFF require two char values
boolean true or false

Legal Conversions Between Primitive Types

Dotted arrows denote conversions that may lose precision.

Legal Conversions Between Primitive Types

Section 6

Lambda Expressions

Functional Interfaces

Interfaces with a single abstract method. Example:

public interface Predicate<T> {
boolean test(T t); 

Implementations of this interface can be supplied in-line as a lambda expression:

  • Anonymous implementations of functional interfaces
  • Parameters and body are separated by an “arrow sign” ("->")
  • Parameters of the abstract method are on the left of the arrow
  • The implementation is on the right of the arrow

Typical usage of lambda expressions:

JButton button = new JButton("MyButton"); button.addActionListener(event -> doSomeImportantStuff(event));

Method References

Lambda expressions represent anonymous functions. You can pass them as method parameters or return them. The same can be done with named methods using method references.

Typical usage of method references:

Without Method Reference

With Method Reference

button.addActionListener(event -> doSomeImportantStuff(event));

button.addActionListener( this::doSomeImportantStuff);

list.forEach(element -> System.out.println(element));

list.forEach(System. out::println);

There are four kinds of method references:

Kind of Method Reference


To a static method


To an instance method of a particular (named) object


To an instance method of an arbitrary object (to be named later) of a given type


To a constructor


Section 7

Collections and Common Algorithms

ArrayList An indexed sequence that grows and shrinks dynamically
LinkedList An ordered sequence that allows efficient insertions and removal at any location
ArrayDeque A double-ended queue that is implemented as a circular array
HashSet An unordered collection that rejects duplicates
TreeSet A sorted set
EnumSet A set of enumerated type values
LinkedHashSet A set that remembers the order in which elements were inserted
PriorityQueue A collection that allows efficient removal of the smallest element
HashMap A data structure that stores key/value associations
TreeMap A map in which the keys are sorted
EnumMap A map in which the keys belong to an enumerated type
LinkedHashMap A map that remembers the order in which entries were added
WeakHashMap A map with values that can be reclaimed by the garbage collector if they are not used elsewhere
IdentityHashMap A map with keys that are compared by ==, not equals

Common Tasks

List<String> strs = new ArrayList<>(); Collect strings
strs.add("Hello"); strs.add("World!"); Add strings
for (String str : strs) System.out.println(str); Do something with all elements in the collection
Iterator<String> iter = strs.iterator(); while (iter.hasNext()) { String str = iter.next(); if (someCondition(str)) iter.remove(); } Remove elements that match a condition. The remove method removes the element returned by the preceding call to next
strs.addAll(strColl); Add all strings from another collection of strings
strs.addAll(Arrays.asList(args)) Add all strings from an array of strings. Arrays.asList makes a List wrapper for an array
strs.removeAll(coll); Remove all elements of another collection. Uses equals for comparison
if (0 <= i && i < strs.size()) { str = strs.get(i); strs.set(i, "Hello"); } Get or set an element at a specified index
strs.insert(i, "Hello"); str = strs.remove(i); Insert or remove an element at a specified index, shifting the elements with higher index values
String[] arr = new String[strs.size()]; strs.toArray(arr); Convert from collection to array
String[] arr = ...; List<String> lst = Arrays.asList(arr); lst = Arrays.asList("foo", "bar", "baz"); Convert from array to list. Use the varargs form to make a small collection
List<String> lst = ...; lst.sort(); lst.sort(new Comparator<String>() { public int compare(String a, String b) { return a.length() - b.length(); } } Sort a list by the natural order of the elements, or with a custom comparator
Map<String, Person> map = new LinkedHashMap<String, Person>(); Make a map that is traversed in insertion order (requires hashCode for key type). Use a TreeMap to traverse in sort order (requires that key type is comparable)
for (Map.Entry<String, Person> entry : map.entrySet()) { String key = entry.getKey(); Person value = entry.getValue(); ... } Iterate through all entries of the map
Person key = map.get(str); // null if not found map.put(key, value); Get or set a value for a given key

Bulk Operations With Stream API


Do something with all elements in the collection

List<String> filteredList = strs .stream() .filter(this::someCondition) .collect(Collectors.toList());

Filter elements that match a condition

String concat = strs
.stream() .collect(Collectors.joining(", "));

Concatenate the elements of a stream

List<User> users = ...; List<String> firstNames = users

.stream() .map(User::getFirstName) .collect(Collectors.toList());

Create a new list that maps to the original one

List<String> adminFirstNames = users .stream()
.filter(User::isAdmin) .map(User::getFirstName) .collect(Collectors.toList());

Combine operations This will not result in two traversals of the list elements

int sumOfAges = users .stream() .mapToLong(User::getAge) .sum();

Simple reduction operation

Map<Role, List<User>> byRole = users .stream()


Group users by a certain attribute

int sumOfAges = users .parallelStream() .mapToLong(User::getAge) .sum();

All the above operations can be done in parallel

Section 8

Character Escape Sequences

\b backspace \u0008
\t tab \u0009
\n newline \u000A
\f form feed \u000C
\r carriage return \u000D
\" double quote
\' single quote
\\ backslash
\uhhhh (hhhh is a hex number between 0000 and FFFF) The UTF-16 code point with value hhhh
\ooo (ooo is an octal number between 0 and 377) The character with octal value ooo
Note: Unlike in C/C++, \xhh is not allowed
Section 9

Formatted Output With printf

Typical Usage

System.out.printf("%4d %8.2f", quantity, price);
String str = String.format("%4d %8.2f", quantity, price);

Each format specifier has the following form. See the tables for flags and conversion characters.

Typical usage


Flag Description Example
+ Prints sign for positive and negative numbers +3333.33
space Adds a space before positive numbers | 3333.33|
0 Adds leading zeroes 003333.33
- Left-justifies field |3333.33 |
( Encloses negative number in parentheses (3333.33)
, Adds group separators 3,333.33
# (for f format) Always includes a decimal point 3,333.
# (for x or o format) Adds 0x or 0 prefix 0xcafe
$ Specifies the index of the argument to be formatted; for example, %1$d %1$x prints the first argument in decimal and hexadecimal 159 9F
< Formats the same value as the previous specification; for example, %d %<x prints the same number in decimal and hexadecimal 159 9F

Conversion Characters

Conversion Character Description Example
d Decimal integer 159
x Hexadecimal integer 9f
o Octal integer 237
f Fixed-point floating-point 15.9
e Exponential floating-point 1.59e+01
g General floating-point (the shorter of e and f)
a Hexadecimal floating-point 0x1.fccdp3
s String Hello
c Character H
b boolean true
h Hash code 42628b2
tx Date and time See the next table
% The percent symbol %
n The platform-dependent line separator
Section 10

Formatted Output With MessageFormat

Typical usage:

String msg = MessageFormat.format("On {1, date,
long}, a {0} caused {2,number,currency} of damage.",
"hurricane", new GregorianCalendar(2009, 0, 15).
getTime(), 1.0E8);

Yields "On January 1, 1999, a hurricane caused $100,000,000 of damage"

  • The nth item is denoted by {n,format,subformat} with optional formats and subformats shown below
  • {0} is the first item
  • The following table shows the available formats
  • Use single quotes for quoting, for example '{' for a literal left curly brace
  • Use '' for a literal single quote
Format Subformat Example
number none 1,234.567
integer 1,235
currency $1,234.57
percent 123,457%
date none or medium Jan 15, 2009
short 1/15/09
long January 15, 2009
full Thursday, January 15, 2009
time none or medium 3:45:00 PM
short 3:45 PM
long 3:45:00 PM PST
full 3:45:00 PM PST
choice List of choices, separated by |. Each choice has
  • a lower bound (use -\u221E for -)
  • a relational operator: < for "less than", # or \u2264 for .
  • a message format string
For example, {1,choice,0#no houses|1#one house|2#{1} houses}
no houses
one house
5 houses
Section 11

Regular Expressions

Common Tasks

String[] words = str.split("\\s+"); Split a string along white space boundaries
Pattern pattern = Pattern.compile("[0-9]+"); Matcher matcher = pattern.matcher(str); String result = matcher.replaceAll("#"); Replace all matches. Here we replace all digit sequences with a #.
Pattern pattern = Pattern.compile("[0-9]+"); Matcher matcher = pattern.matcher(str); while (matcher.find()) { process(str.substring(matcher.start(), matcher.end())); } Find all matches.
Pattern pattern = Pattern.compile( "(1?[0-9]):([0-5][0-9])[ap]m"); Matcher matcher = pattern.matcher(str); for (int i = 1; i <= matcher.groupCount(); i++) { process(matcher.group(i)); } Find all groups (indicated by parentheses in the pattern). Here we find the hours and minutes in a date.

Regular Expression Syntax

c The character c
\unnnn, \xnn, \0n, \0nn, \0nnn The code unit with the given hex or octal value
\t, \n, \r, \f, \a, \e The control characters tab, newline, return, form feed, alert, and escape
\cc The control character corresponding to the character c
Character Classes
[C1C2 . . .] Union: Any of the characters represented by C1C2 , . . . The Ci are characters, character ranges c1-c2, or character classes. Example: [a-zA-Z0-9_]
[^C1C2 . . .] Complement: Characters not represented by any of C1C2 , . . . Example: [^0-9]
[C1&&C2 &&. . .] Intersection: Characters represented by all of C1C2 , . . . Example: [A-f&&[^G-`]]
Predefined Character Classes
. Any character except line terminators (or any character if the DOTALL flag is set)
\d A digit [0-9]
\D A nondigit [^0-9]
\s A whitespace character [ \t\n\r\f\x0B]
\S A nonwhitespace character
\w A word character [a-zA-Z0-9_]
\W A nonword character
\p{name} A named character class-see table below
\P{name} The complement of a named character class
Boundary Matchers
^ $ Beginning, end of input (or beginning, end of line in multiline mode)
\b A word boundary
\B A nonword boundary
\A Beginning of input
\z End of input
\Z End of input except final line terminator
\G End of previous match
X? Optional X
X* X, 0 or more times
X+ X, 1 or more times
X{n} X{n,} X{n,m} X n times, at least n times, between n and m times
Quantifier Suffixes
? Turn default (greedy) match into reluctant match
+ Turn default (greedy) match into reluctant match
Set Operations
XY Any string from X, followed by any string from Y
X |Y Any string from X or Y
(X) Capture the string matching X as a group
\g The match of the gth group
\c The character c (must not be an alphabetic character)
\Q . . . \E Quote . . . verbatim
(? . . . ) Special construct-see API notes of Pattern class

Predefined Character Class Names

Lower ASCII lower case [a-z]
Upper ASCII upper case [A-Z]
Alpha ASCII alphabetic [A-Za-z]
Digit ASCII digits [0-9]
Alnum ASCII alphabetic or digit [A-Za-z0-9]
XDigit Hex digits [0-9A-Fa-f]
Print or Graph Printable ASCII character [\x21-\x7E]
Punct ASCII nonalpha or digit [\p{Print}&&\P{Alnum}]
ASCII All ASCII [\x00-\x7F]
Cntrl ASCII Control character [\x00-\x1F]
Blank Space or tab [ \t]
Space Whitespace [ \t\n\r\f\0x0B]
javaLowerCase Lower case, as determined by Character.isLowerCase()
javaUpperCase Upper case, as determined by Character.isUpperCase()
javaWhitespace White space, as determined by Character.isWhitespace()
javaMirrored Mirrored, as determined by Character.isMirrored()
InBlock Block is the name of a Unicode character block, with spaces removed, such as BasicLatin or Mongolian
Category or InCategory Category is the name of a Unicode character category such as L (letter) or Sc (currency symbol)

Flags for Matching

The pattern matching can be adjusted with flags, for example:

Pattern pattern = Pattern.compile(patternString,
Flag Description
CASE_INSENSITIVE Match characters independently of the letter case. By default, this flag takes only US ASCII characters into account
UNICODE_CASE When used in combination with CASE_INSENSITIVE, use Unicode letter case for matching
MULTILINE ^ and $ match the beginning and end of a line, not the entire input
UNIX_LINES Only '\n' is recognized as a line terminator when matching ^ and $ in multiline mode
DOTALL When using this flag, the . symbol matches all characters, including line terminators
CANON_EQ Takes canonical equivalence of Unicode characters into account. For example, u followed by ¨ (diaeresis) matches ü
LITERAL The input string that specifies the pattern is treated as a sequence of literal characters, without special meanings for . [ ] etc.
Section 12


Common Tasks

Logger logger = Logger.getLogger("com.mycompany.myprog.mycategory"); Get a logger for a category
logger.info("Connection successful."); Logs a message of level FINE. Available levels are SEVERE, WARNING,INFO,CONFIG,FINE, FINER, FINEST, with corresponding methods severe, warning, and so on
logger.log(Level.SEVERE, "Unexpected exception", Throwable); Logs the stack trace of a Throwable
logger.setLevel(Level.FINE); Sets the logging level to FINE. By default, the logging level is INFO, and less severe logging messages are not logged
Handler handler = new FileHandler("%h/myapp.log", SIZE_LIMIT, LOG_ROTATION_COUNT); handler.setFormatter(new SimpleFormatter()); logger.addHandler(handler); Adds a file handler for saving the log records in a file. See the table below for the naming pattern. This handler uses a simple formatter instead of the XML formatter that is the default for file handlers

Logging Configuration Files

The logging configuration can be configured through a logging configuration file, by default jre/lib/logging.properties. Another file can be specified with the system property java.util.logging.config.file when starting the virtual machine. (Note that the LogManager runs before main.)

Configuration Property Description Default
loggerName.level The logging level of the logger by the given name None; the logger inherits the handler from its parent
handlers A whitespace or comma-separated list of class names for the root logger. An instance is created for each class name, using the default constructor java.util.logging. ConsoleHandler
loggerName.handlers A whitespace or comma-separated list of class names for the given logger list of class names for the given logger None
loggerName. useParentHandlers false if the parent logger's handlers (and ultimately the root logger's handlers) should not be used true
config A whitespace or comma-separated list of class names for initialization None
The default handler level Level.ALL for FileHandler, Level.INFO for ConsoleHandler
The class name of the default filter None
The class name of the default formatter formatter java.util.logging. XMLFormatter for FileHandler, java.util.logging. SimpleFormatter for ConsoleHandler
The default encoding default platform encoding
java.util.logging.FileHandler.limit The default limit for rotating log files, in bytes 0 (No limit), but set to 50000 in jre/lib/ logging.properties
java.util.logging.FileHandler.count The default number of rotated log files 1
java.util.logging.FileHandler.pattern The default naming pattern for log files. The following tokens are replaced when the file is created:
Token Description
/ Path separator
%t System temporary directory
%h Value of user.home system property
%g The generation number of rotated logs
%u A unique number for resolving naming conflicts
%% The % character
java.util.logging. FileHandler.append The default append mode for file loggers; true to append to an existing log file false
Section 13

Property Files

  • Contain name/value pairs, separated by =, :, or whitespace
  • Whitespace around the name or before the start of the value is ignored
  • n Lines can be continued by placing an \ as the last character; leading whitespace on the continuation line is ignored
    button1.tooltip = This is a long \
    tooltip text.
  • \t \n \f \r \\ \uxxxx escapes are recognized (but not \b or octal escapes)
  • Files are assumed to be encoded in ISO 8859-1; use native2ascii to encode non-ASCII characters into Unicode escapes
  • Blank lines and lines starting with # or ! are ignored

Typical Usage:

Properties props = new Properties();
props.load(new FileInputStream("prog.properties"));
String value = props.getProperty("button1.tooltip");
// null if not present

Also Used for Resource Bundles:

ResourceBundle bundle = ResourceBundle.getBundle("prog");
// Searches for prog_en_US.properties,
// prog_en.properties, etc.
String value = bundle.getString("button1.tooltip");
Section 14

JAR Files

  • Used for storing applications, code libraries
  • By default, class files and other resources are stored in ZIP file format
  • META-INF/MANIFEST.MF contains JAR metadata
  • META-INF/services can contain service provider configuration
  • Use the jar utility to make JAR files

JAR Utility Options

Option Description
c Creates a new or empty archive and adds files to it. If any of the specified file names are directories, the jar program processes them recursively
C Temporarily changes the directory. For example, jar cvfC myprog.jar classes *.class changes to the classes subdirectory to add class files
e Creates a Main-Class entry in the manifest jar cvfe myprog.jar com.mycom.mypkg.MainClass files
f Specifies the JAR file name as the second command-line argument. If this parameter is missing, jar will write the result to standard output (when creating a JAR file) or read it from standard input (when extracting or tabulating a JAR file)
i Creates an index file (for speeding up lookups in a large archive)
m Adds a manifest to the JAR file. jar cvfm myprog.jar mymanifest.mf files
M Does not create a manifest file for the entries
t Displays the table of contents. jar tvf myprog.jar
u Updates an existing JAR file jar uf myprog.jar com/mycom/mypkg/SomeClass.class
v Generates verbose output
x Extracts files. If you supply one or more file names, only those files are extracted. Otherwise, all files are extracted. jar xf myprog.jar
O Stores without ZIP compression
Section 15

Common javac Options

Option Purpose
-cp or -classpath Sets the class path, used to search for class files. The class path is a list of directories, JAR files, or expressions of the form directory/'*' (Unix) or directory\* (Windows). The latter refers to all JAR files in the given directory. Class path items are separated by : (Unix) or ; (Windows). If no class path is specified, it is set to the current directory. If a class path is specified, the current directory is not automatically included-add a . item if you want to include it
-sourcepath Sets the path used to search for source files. If source and class files are present for a given file, the source is compiled if it is newer. If no source path is specified, it is set to the current directory
-d Sets the path used to place the class files. Use this option to separate .java and .class files
-source Sets the source level. Valid values are 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 5, 6, 7, 8
-deprecation Gives detail information about the use of deprecated features
-Xlint:unchecked Gives detail information about unchecked type conversion warnings
-cp or -classpath Sets the class path, used to search for class files. See the previous table for details. Note that javac can succeed when java fails if the current directory is on the source path but not the class path.
-ea or -enableassertions Enable assertions. By default, assertions are disabled.
-Dproperty=value Sets a system property that can be retrieved by System. getProperty(String)
-jar Runs a program contained in a JAR file whose manifest has a Main-Class entry. When this option is used, the class path is ignored.
-verbose Shows the classes that are loaded. This option may be useful to debug class loading problems.
Sets the initial or maximum heap size. The size is a value in bytes. Add a suffix k or m for kilobytes or megabytes, for example, -Xmx10m
Section 16

Common java Options



-cp or -classpath

Sets the class path, used to search for class files. See the previous table for details. Note that javac can succeed when java fails if the current directory is on the source path but not the class path

-ea or -enableassertions

Enable assertions. By default, assertions are disabled


Sets a system property that can be retrieved by



Runs a program contained in a JAR file whose manifest has a Main-Class entry. When this option is used, the class path is ignored


Shows the classes that are loaded. This option may be useful to debug class loading problems

-Xmssize -Xmxsize

Sets the initial or maximum heap size. The size is a value in bytes. Add a suffix k or m for kilobytes or megabytes, for example, -Xmx10m


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125.2k 339.6k
Getting Started with Ajax
Introduces Ajax, a group interrelated techniques used in client-side web development for creating asynchronous web applications.
101.3k 209.8k
Getting Started with Git
This updated Refcard explains why so many developers are migrating to this exciting platform. Learn about creating a new Git repository, cloning existing projects, the remote workflow, and more to pave the way for limitless content version control.
114.4k 266.1k
Spring Configuration
Catalogs the XML elements available as of Spring 2.5 and highlights those most commonly used: a handy resource for Spring context configuration.
102.9k 260.1k
Foundations of RESTful Architecture
The Representational State Transfer (REST) architectural style is a worldview that elevates information into a first-class element of architectures. REST allows us to achieve the architectural properties of performance, scalability, generality, simplicity, modifiability, and extensibility. This newly updated Refcard explains main HTTP verbs, describes response codes, and lists libraries and frameworks. It also gives additional resources to further explore each topic.
98.8k 158.7k
Core CSS: Part I
Covers Core principles of CSS that will expand and strengthen your professional ability to work with CSS. Part one of three.
89.8k 195.5k
jQuery Selectors
Introduces jQuery Selectors, which allow you to select and manipulate HTML elements as a group or as a single element in jQuery.
92.9k 353.5k
Scrum is a framework that allows people to productively and creatively deliver products of the highest possible value. With over 70% of Agile teams using Scrum or Scrum hybrid, learn more about its benefits in managing complex product development. This newly updated Refcard explores the details of Scrum, including theory, values, roles, and events. It also includes a sample of a popular approach to deliver Integrated Increments in a scaled environment.
90.3k 237.7k
Core Java Concurrency
Helps Java developers working with multi-threaded programs understand the core concurrency concepts and how to apply them.
89.6k 187.4k
Getting Started with Eclipse
Eclipse IDE is a cross-platform, multi-purpose, open-source Integrated Development Environment. It is widely used to develop projects in Java, JavaScript, PHP, C++, Scala, and many others. This newly updated Refcard breaks down installing, setting up, and getting started with Eclipse. It also covers productivity tips, creating new projects and files, accessing Source Control Managers, and debugging configurations.
78.5k 209.4k
Core CSS: Part II
Covers Core principles of CSS that will expand and strengthen your professional ability to work with CSS. Part two of three.
73.3k 140.2k
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