refcardz-core-java.pdf

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tech facts at your fingertips
CONTENTS INCLUDE:
n Java Keywords
n Standard Java Packages
n Character Escape Sequences
n Collections and Common Algorithms
n Regular Expressions
n JAR Files
Core Java
By Cay S. Horstmann
AbOUT CORE JAVA
Java Keywords, continued
Keyword
Description
Example
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 ).
inally
the part of a try block
that is always executed
see try
loat
the single-precision
loating-point type
loat 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
JAVA KEywORDS
goto
not used
Keyword
Description
Example
if
a conditional statement if (input == 'Q')
System.exit(0);
else
more = true;
abstract an abstract class or
method
abstract class Writable {
public abstract void write(Writer out);
public void save(String ilename) { ... }
}
implements deines the interface(s)
that a class implements
class Student
implements Printable {
...
}
assert with assertions enabled,
throws an error if
condition not fulilled
assert param != null;
Note: Run with -ea to enable assertions
import
imports a package
import java.util.ArrayList;
import com.dzone.refcardz.*;
boolean the Boolean type with
values true and false
boolean more = false;
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
break
breaks out of a switch
or loop
while ((ch = in.next()) != -1) {
if (ch == '\n') break;
process(ch);
}
Note: Also see switch
interface Printable {
void print();
}
byte
the 8-bit integer type byte b = -1; // Not the same as 0xFF
Note: Be careful with bytes < 0
long
the 64-bit long integer
type
long worldPopulation = 6710044745L;
case
a case of a switch
see switch
native
a method implemented
by the host system
catch
the clause of a try block
catching an exception
see try
new
allocates a new object
or array
Person fred = new Person("Fred");
char
the Unicode character
type
char input = 'Q';
null
a null reference
Person optional = null;
class
deines a class type
class Person {
private String name;
public Person(String aName) {
name = aName;
}
public void print() {
System.out.println(name);
}
}
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;
...
}
const not used
continue continues at the end of
a loop
→
while ((ch = in.next()) != -1) {
if (ch == ' ') continue;
process(ch);
}
(They’re free!)
default the default clause of a
switch
see switch
do
the top of a do/while
loop
do {
ch = in.next();
} while (ch == ' ');
n Authoritative content
n Designed for developers
n Written by top experts
n Latest tools & technologies
n Hot tips & examples
n Bonus content online
n New issue every 1-2 weeks
double the double-precision
loating-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 deines the parent class
of a class
class Student extends Person {
private int id;
public Student(String name, int anId) { ... }
public void print() { ... }
}
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inal
a constant, or a class or
method that cannot be
overridden
public static inal int DEFAULT_ID = 0;
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instanceof tests if an object is an
instance of a class
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2
Core Java
tech facts at your fingertips
Java Keywords, continued
OpERATOR pRECEDENCE
Keyword
Description
Example
public
a feature that is
accessible by methods
of all classes
see class
Operators with the
same precedence
Notes
return
returns from a method int getId() { return id; }
[] . () (method call)
Left to right
short
the 16-bit integer type short skirtLength = 24;
! ~ ++ -- + (unary) –
(unary) () (cast) new
Right to left
~ lips each bit of a number
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 ilename);
public static inal String DEFAULT_EXT = ".dat";
}
* / %
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.
strictfp Use strict rules
for loating-point
computations
+ -
Left to right
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);
}
<< >> >>> 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
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 .
== !=
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
synchronized a method or code
block that is atomic to
a thread
public synchronized void addGrade(String gr) {
grades.add(gr);
}
&&
Left to right
this
the implicit argument
of a method, or a
constructor of this class
public Student(String id) { this .id = id;}
public Student() { this (""); }
||
Left to right
?:
Right to left
throw
throws an exception if (param == null)
throw new IllegalArgumentException();
= += -= *= /= %= &=
|= ^= <<= >>= >>>=
Right to left
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;
...
}
pRImITIVE TypES
try
a block of code that
traps exceptions
try {
try {
fred.print(out);
} catch (PrinterException ex) {
ex.printStackTrace();
}
} inally {
out.close();
}
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 ).
void
denotes a method
that returns no value
public void print() { ... }
byte
1 byte
–128 to 127
Note that the range is not
0 ... 255.
volatile ensures that a ield is
coherently accessed
by multiple threads
class Student {
private volatile int nextId;
...
}
loat
4 bytes
approximately
±3.40282347E+38F (6–7
signiicant decimal digits)
Literals end with F (e.g. 0.5F )
while
a loop
while (in.hasNext())
process(in.next());
double 8 bytes
approximately
±1.79769313486231570E+308
(15 signiicant decimal digits)
Use BigDecimal for arbitrary
precision loating-point
numbers.
STANDARD JAVA pACKAgES
char
2 bytes \u0000 to \uFFFF
The wrapper type is
Character. Unicode
characters > U+FFFF require
two char values.
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
boolean
true or false
Legal conversions between primitive types
Dotted arrows denote conversions that may lose precision.
Database support
java.text
Internationalized formatting of text and numbers
java.util
Utilities (including data structures, concurrency, regular expressions,
and logging)
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3
Core Java
tech facts at your fingertips
COLLECTIONS AND COmmON ALgORIThmS
FORmATTED OUTpUT wITh printf
ArrayList An indexed sequence that grows and shrinks dynamically
LinkedList An ordered sequence that allows eficient 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 eficient 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
Typical usage
System.out.printf("%4d %8.2f", quantity, price);
String str = String.format("%4d %8.2f", quantity, price);
Each format speciier has the following form. See the tables for
lags and conversion characters.
Flags
Flag
Description
Example
Common Tasks
+
Prints sign for positive and negative numbers
+3333.33
space
Adds a space before positive numbers
| 3333.33|
List<String> strs = new ArrayList<String>(); 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
0
Adds leading zeroes
003333.33
-
Left-justiies ield
|3333.33 |
(
Encloses negative number in parentheses
(3333.33)
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 .
,
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 preix
0xcafe
strs.addAll(strColl);
Add all strings from another
collection of strings
$
Speciies the index of the argument to be formatted;
for example, %1$d %1$x prints the irst argument in
decimal and hexadecimal
159 9F
strs.addAll(Arrays.asList(args))
Add all strings from an array of
strings. Arrays.asList makes a
List wrapper for an array
<
Formats the same value as the previous speciication;
for example, %d %<x prints the same number in decimal
and hexadecimal
159 9F
strs.removeAll(coll);
Remove all elements of another
collection. Uses equals for
comparison
Conversion characters
if (0 <= i && i < strs.size()) {
str = strs.get(i);
strs.set(i, "Hello");
}
Get or set an element at a
speciied index
Conversion
Character
Description
Example
strs.insert(i, "Hello");
str = strs.remove(i);
Insert or remove an element at
a speciied index, shifting the
elements with higher index values
d
Decimal integer
159
x
Hexadecimal integer
9f
String[] arr = new String[strs.size()];
strs.toArray(arr);
Convert from collection to array
o
Octal integer
237
f
Fixed-point loating-point
15.9
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.
e
Exponential loating-point
1.59e+01
g
General loating-point (the shorter of e and f)
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.
a
Hexadecimal loating-point
0x1.fccdp3
s
String
Hello
c
Character
H
b
boolean
true
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).
h
Hash code
42628b2
t x
Date and time
See the next table
%
The percent symbol
%
n
The platform-dependent line separator
for (Map.Entry<String, Person> entry :
map.entrySet()) {
String key = entry.getKey();
Person value = entry.getValue();
...
}
Iterate through all entries of the
map
FORmATTED OUTpUT wITh MessageFormat
Person key = map.get(str); // null if not found
map.put(key, value);
Get or set a value for a given key
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"
n The n th item is denoted by {n,format,subformat} with
optional formats and subformats shown below
n {0} is the irst item
n The following table shows the available formats
n Use single quotes for quoting, for example '{' for a literal
left curly brace
n Use '' for a literal single quote
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
→
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4
Core Java
tech facts at your fingertips
Formatted Output with MessageFormat , continued
Regular Expression Syntax, continued
Format
Subformat
Example
Boundary Matchers
^ $
number none
1,234.567
Beginning, end of input (or beginning, end of line in multiline mode)
integer
1,235
\b
A word boundary
currency
$1,234.57
\B
A nonword boundary
percent
123,457%
\A
Beginning of input
date
none or medium
Jan 15, 2009
\z
End of input
short
1/15/09
\Z
End of input except inal line terminator
\G
End of previous match
long
January 15, 2009
full
Thursday, January 15, 2009
Quantiiers
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
Quantiier Sufixes
?
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
n a lower bound (use -\u221E for -∞ )
n a relational operator: < for “less than”, # or
\u2264 for ≤
n a message format string
For example, {1,choice, 0#no houses|1#one
house|2#{1} houses }
no house
Turn default (greedy) match into reluctant match
one house
+
Turn default (greedy) match into reluctant match
5 houses
Set Operations
XY
Any string from X , followed by any string from Y
X | Y
Any string from X or Y
Grouping
( X )
Capture the string matching X as a group
REgULAR ExpRESSIONS
\g
The match of the g th group
Common Tasks
Escapes
\ c
The character c (must not be an alphabetic character)
String[] words = str.split("\\s+");
Split a string along white
space boundaries
\Q . . . \E
Quote . . . verbatim
(? . . . )
Special construct— see API notes of Pattern class
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 #.
Predeined Character Class Names
Pattern pattern = Pattern.compile("[0-9]+");
Matcher matcher = pattern.matcher(str);
while (matcher.ind()) {
process(str.substring(matcher.start(), matcher.end()));
}
Find all matches.
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()
In Block Block is the name of a Unicode character block, with spaces
removed, such as BasicLatin or Mongolian .
Category or In Category Category is the name of a Unicode character category such
as L (letter) or Sc (currency symbol).
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 ind
the hours and minutes
in a date.
Regular Expression Syntax
Characters
c
The character c
\u nnnn , \x nn ,
\0 n , \0 nn ,
\0 nnn
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
\c c
The control character corresponding to the character c
Character Classes
[ C 1 C 2 . . . ] Union: Any of the characters represented by C 1 C 2 , . . .
The C i are characters, character ranges c 1 - c 2 , or character classes.
Example: [a-zA-Z0-9_]
[ ^ C 1 C 2 . . . ] Complement: Characters not represented by any of C 1 C 2 , . . .
Example: [^0-9]
[ C 1 && C 2 && . . . ] Intersection: Characters represented by all of C 1 C 2 , . . .
Example: [A-f&&[^G-`]]
Predeined Chara cter Classes
.
Flags for matching
The pattern matching can be adjusted with lags, for example:
Pattern pattern = Pattern.compile(patternString,
Pattern.CASE_INSENSITIVE + Pattern.UNICODE_CASE)
Flag
Description
CASE_INSENSITIVE
Match characters independently of the letter case. By default,
this lag takes only US ASCII characters into account.
Any character except line terminators (or any character if the DOTALL
lag is set)
UNICODE_CASE
When used in combination with CASE_INSENSITIVE , use Unicode
letter case for matching.
\d
A digit [0-9]
\D
A nondigit [^0-9]
MULTILINE
^ and $ match the beginning and end of a line, not the entire input.
\s
A whitespace character [ \t\n\r\f\x0B]
UNIX_LINES
Only '\n' is recognized as a line terminator when matching ^
and $ in multiline mode.
\S
A nonwhitespace character
DOTALL
When using this lag, the . symbol matches all characters,
including line terminators.
\w
A word character [a-zA-Z0-9_]
\W
A nonword character
CANON_EQ
Takes canonical equivalence of Unicode characters into account.
For example, u followed by ¨ (diaeresis) matches ü.
\p{ name }
A named character class—see table below
LITERAL
The input string that speciies the pattern is treated as a sequence
of literal characters, without special meanings for . [ ] etc.
\P{ name }
The complement of a named character class
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5
Core Java
tech facts at your fingertips
LOggINg
pROpERTy FILES
Common Tasks
n Contain name/value pairs, separated by = , : , or whitespace
n 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.
n \t \n \f \r \\ \uxxxx escapes are recognized (but not \b
or octal escapes)
n Files are assumed to be encoded in ISO 8859-1; use
native2ascii to encode non-ASCII characters into
Unicode escapes
n 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");
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 ile handler for saving the
log records in a ile. See the table
below for the naming pattern. This
handler uses a simple formatter
instead of the XML formatter that
is the default for ile handlers.
Logging Coniguration Files
The logging coniguration can be conigured through a logging
coniguration ile, by default jre /lib/logging.properties .
Another ile can be speciied with the system property java.
util.logging.conig.ile when starting the virtual machine.
(Note that the LogManager runs before main .)
Coniguration 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
JAR FILES
loggerName .handlers A whitespace or comma-separated list
of class names for the given logger
None
n Used for storing applications, code libraries
n By default, class iles and other resources are stored in
ZIP ile format
n META-INF/MANIFEST.MF contains JAR metadata
n META-INF/services can contain service provider
coniguration
n Use the jar utility to make JAR iles
loggerName .
useParenthandlers
false if the parent logger's handlers
(and ultimately the root logger's
handlers) should not be used
true
conig
A whitespace or comma-separated list
of class names for initialization.
None
java.util.logging.
FileHandler.level
java.util.logging.
ConsoleHandler.level
The default handler level
Level.ALL for
FileHandler,
Level.INFO for
ConsoleHandler
java.util.logging.
FileHandler.formatter
java.util.logging.
ConsoleHandler.formatter
The class name of the default ilter
None
jar Utility Options
Option Description
c
java.util.logging.
FileHandler.formatter
java.util.logging.
ConsoleHandler.formatter
The class name of the default formatter java.util.logging.
XMLFormatter for
FileHandler,
java.util.logging.
SimpleFormatter for
ConsoleHandler
Creates a new or empty archive and adds iles to it. If any of the speciied ile
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 iles.
java.util.logging.
FileHandler.encoding
java.util.logging.
ConsoleHandler.encoding
The default encoding
default platform
encoding
e
Creates a Main-Class entry in the manifest
jar cvfe myprog.jar com.mycom.mypkg.MainClass iles
f
Speciies the JAR ile name as the second command-line argument. If this
parameter is missing, jar will write the result to standard output (when creating a
JAR ile) or read it from standard input (when extracting or tabulating a JAR ile).
java.util.logging.
FileHandler.limit
The default limit for rotating log iles,
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 iles
1
i
Creates an index ile (for speeding up lookups in a large archive)
m
Adds a manifest to the JAR ile.
jar cvfm myprog.jar mymanifest.mf iles
java.util.logging.
FileHandler.pattern
The default naming pattern for log iles.
The following tokens are replaced when
the ile is created:
%h/java%u.log
M
Does not create a manifest ile for the entries.
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 conlicts
%% The % character
t
Displays the table of contents.
jar tvf myprog.jar
u
Updates an existing JAR ile
jar uf myprog.jar com/mycom/mypkg/SomeClass.class
v
Generates verbose output.
x
Extracts iles. If you supply one or more ile names, only those iles are
extracted. Otherwise, all iles are extracted.
jar xf myprog.jar
java.util.logging.
FileHandler.append
The default append mode for ile loggers;
true to append to an existing log ile
false
O
Stores without ZIP compression
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