conversion : Java Glossary

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conversion

How do you convert Strings to ints, vice versa and the like? You can’t always simply cast them. Java has a rather chaotic naming convention for the various routines to do the conversions. Look for candidate static and dynamic methods on both the source and target classes, and related wrapper classes like Integer. Likely conversion method names are valueOf, toString, parse???, ???Value, to???.

I have also written the Conversion Amanuensis Applet in Java, available with source code, that will generate Java source code for any of the 400 possible conversions between the 20 basic types. You can then cut and paste the result into your code.

Conversion Amanuensis Rounding
Conversion Matrix for Java 1.4- Left Zeros
Conversion Matrix for Java 1.5+ Formatted Output
Primitives Reading Numeric Data
Basic Types StreamTokenizer
Truncation StringTokenizer
byte[] ⇔ String Binary
char[] ⇔ String Metric
Hexadecimal Links
StringBuffers & StringBuilders

Primitives

Primitive variables include Boolean, char, byte, short, int, long, float and double. Strings, arrays and Objects are not considered primitives.
Java Primitives
Type Signed? Bits Bytes Digits Lowest Highest Mnemonic
Boolean n/a 1 1 1 false true zero/one
char unsigned Unicode 16 2 4:5 '\u0000' [0] aka Character. MIN_VALUE '\uffff' [216-1] aka Character. MAX_VALUE Unicode chars are twice as big as C’s.
byte signed 8 1 2:3 -128 [-27] aka Byte. MIN_VALUE +127 [27-1]aka Byte. MAX_VALUE Bytes are signed, so half the usual 255 range.
short signed 16 2 4:5 -32,768 [-215] aka Short. MIN_VALUE +32,767 [215-1] aka Short. MAX_VALUE 32K
int signed 32 4 9:10 -2,147,483,648 [-231] aka Integer.MIN_VALUE aka -2 gig, roughly -2 billion +2,147,483,647 [231-1] aka Integer. MAX_VALUE. aka 2 gig, roughly 2 billion 2 gig
long signed 64 8 1:19 -9,223,372,036,854,775,807 [-263] aka Long. MIN_VALUE about -9×1018 9,223,372,036,854,775,808 [+263-1] aka Long. MAX_VALUE about 9×1018 9 exabytes, or 9 billion gig
float signed exponent and mantissa 32 4 7 ±1.40129846432481707e-45 aka Float. MIN_VALUE ±3.40282346638528860e+38 aka Float. MAX_VALUE
or roughly ±2127
with about 7 significant digits of accuracy.
A float can exactly represent integers
in the range -224 to +224.
rough, compact float
double signed exponent and mantissa 64 8 16 ±4.94065645841246544e-324 aka Double. MIN_VALUE ±1.79769313486231570e+308 aka Double. MAX_VALUE
or roughly ±21023
with 15 significant digits of accuracy, almost 16 with 15.95 significant digits.
A double can exactly represent integers
in the range -253 to +253.
high precision float

Primitives vs Immutable Wrapper Objects

Contrast that table of primitives, with this table of basic Java types:

Immutable Primitives Immutable Objects
Boolean Boolean
ordinary signed byte Byte
unsigned byte Byte
short Short
char Character
int Integer
long Long
float Float
double Double
char[] String

Truncation

If a long is converted to an int, or an int to a byte, the high order bits are simply truncated. This can result in surprising results including sign reversal.

byte[] ⇔ String

Interconverting byte [] and String is tricky because there is always an implied encoding translation. "8859_1" encoding simply chops the high byte off or pads the high byte with 0s, what newbies erroneously imagine happens all the time.

String s = "abc";

// string -> byte[]
byte [] b = s.getBytes( "8859_1" /* encoding */ );

// byte[] -> String
String t = new String( byteArray, "Cp1252" /* encoding */ );

// subset of byte[] -> String
String t = new String( byteArray, offset, length, "UTF-8" /* encoding */ );

char[] ⇔ String

Interconverting char [] and String is easier because there is no encoding involved. Encoding is about how to represent 16-bit Unicode in 8-bit bytes.

String s = "abc" ;
// string -> char[]
char[] ca = s.toCharArray();
// char[] -> String
String s = new String( ca );

Beware of char[].toString(). It does not convert the character array to String. It prints out the address of the array — not useful for anything. Use new String ( chararray ) instead.

Hexadecimal

The radix feature of the toString and parseInt methods lets you handle hexadecimal numbers. Just set the radix to 16.

StringBuffers & StringBuilders

Strings and StringBuffers can be interconverted, as can Strings and StringBuilders

Rounding

Rounding often surprises because fractions like 0.1 cannot be precisely be represented in IEEE (Institute of Electrical & Electronics Engineers) floating point format. Often you find yourself having to add tiny numbers just prior to printing to get the desired effects.

// Rounding to an integer:
long n = Math.round(d);
double d = Math.rint(d);

// Rounding to two decimal places:
long n = Math.round(d *100.); /* keep as "pennies" */
double d = Math.rint (d *100.)/100.;

floating point for more details.

Left Zeros

The standard conversions give you no leading blanks or leading zeroes. Here is a code snippet to convert an int to a String padded with leading left zeroes. With an obvious modification it would give you lead blanks.

Formatted Output

JDK (Java Development Kit) 1.0.2 has no function like C’s printf that lets you control how many positions and decimal places you want in your output. You have to roll your own.

Java version 1.1 or laterhas formatting picture classes such as java.util.DateFormat, SimpleDateFormat and DecimalFormat.

Formatting is such a common request, you might attain sainthood if you wrote a formatting class that gives you all the power of printf without the overhead of parsing strings for % produce a string, e. g.

Java 1.5+, java.io. PrintWriter. printf, java.io. PrintStream. printf and the java.util. Formatter class give you abilities similar to C’s printf. See printf for more details.

Reading Numeric Data From an ASCII (American Standard Code for Information Interchange) file

Java has no built-in methods for reading data of the form: 123 456,-4.

You have to roll your own method. You may be able to use my CSVReader class. Or use java.io.StreamTokenizer or java.util.StringTokenizer, perhaps in combination with readLine to get your data into strings. StreamTokenizer has bells and whistles to deal with parsing source code, including white space, comments and numbers. StringTokenizer just splits the text up based on delimiter characters. Then use the conversion methods in the table above to convert to integers etc. See below for a simplified examples of how you would do this.

StreamTokenizer Method of Reading Integers From An ASCII File

StringTokenizer Method of Reading Integers From An ASCII file

Binary

Binary is a compact, machine-friendly, human-unintelligible format. For human readable i/o, Java works with Strings of characters. You separately convert these to and from internal binary format e.g. int. See the conversion Amanuensis for how.

Metric Conversions

You can convert metric to Imperial or other measurements by putting expressions in ordinary Google search boxes:

Learning More

Oracle’s Javadoc on Byte class : available:
Oracle’s Javadoc on Character class : available:
Oracle’s Javadoc on Double class : available:
Oracle’s Javadoc on Float class : available:
Oracle’s Javadoc on Integer class : available:
Oracle’s Javadoc on Long class : available:
Oracle’s Javadoc on Short class : available:
Oracle’s Javadoc on String class : available:

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