Java IO.pdf - Nguyen Dang Binh

16.6.2.1 Utility methods
Java I/O
For completeness, I'll note that DecimalFormatSymbols overrides the usual three utility
methods from java.lang.Object, hashCode(), clone(), and equals():
public int hashCode()
public boolean equals(Object obj)
public Object clone()
One DecimalFormatSymbols object is equal to another DecimalFormatSymbols object if
they're instances of the same class and all their symbols are the same.
16.6.3 Constructing Decimal Formats with Patterns and Symbols
Most of the time, you use the factory methods in NumberFormat to get DecimalFormat
instances. However, there are three public DecimalFormat constructors you can use to create
DecimalFormat instances directly:
public DecimalFormat()
public DecimalFormat(String pattern)
public DecimalFormat(String pattern, DecimalFormatSymbols symbols)
The no-argument constructor creates a decimal format that uses the default pattern and
symbols for the default locale. The second constructor creates a decimal format that uses the
specified pattern and the default symbols for the default locale. The third constructor creates a
decimal format that uses the specified pattern and the specified symbols for the default locale.
These are useful for special cases that aren't handled by the default patterns and symbols.
16.7 An Exponential Number Format
The DecimalFormat class is useful for medium-sized numbers, but it doesn't work very well
for exceptionally large numbers like Avogadro's number
(6,022,094,300,000,000,000,000,000) or exceptionally small numbers like Planck's constant
(0.00000000000000000000000000625 erg-seconds). These are traditionally written in
scientific notation as a decimal number times 10 to a certain power, positive or negative; for
example, 6.0220943 × 10 23 and 6.25 × 10 -27 erg-seconds. In most programming languages,
including Java, an E followed by either a + or a - is used to represent "× 10 to the power"; for
example, 6.0220943E+23 or 6.25E-27 erg-seconds.
The java.text package does not provide support for formatting numbers in scientific
notation, [3] so as the final example of this chapter, I'll develop a new subclass of
NumberFormat that does use scientific notation. Technically, scientific notation requires
exactly one nonzero digit before the decimal point, but I'll be a little more general than that,
providing for numbers like 13.2E-8 as well.
The NumberFormat class is abstract. It declares three abstract methods any subclass must
implement:
3 The java.lang.Double class's toString() methods do format numbers less than 0.001 or greater than 10 million in scientific
notation.
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