Download 6.1 System Class

Chapter 6
6.1.1 Use of the System class for
input and output
• Data traveling to and
from objects is called
a stream
• Stream objects can
be used to represent
and control the flow
of data
• java.lang.System has
3 stream objects that
provide basic
input/output
capabilities
6.1.2 Input and output using
java.lang.System
• 3 streams of basic I/O
• Standard Output (stdout) is used to send
messages to the console
• Standard Error (stderr) is used to send
messages to the error logging device
• Standard Input (stdin) is used to read input
from the keyboard
6.2 Strings
• No string primitive
• Created by assigning a reference to a literal, or
by instantiating a String object
String name = “Jody”;
String name = new String(“Jody”);
• Strings are immutable – They can never
change
• A reference to a String can be assigned to
another string
6.2.1 String storage & methods
• If a String is created using a literal, it is stored
in the String pool
• If a String object is instantiated, it is stored on
the heap
• The String class has many useful methods
• The ‘+’ operator can be used to concatenate
strings
• Use the String.equals() method to test for
equality. Using ‘==‘ compares the reference
variables
6.2.3 Casting, conversion &
StringBuffer
• Primitive values can be assigned to a String
reference by concatenating the primitive with a
literal String
int x = 5;
string s1 = “” + x;
• Objects are converted to a String using their
“toString()” method
• The StringBuffer class can be used to manage
strings that need to change
• StringBuffer contains methods for changing the
string
6.3.1 StringBuffer methods
• The StringBuffer class can be used to manage
strings that need to change
• StringBuffer contains methods for changing the
string
6.5.1 Introduction to wrapper
classes
• Java does not treat primitives as objects
• Wrapper classes exist to “wrap” the primitive in
an object
• Wrapper class have methods useful with
primitives
• Wrapper classes are immutable
• Many wrapper methods are static
6.6.1 The Math class
• The Math class is useful for performing
mathematical functions
• Methods are static, so no instance is required
• The BigNumber and BigDecimal classes are
used to store very large values
6.8.1 Creating dates
• 3 classes for storing and manipulating dates
java.util.Date
java.util.Calendar
java.text.DateFormat
• Date represents a date, Calendar sets or
changes the date, and DateFormat is used to
display the Date in different formats
• Use DateFormat.parse() to obtain an instance
of Date
Date someDate;
DateFormat df = DateFormat.getInstance(DateFormat.FULL,
Locale.US);
someDate = rf.parse(“Monday, May 30, 2002”);
6.8.2 Setting dates
• Date methods that change the date should not
be used
• The Calendar class can be used to set the date
6.8.3 Formatting dates
• The DateFormat class can be used to convert a
Date in to a readable String
6.8.4 Random numbers
• The static Math.random() method returns a
double between 0.0 and 1.0
• The java.util.Random class can be used to
work with random numbers of int, long, float or
double
• Random number generators require a seed. If
the same seed is used, the results will be the
same
• The default constructor of the Random class
uses the system clock as the seed