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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