Lecture 9: More on objects, classes, strings

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Lecture 9 More on objects, classes, strings

• discuss hw3
• assign hw4
• default values for variables
• scope of variables and shadowing
• null reference and NullPointerException
• garbage collection
• more on strings
• equals and
• explicit and implicit parameters
• side effects
• immutable classes
• interned string values
• console input
• your input!

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HW3
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HW4

• Builds on our Date class
• dayOfWeek method

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Default values for variables

• Data fields (instance variables) are assigned
  default initial values
• 0 for numeric types (int, double, etc)
• false for boolean
• \u0000 for char
• null for object references
• Local variables declared inside methods are not
  assigned default initial values

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Scope of variables

• A variable declared inside a method is called a
  local variable
• Its scope starts from its declaration and
  continues to the end of the smallest block
  that contains the variable.
• Before and after that it is not accessible.
• Method parameters are also variables their scope
  is the entire method
• The scope of a data field (static or instance
  variable in a class) is the entire class,
  regardless of where its declared good style to
  declare them all before the methods
• They can be declared in any order unless the
  initial value of one depends on the initial value
  of another, but this is not good style anyway
• See Oct10.java

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Shadowing

• Occurs when access to an instance variable is
  prevented by declaration of a local variable with
  the same name
• A local variable cannot be shadowed by another
  local variable because their scopes are not
  allowed to overlap

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Null reference

• Player jack null
• means jack does not point to any object
• If we now invoke jack.flip(), a
  NullPointerException occurs
• Player jill jack // now jill is null too

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Garbage and garbage collection

• Player jack new Player()
• Player jill new Player()
• jill jack // jill object is inaccessible
• but if we first did
• Player jasmine jill // copy of jill
• the object IS accessible
• Figuring out what is and what is not accessible
  is the job of the Java garbage collector it
  reclaims wasted space
• In C, have to explicitly release space
• In Java, dont, but can help by setting reference
  values to null when no longer needed

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More on strings

• A string is an object
• Only thing special about a string is special
  syntax in the language
• Concatenation operator
• Use of to construct a string

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equals and

• String s1 abc
• String s2 new String(abc)
• String s3 new String(abc)
• System.out.println(s1 s2)
• System.out.println(s1.equals(s2))
• System.out.println(s2 s3)
• System.out.println(s2.equals(s3))

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Explicit and implicit parameters

• The explicit parameters in a method call are the
  actual parameters (arguments) matching the
  parameters in the signature
• The implicit parameter in a call to an instance
  method is the object instance name
• jack.multipleFlip(n)
• s1.equals(s2)
• complete lack of symmetry between these can be
  very confusing
• our text book does not use these terms, but they
  are standard

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Side effects

• Something that happens as the result of a method
  call which is not the main purpose of the method
  is called a side effect
• For example, a method intended to display
  information makes changes to an implicit or
  explicit parameter object
• Generally it is undesirable to make a change to
  an explicit parameter object
• Usually, if a change to an object is desired this
  is done by passing it as an implicit parameter,
  as in jack.multipleFlip()

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Undesirable vs Impossible

• It is impossible to change the value of a
  primitive type variable in a method by passing it
  as an explicit parameter to another method
• However, it is often desirable to change the
  values in an array by passing it as an explicit
  parameter to another method
• It is generally undesirable to change the data
  fields of an object by passing it as an explicit
  parameter to another method usually better to
  pass it as an implicit parameter
• However, when an object is immutable it is
  impossible to make changes to it by passing it as
  either an explicit or implicit parameter

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Immutable objects

• An object is immutable if it is impossible to
  make changes to it once it is constructed
• The class is also called immutable
• Strings are immutable
• String s1 new String(abc)
• s1 new String(def)
• The string that was constructed to contain abc
  is not changed it is lost
• Requirements for an immutable class include
• all data fields (instance variables) are private
• there are no mutator methods, only accessor
  methods
• But this is not enough! See p. 232 of text

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Interned string values

• String s1 abc
• String s2 abc
• System.out.println(s1 s2)
• Both s1 and s2 point to the same object
  constructed to contain abc
• String s3 new String(abc)
• System.out.println(s1 s3)
• System.out.println(s1.equals(s3))
• String s4 s3.intern() // get interned string
• System.out.println(s1 s4)

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String methods

• You only need to know charAt
• subString is also useful
• There are lots of others

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Passing strings to main

• public static void main(String args)
• up until now we have never accessed args, which
  is an array of String objects (any name can be
  used)
• this can be used to pass information into main
  from the command line

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Other information in Chapter 8

• A lot of technical detail that you do not need to
  learn, though you may find some of it useful
• Well return to files later
• The Scanner class use this if you want to read
  from the console instead of using JOptionPane
• Scanner builds on System.in, which is very
  complicated to use directly