Static Methods vs' NonStatic Methods

1
Static Methods vs.Non-Static Methods
An Introduction to Objects using the String Class
2
Review

• When WRITING (not using) a static method, you
  need
• modifiers (public static)
• return type (void, int, double, char, etc)
• method identifer (name of the method)
• arguments/parameters (declare the variables you
  need to get from the user to work)
• body (the stuff inside the )

3
Review

• How do you call a static method?
• How do you call the Math classs pow method?
• How do you call the Math class's square root
  method?
• Explain the difference between CALLING a method
  and WRITING a method.

4
Review

• Where can you go to find out more about the
  methods of a pre-written Java classes?
• What is a static class data field (or class
  variable) and how do you use it?
• Where can you find out more about the data fields
  of a pre-written Java class?
• What is the difference between a data field
  (class variable) and a method?

5
Static Methods (again)
A static (class) method is called by
sayingClassName.methodName(inputParameterValues
)

• A static method is one you can call just by
  giving the name of the class it belongs to, a
  period, then the name of the method.
• ClassName.methodName(arg1, arg2)
• Math.pow(2,4) //24
• static means unchanging.
• The result you get by calling them doesnt depend
  on any circumstances except their input
  parameters

6
Static Methods

• In the API, you can tell if a method is static or
  not by going to the method summary, and looking
  at the left-hand column of the method you want to
  check (where it also gives the return type for
  the method). If it says static, it is a static
  method.
• If it doesnt say static, it is an object,
  non-static method

7
Objects

• A few weeks ago, we mentioned that Java has two
  types of data types
• primitive data types boolean, char, int, double
• Objects user-defined data types that can be for
  anything you imagine.

8
Objects

• Primitive data types are "basic". They consist of
  a single value, and that is it.
• Objects are different. They consist of two
  things
• properties the things that decide their "value"
• behaviors things that they can do, or things
  that can be done to them. These behaviors come in
  the form of special non-static methods they can
  use called "object methods"

9
Objects

• There are a TON of Objects that are already
  written for you to use in the Java API.
• Today, we will start small, and use one of the
  most common objects in any programming language
  Strings.

10
Strings

• In a lot of the examples we have done so far, we
  have used the String class, maybe even without
  realizing it.
• When we say public static void main(String
  args) the parameter args is a String array
  (dont worry about what the are for now).
• When we say System.out.println("afa dsgsdg
  ad")the text between the quotation marks is a
  String literal, or String value.

11
String

• Now it is time to take a look at the String type,
  and figure out how it works
• So what is a String?
• String is a class (so it is not a primitive data
  type).
• Because java.lang.String is automatically
  imported into every Java program (like the Math
  class), you can use it without having to import
  it.
• Just like you did with primitive data types, you
  can make variables of type String that represent
  String values

12
String

• String variable declarations and assignments
• String name Cerny
• final String COURSE Computer Science
• Just like with primitive data types, when you
  declare a String variable you give the type
  (String) and identifier.
• When you assign the variable a value, you still
  use the sign and set it equal to a value (just
  like you did with the primitive data types).

13
String literals (String values)

• For integer literals (values), you wrote a
  number.
• For double literals, you wrote a number with a
  decimal.
• For boolean literals, you wrote true or false.
• For char literals, you wrote the character value
  in single quotes, like 'a'.
• For String literals, though, you must write the
  value in double quotes, like a.
• 'a' is a char value
• "a" is a String value

14
String

• When you use the sign with numbers, it adds
  them together. When you use the sign with
  Strings, though, it concatenates them together
  (or joins the two Strings together to form a new
  String)
• 100 200 300
• "100" "200" "100200"

15
String Concatenation

• Whenever you add anything to a String, it turns
  the anything into a String for the result.
• Remember when we talked about precision, and said
  the result of an operation is always the same
  precision as the most precise number in the
  operation?
• It works kind of the same way with Stringanytime
  you use the sign, and one of the things being
  added together is a string, the result is
  automatically a String

16
String Concatenation

• "ab" 1 "ab1"
• 1 "" "1"
• "My number is " 3 "My number is 3"
• Looking at these examples, it is a little easier
  to understand why, in our System.out.println's,
  we have been using the sign everytime we wanted
  to print out text and the value of a variable on
  the same line...the println() method takes in a
  String as its input parameter.

17
String Concatenation

• int a 3
• System.out.println("My variable is equal to
  "a)
• This statement takes the String "My variable is
  equal to " and concatenates it with the value of
  variable a, which in this case is the int 3.
• So on the screen, it will print out "My variable
  is equal to 3"

18
String an object, not a primitive data type

• With all of the things we have done so far with
  String, it might not seem like it is all that
  different from a primitive data type like int.
• We said that, unlike primitive data types,
  Objects like String have special non-static
  object methods that they can use.
• Non-static methods are all the methods in the API
  that DO NOT have the word "static" next to them

19
Non-Static Object Methods
A non-static (object) method is called by
sayingvariableName.methodName(inputParameterVal
ues)

• A non-static method is called an "object" method
  because it only works if you have an object to
  use it with.
• To call it, you give the name of a variable
  belonging to the same class as the method, a
  period, the name of the method, and then the
  arguments, like this
• String a "computer"
• String b a.toUpperCase()

20
Non-Static Methods

• We said before that System.out is the name of a
  static variable of type PrintStream.
• Since System.out is a PrintStream variable, we
  can go to the PrintStream class and use any
  non-static methods there on it. One non-static
  method we have already used from this class is
  println().
• System.out.println("Hello")

variable name (System.out is a static variable
of type PrintStream)
method name (println is a non-static method in
the PrintStream class)
input parameter values (the println takes in a
String, so we have to give it either a String
variable or a String literal (anything in "")
21
String's Non-Static Methods

• There are a bunch of them in the API, but here is
  some examples of a few useful ones.
• String example "Bob"
• example.toUpperCase() //returns "BOB"
• example.toLowerCase() //returns "bob"
• example.length() //returns the int 3
• example.replace('o', 'a') //returns "Bab"
• example.charAt(0) /returns 'B' (the first
  character in "Bob"... Java
  always starts with 0 instead of 1. /

22
String's Non-Static Methods

• There are a bunch of them in the API, but here is
  some examples of a few useful ones.
• String example "Bobby"
• example.substring(0,3) //returns "Bob"
• example.substring(3) //returns "by"
• example.length() //returns the int 5
• "Pizza".toUpperCase() //returns "PIZZA"

23
Static vs Non-Static

• We will work more with this later, but for now
  just know
• a STATIC method is called by putting the class
  name, a period, the identifier, and the
  arguments
• Math.sqrt(4) //returns 2
• a NON-STATIC method is called by putting the
  variable name, a period, the identifier, and the
  arguments.
• String x "Bob"x.toUpperCase() //returns
  "BOB"
• For right now, we will not be writing non-static
  methods
• We will get to that during the third six weeks.

24
StringPractice Lab

• This next lab is a short little lab that is meant
  to
• introduce Strings
• introduce non-static methods