Transcript Static and main

Static, main, math, Strings
What helps me learn Java the
most:
A.
B.
C.
D.
E.
F.
Going to the Mac lab
Writing Java on the Board
Doing the Homework
Watching the Instructor code in BlueJ
Doing the Programming Assignments
clickers
Review reading javadoc and
methods
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java.lang.Math
static double sqrt(double a)
Returns the correctly rounded positive square root of a
double value.
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All methods in java.lang.Math are static
To call sqrt, do the following:
A. int x = Math.sqrt(4);
B. int x = sqrt(4);
C. Math.sqrt(4);
D. int x = new Math( ); x.Math.sqrt(4);
E. int x = new Math( ); x.sqrt(4);
static
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static means "class level", not "object
level"
public int x; // note: public just for demo
// instance variables should be private
exists at class level
public static int y;
before any objects are
object is created;
on object bench
created
Using static variables/methods
For each line ok? A. YES B. NO
class MyClass {public int x;
public static int y;}
MyClass c = new MyClass( ); // ok?
c.x = 4; // ok?
c.y = 8; // ok?
MyClass d = new MyClass( ); //ok?
// c.x is A. 0 B. 4 C. neither D. undefined
// c.y is A. 0 B. 8 C. neither D. undefined
More on using static
class MyClass {public int x;
public static int y;}
MyClass c = new MyClass( );
c.y = 8; // ok? A. YES B. NO
MyClass.y = 8; // ok? A. YES B. NO
MyClass.x = 4; // ok? A. YES B. NO
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For static variables/names, use name of class, not object
name. can use either
For object variables, use name of object, not class name.
canNOT use either
Why have static?
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If instance variables are not needed
Math.PI, Math.sqrt,
JOptionPane.showMessageDialog
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when in doubt, do NOT use static
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static methods
static methods must use static everything
since they exist even if an object doesn't
SO
 static methods can only call static
methods
 static methods cannot modify instance
variables
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main
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main is the first method that is called when a
program is run
main must be static in Java because it is created
before an object is created
because main is static, all methods it calls must
be static
moral: main should create an object, and that
object should do the work of the program
program with main
public class MyClass
{
// do stuff with methods and constructors
public static void main(String[ ] args)
{
new MyClass( ); // call constructor
}
String[ ] args
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You can specify arguments at the
command line
public static void main(String[ ] args)
{
for (String mystring : args)
// Note: NO references to the array in foreach
System.out.println("You entered " + mystring);
/* the same loop with a for would be:
for (int i=0;i<args.length;i++)
System.out.println("You entered " + args[i]);
*/
}
Wrapper Classes
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Sometimes, primitives
need to act like
objects and the
reverse
Each primitive has a
"wrapper" class that
makes it look like an
object:
Wrappers are
instances of Classes,
so are capital
Wrapper
Type
Boolean
Primitive
Type
boolean
Integer
int
Double
double
Character
char
primitives
are all
lowercase
this: refers to the class
or class's instance variable
public class Member
{
private String name;
private String motto;
private<Member> friends = new ArrayList<Member> ( );
public void addMyBestFriend( )
{
// want to say:
// friends.add(me);
friends.add(this); // will add the current Member as a friend
}
}
More practice using javadoc: String
(expand S.o.p to System.out.println)
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int compareTo(String anotherString)
Compares two strings lexicographically.
Returns: the value 0 if the argument string is equal to this string; a
value less than 0 if this string is lexicographically less than the string
argument; and a value greater than 0 if this string is lexicographically
greater than the string argument.
Call to compareTo:
A.
String s1, s2; if (s1 < s2) S.o.p("s1 < s2");
B.
String s1, s2; if (s1.compareTo(s2) ) S.o.p ("s1 < s2");
C.
String s1, s2; if (compareTo(s1, s2)) S.o.p("s1 < s2");
D.
String s1, s2; if (s1.compareTo(s2) < 0) S.o.p("s1 < s2");
E.
String s1, s2; int x = 0;
if (x = s1.compareTo(s2) ) S.o.p("s1 < s2");