From C to Java

1
From C to Java

• A whirlwind tour of Java
• for
• C programmers

2
Java statements

• Identical to those of C
• Assignment
• Decision
• if else
• switch
• Repetition
• while
• for
• do while
• break, continue
• return
• catch, throw

3
Java scope rules

• The scope of a local variable extends from the
  point where the variable is declared to the end
  of the block containing the declaration.
• The scope of a formal parameter is the entire
  definition of the method.
• Blocks may be nested
• Variables may be declared anywhere within the
  block
• A variable declared within a block may not have
  the same name as any identifier within an
  enclosing block.

4
Java scope rules

• Scope of a for loop index variable is the body of
  the loop
• All variable definitions must occur within a
  class declarationthere are no global variables
  in Java!

5
Differences between C and Java

• Conditions in Java control statements must be
  boolean (C allows arithmetic and assignment
  expressions).
• There are no standalone functions in Java, only
  methods that are defined within classes.
• There are no global variables (variables defined
  outside of a class) in Java.
• There are no pointers in Java however, objects
  are accessed via reference variables

6
Differences between C and Java

• A .java file usually contains a single class
  definition, and the name of the class is the same
  as the name of the file.
• For example, the class HelloWorld is defined in
  the file HelloWorld.java
• In Java, all parameters are passed by value
  there is no operator for passing parameters
  by reference.
• Operators cannot be overloaded in Java.
• There are no templates in Java.

7
Simple console output in Java

• Use System.out.print and System.out.println
• int x 5
• double y 3.2e4
• String name "Bob"
• System.out.println("x " x)
• System.out.println("y " y)
• System.out.println("name " name)
• Output
• x 5
• y 32000.0
• name Bob

8
Simple console input in Java

• Not so simple, unfortunately
• int ndouble xBufferedReader inData new
  BufferedReader( new InputStreamReader(System
  .in))n Integer.parseInt(inData.readLine())x
  Double.parseDouble(inData.readLine())
• Fortunately, we will not be doing that much
  console iothe bulk of our applications will be
  GUI-based.

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Howdy.java
import java.io. public class Howdy public
static void main(String args) throws
IOException BufferedReader inData
new BufferedReader( new
InputStreamReader(System.in))
System.out.print(What is your name? )
String name inData.readLine()
System.out.println(Howdy there, name)
10
Primitive Data Types

• Java primitive types include
• byte
• short
• int
• long
• float
• double
• char
• boolean

11
Java classes

• All Java classes are descendants of the Object
  class.
• All classes inherit certain methods from Object.
• Variables of primitive types are not objects.
• There are hundreds of Java classes available to
  the programmer.

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Java Strings

• Strings are sequences of characters, such as
  hello
• Java does not have a built in string type, but
  the standard Java library has a class called
  String
• String declarationsString s // s is initially
  null
• String greeting "Howdy!"

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String concatenation

• is the concatenation operator
• String s1 "Jim "
• String s2 "Bob"
• String name s1 s2
• String clone name 2

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substring() and length()
String s "abcdefgh" String sub
s.substring(3,7) // sub is "defg" String sub2
s.substring(3) // sub2 is "defgh" int len
s.length() // len is 8
15
Strings are immutable

• Objects of the String class are immutable, which
  means you cannot change the individual characters
  in a String.
• To change a String, use assignment to make the
  object point to a different String
• String s "Mike" s s.substring(0,2)
  "lk"
• // s is now "Milk"

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Testing Strings for equality

• The equals method tests for equality
•   String s1 "Hello",
• s2 "hello"
• s1.equals(s2) returns falses1.equals("Hello")
  is true
• The equalsIgnoreCase method returns true if 2
  Strings are identical except for upper/lower
  case
• s1.equalsIgnoreCase(s2) returns true
• Do not use to compare stringsyou are
  comparing string locations when you do!

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Useful String methods

• char charAt(int index)
• returns the character at the specified index
• int compareTo(String s)
• returns
• negative value if the String is alphabetically
  less than s,
• positive value if the String is alphabetically
  greater than s
• 0 if the strings are equal
• boolean equals(String s)
• returns true if the String equals s

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Useful String methods

• boolean equalsIgnoreCase(String s)
• returns true if the String equals s, except for
  upper/lower case differences
• int indexOf(String s)
• int indexOf(String s, int fromIndex)
• return the start of the first substring equal to
  s, starting at index 0 or at fromIndex
• if s is not contained in String, return 1.

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Useful String methods

• int length()
• returns the length of the string
• String substring(int beginNdx) String
  substring(int beginNdx,
  int endNdx)
• return a new string consisting of all characters
  from beginNdx to the end of the string or until
  endNdx (exclusive)

20
Useful String methods

• String toLowerCase()
• returns a new string with all characters
  converted to lower case
• String toUpperCase()
• returns a new string with all characters
  converted to upper case
• String trim()
• returns a new string by eliminating leading and
  trailing blanks from original string

21
Java arrays

• There are 2 equivalent notations for defining an
  array
• int a
• int a
• Note that space for the array is not yet
  allocated
• In Java, steps for creating an array are
• Define the array
• Allocate storage
• Initialize elements

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Allocating storage for an array

• Allocate a 100-element array
• a new int100
• You can specify initial values like this
• a new int1,2,3,4,5
• You can declare and initialize all in a single
  step
• int a 1,2,3,4,5

23
Be careful!

• String names new String4
• At this point, names contains 4 elements, all of
  which have the value null.
• The elements of names must be initialized before
  they can be used. For example
• names0 bob
• This situation arises whenever you have an array
  of objects. Remember to
• Allocate storage for the array, and
• Initialize the elements

24
Array operations

• The member variable length contains the length of
  the array
• for(int i0 i lt a.length i)
  System.out.println(ai)

25
Array operations

• Array assignment is permitted
• int a 1,2,3,4
• int b
• b a
• for(int i 0 i lt b.length i)
  System.out.println(bi)

26
Arrays of objects

• When creating arrays of objects, keep in mind
  that you must create the individual objects
  before accessing each one.
• The following example illustrates the process of
  using arrays of objects.
• In the example we use a class named MyClass
  (defined on the next slide)

27
MyClass
public class MyClass static int count0
private int data public MyClass()
count data count public
String toString() return data

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An array of MyClass objects
// declare an array MyClass arr new
MyClass5 // Now the array holds references
to // MyClass objects, not objects itself. //
The following code produces a // runtime error
System.out.println(arr0) // arr0 is null!
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An array of MyClass objects

• // To fix this error, we create objects
• MyClass arr new MyClass new
  MyClass(), new MyClass(), new
  MyClass(), new MyClass()
• // alternately, we could initialize the
• // array with a for loop.

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Multidimensional arrays
int arr 1, 2, 3, 4, 5, 6 for(int
i 0 i lt arr.length i) for(int j 0
j lt arri.length j)
System.out.println(arrij)
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Multidimensional arrays
MyClass arr new MyClass25 for (int i
0 i lt 2 i) for (int j 0 j lt 5
j) arrij new MyClass()
// create the objects before you use
the // array!
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Ragged arrays
int arr 1,2, null, 3,4,5, 6 for
(int i 0 i lt arr.length i) if (arri
! null) for (int j 0 j lt
arri.length j)
System.out.print(arrij " ")
System.out.println()
33
Methods can return arrays
public static String getNames(int n) throws
IOException BufferedReader inData
new BufferedReader( new InputStreamReader(Sy
stem.in)) String names new Stringn
for (int i 0 i lt n i) namesi
inData.readLine() return names
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End of the Tour