CS 4773 Object Oriented Systems
The Java Language

The examples Test01.java through Test15.java are available in /usr/local/courses/cs4773/spring99/examples/set1

Primitive Types
Promotion in Expressions
Assignment Conversion
Type Casting
Arrays
Java Types and Default Values
Arrays with Multiple Dimensions
Strings
Arrays of Characters
String Buffers:
Operators
Flow Control
Classes
Exceptions
Packages, Classes, Interfaces, Methods, Fields, and Objects
Packages and Name Space Resolution
Class Modifiers
Method and Field Modifiers

Next Topic: Object Oriented Programming Concepts

Primitive Types:

Integers (all signed)
- byte (8 bits)
- short (16 bits)
- int (32 bits)
- long (64 bits)
Floating Point
- float (32 bits, IEEE 754)
- double (64 bits, IEEE 754)
Characters
- char (16-bit Unicode)
  The first 128 of these are ASCII}
Boolean
- boolean
  values true and false
  Integers may not be used in place of logical variables
  May not be cast to an integer

Type Promotion in Expressions

This is similar to C but it is more important in Java. Example:

class Test01 {
   public static void main(String args[]) {
      byte b = 25;
      b = b*4;
      System.out.println("Hello World! b = " + b);
   }
}
 
make Test01.class
javac Test01.java
Test01.java:4: Incompatible type for =.
Explicit cast needed to convert int to byte.
      b = b*4;
        ^
1 error
*** Error code 1
make: Fatal error: Command failed for target `Test01.class'

If you change byte to int everything works!

class Test02 {
   public static void main(String args[]) {
      int b = 25;
      b = b*4;
      System.out.println("Hello World! b = " + b);
   }
}
 
make Test02.class
javac Test02.java
 
java Test02
Hello World! b = 100

Assignment Conversion:

An automatic conversion will take place on assignment if the conversion is a widening.

A narrowing conversion is only allowed if:

the expression is of type int
the variable is of type byte, short, or char
the value of the expression is known at compile time and fits in the variable.

The following is OK:

class Test03 {
   public static void main(String args[]) {
      byte b = 25;
      b = 100;
      System.out.println("Hello World! b = " + b);
   }
}

However, since Java is interpreted and there is no preprocessor, what do you think happens when you try to compile the following:

class Test04 {
   public static void main(String args[]) {
      byte b;
      b = 25*4;
      System.out.println("Hello World! b = " + b);
   }
}
 
make Test04.class
javac Test04.java
Test04.java:4: Incompatible type for =.
Explicit cast needed to convert int to byte.
      b = 25*4;
        ^
1 error
*** Error code 1
make: Fatal error: Command failed for target `Test04.class'

This is what happens under JDK 1.0x. However, the JDK 1.1.6 is smart enough to figure this out and it compiles it correctly.

However, if we make a small change:

class Test05 {
   public static void main(String args[]) {
      byte b;
      byte a=25;
      b = a*4;
      System.out.println("Hello World! b = " + b);
   }
}

We get the error even under JDK 1.1.6.

Does the following work?

class Test06 {
   public static void main(String args[]) {
      byte b;
      final byte a=25;
      b = a*4;
      System.out.println("Hello World! b = " + b);
   }
}

Yes!

Type Casting

Type Casting is severely limited.

Anything can be cast to a String

boolean values cannot be cast to anything else.

You can cast between any arithmetic types (integer, floating point, character)

Basically for objects, you can only cast if one is a subclass of the other.

Java Types and Default Values

No variable in Java ever has an undefined value.

All variables are given a value when they are created.

There are two classifications of types in Java.

Primitive Types
- integer types: byte, short, int, long
- floating point types: float, double
- character type: char
- boolean type: boolean
Reference Types
- class types
- interface types
- array types

Variables of primitive type are created by their declaration.
If not explicitly initialized, they are assigned a default value.
boolean variables have default value false.
All other variables of primitive type have default value 0.

All variables of reference type have default value null.

Arrays

This is a reference type.
This means that an array is represented by a pointer.

Unlike in C, arrays are created dynamically.
A declaration does not automatically create the array.
Arrays are created by new.

Array declarations which do not create an array:

   int num[];

   int[] num;

Array declarations which do create an array:

   int num[] = new int[10];

   int[] num = new int[10];

   int num[] = {2,4,6,8};

Array declarations with run time creation:

   int num[];
   int size;
 
   size = 20;
   num = new int[size];

The size of an array is known at run time.

Bounds checking is done on all array references.

A program can determine the size of an array:

class Test07 {
   public static void main(String args[]) {
      int num[];
      int size;
      int i;
 
      size = 3;
      num = new int[size];
      for (i=0; i < num.length; i++)
         num[i] = i*i;
      for (i=0; i < num.length; i++)
         System.out.println("num[" + i + "] = " + num[i]);
   }
}
 
java Test07
num[0] = 0
num[1] = 1
num[2] = 4

Arrays of non-primitive type only contain references

This is not the case in C. In C you can have an array of structs or and array of pointers to structs. In java there are no structs and an array of objects is an array of references to objects.

Consider the following array:

   Point[] A;
   A = new Point[6];
   for (int i=0; i < 6; i++)
      A[i] = new Point(i,i*i);

Point is a simple class with the integer variables, x and y. It is initialized like this:

What is the meaning of the following:
A[4] = A[2]?
If we print out the array we will have:

What are the values after A[4].x = 50?

Click Here to run this applet.

Here is the source code for this applet:

import java.awt.*;
import java.awt.event.*;
import java.applet.*;

public class Test08 extends Applet implements ActionListener {
   Point[] A;
   int A_size;
   Panel p;
   Button Init;
   Button Copy1;
   Button Copy2;
   Button Set2;
   Button Set4;

   public void init() {
      setLayout(new BorderLayout());
      p = new Panel();
      p.setLayout(new GridLayout(1,5));
      p.add(Init = new Button("Init"));
      p.add(Copy1 = new Button("Copy 1"));
      p.add(Copy2 = new Button("Copy 2"));
      p.add(Set2 = new Button("Set 2"));
      p.add(Set4 = new Button("Set 4"));
      add("South",p);
      A_size = 10;
      A = new Point[A_size];
      initialize();
      Init.addActionListener(this);
      Copy1.addActionListener(this);
      Copy2.addActionListener(this);
      Set2.addActionListener(this);
      Set4.addActionListener(this);
      setBackground(new Color(245,245,245));
      Init.setBackground(Color.cyan);
      Copy1.setBackground(Color.yellow);
      Copy2.setBackground(Color.yellow);
      Set2.setBackground(Color.pink);
      Set4.setBackground(Color.pink);
      repaint(1);
   }

   void initialize() {
      for (int i=0; i < A_size; i++)
         A[i] = new Point(i,i*i);
   }

   void copy_points_1(int i, int j) {
      A[i] = A[j];
   }

   void copy_points_2(int i, int j) {
      A[i].x = A[j].x;
      A[i].y = A[j].y;
   }

   void set_points(int i, int x, int y) {
      A[i].x = x;
      A[i].y = y;
   }

   public void paint(Graphics g) {
      int y;
      y = 15;
      g.drawString("i",10,y);
      g.drawString("x",30,y);
      g.drawString("y",50,y);
      for (int i=0;i < A_size;i++) {
         y = i*15+30;
         g.drawString(i+":",10,y);
         g.drawString(A[i].x+" ",30,y);
         g.drawString(A[i].y+" ",50,y);
      }
      g.setColor(Color.blue);
      g.drawString("Init:",80,50);
      g.drawString("Initialize array to A[i].x=i and A[i].y=i*i",140,50);
      g.drawString("Copy 1:",80,75);
      g.drawString("A[4] = A[2]",140,75);
      g.drawString("Copy 2:",80,100);
      g.drawString("A[4].x = A[2].x and A[4].y = A[2].y",140,100);
      g.drawString("Set 2:",80,125);
      g.drawString("A[2].x = 10 and A[2].y = 30",140,125);
      g.drawString("Set 4:",80,150);
      g.drawString("A[4].x = 50 and A[4].y = 80",140,150);
   }

   public void actionPerformed (ActionEvent e) {
      if (e.getSource() == Init) {
         initialize();
      }  
      else if (e.getSource() == Copy1) {
         copy_points_1(4,2);
      }  
      else if (e.getSource() == Copy2) {
         copy_points_2(4,2);
      }  
      else if (e.getSource() == Set2) {
         set_points(2,10,30);
      }
      else if (e.getSource() == Set4) {
         set_points(4,50,80);
      }
      repaint(1);
   }

}

Arrays with Multiple Dimensions

What is the meaning of the expression "Multi-dimensional Array"?

C only has one-dimensional arrays, but you can have arrays of arrays giving you the functionality of multi-dimensional arrays. In C you would do something like this:

   double matrix[10][20];
   int i,j;

   for (i=0;i<10;i++)
      for (j=0;j<20;j++)
         matrix[i][j] = i+j;

Some languages allow you to use multi-dimensional array syntax such as matrix[i,j] but not C or Java.

Like C, Java only has one-dimensional arrays.

class Test09 {
   public static void main(String args[]) {
      double   matrix[][];
      byte[][] bytematrix = {{2,3,4},{6,8,10}};
 
      matrix = new double[3][12];
      int i,j;
 
      System.out.println("matrix has lengths " +
         matrix.length + " and " + matrix[0].length);
      System.out.println("bytematrix has lengths " +
         bytematrix.length + " and " +
         bytematrix[0].length);
      for (i=0; i<2; i++)
         for (j=0; j<3; j++)
         System.out.println(
            "bytematrix[" + i + "][" + j + "] = " +
            bytematrix[i][j]);
   }
}
 
java Test09
matrix has lengths 3 and 12
bytematrix has lengths 2 and 3
bytematrix[0][0] = 2
bytematrix[0][1] = 3
bytematrix[0][2] = 4
bytematrix[1][0] = 6
bytematrix[1][1] = 8
bytematrix[1][2] = 10

Strings

A String in Java is just a predefined class.

once created, a string is read-only
you cannot add characters to a string
strings are not terminated by a terminator
constant strings can be defined using double quotes as in C
the length method returns the length of the string
the + operator can concatenate strings
if only one argument of + is a string, the other will be converted to a string
the charAt method returns individual characters of the string indexed from 0 to length()-1
the equals() method can be used to compare two strings

A String Example

class Test10 {
   public static void main(String args[]) {
      String str1 = "This is a string";
      String str2;
      String str3;
      String str4;
      String str5;
      int i = 5;
      char fourth;

      System.out.println("str1: " + str1);
      System.out.println("str1 has length " + str1.length());
      str2 = str1 + i;
      System.out.println("str2: " + str2);
      str3 = "This is" + " a string";
      str4 = " a string";
      str5 = "This is" + str4;
      System.out.println("str3: " + str3);
      System.out.println("str5: " + str5+"\n");
      if (str1.equals(str3))
          System.out.println("str1 equals str3");
      else
          System.out.println("str1 and str3 are different");
      if (str1 == str3)
          System.out.println("str1 == str3");
      else
          System.out.println("str1 != str3");
      if (str1.equals(str5))
          System.out.println("str1 equals str5");
      else
          System.out.println("str1 and str5 are different");
      if (str1 == str5)
          System.out.println("str1 == str5");
      else
          System.out.println("str1 != str5");

      str1 = "This is a new string 1";
      System.out.println("\nstr1: " + str1);
      if (str1.equals(str3))
          System.out.println("str1 equals str3");
      else
          System.out.println("str1 and str3 are different");
      if (str1 == str3)
          System.out.println("str1 == str3");
      else
          System.out.println("str1 != str3");
      fourth = str1.charAt(3);
      System.out.println(
         "\nThe fourth character in str1 is " + fourth);
   }
}
str1: This is a string
str1 has length 16
str2: This is a string5
str3: This is a string
str5: This is a string

str1 equals str3
str1 == str3
str1 equals str5
str1 != str5

str1: This is a new string 1
str1 and str3 are different
str1 != str3

The fourth character in str1 is s

A Command Line Arguments Example

class Test11 {
   public static void main(String args[]) {
      int numargs;
      int i;
 
      numargs = args.length;
      System.out.println(
         "Number of command line arguments: " + numargs);
      for (i=0; i< numargs; i++)
         System.out.println("   " + i + ": " + args[i]);
   }
}
 
java Test11
Number of command line arguments: 0
 
java Test11 abc def ghij
Number of command line arguments: 3
   0: abc
   1: def
   2: ghij
 
java Test11 abc "def ghij"
Number of command line arguments: 2
   0: abc
   1: def ghij

Arrays of Characters

The copyValueOf(char[]) static method of the class String returns a string made from a character array. The method
getChars(int srcBegin, int srcEnd, char dst[], int dstBegin)
copies characters from the string into the array starting at srcBegin and ending before srcEnd.

class Test12 {
   public static void main(String args[]) {
      String str1 = "A string";
      String str2;
      char arr[];
      int strlen;
      int i;
 
      strlen = str1.length();
      System.out.println("str1: " + str1);
      System.out.println("str1 has length " + strlen);
      arr = new char[strlen];
      str1.getChars(0,strlen,arr,0);
      for (i=0; i< strlen; i++)
         System.out.println("   " + i + ": " + arr[i]);
      arr[2] = 'S';
      str2 = String.copyValueOf(arr);
      System.out.println("str2: " + str2);
   }
}
 
java Test12
str1: A string
str1 has length 8
   0: A
   1:
   2: s
   3: t
   4: r
   5: i
   6: n
   7: g
str2: A String

String Buffers

The StringBuffer class does not require prior knowledge of the size of the string.

class Test13 {
   public static void main(String args[]) {
      StringBuffer mystrbuf = new StringBuffer();
      String str1 = "A string of any size";
      String str2 = " Another string of any length";
      String str3;
 
      System.out.println("str1: " + str1);
      System.out.println("str2: " + str2);
      mystrbuf.append(str1);
      mystrbuf.append(str2);
      str3 = mystrbuf.toString();
      System.out.println("str3: " + str3);
   }
}
 
java Test13
str1: A string of any size
str2:  Another string of any length
str3: A string of any size Another string of any length

When to use String, StringBuffer, char[]

String supports:

Literal strings in double quotes
Comparison of strings
Location of a substring
Conversion of almost anything to a String

StringBuffer supports:

Inserting and appending
Setting the length

char[] supports:

Inserting and Reading individual characters

Operators

Java operators are similar to C operators.

Here are a few of the differences.

No unary & * operators
No -> or sizeof operators
The comma operator can only be used in the initialization and continuation sections of a for loop.
If one argument of the binary + operator is a string the other argument will be converted to a string and the strings will be concatenated.
The >>> operator is an unsigned shift right.

Flow Control

The following are similar to C:

if-else
switch
while and do-while
for
break
continue
return

but no goto and a break or continue can refer to a label and transfer control out of a block from inside a nested block.

double arr[][];
 
outer:
   for (i=0; i < arr.length; i++) {
      for (j=0; i < arr[0].length; j++) {
         if (arr[i][j] == val) {
            foundit = true;
            break outer;
         }
      }  
   }

Like in C++, you can declare a variable in the initialization section of a for loop, but the scope of this variable is the body of the for loop.

Classes

When the following is compiled, two files are created:
Test14.class and DPoint.class.

Note the two constructors for the class Point.

class DPoint {
   protected double x;
   protected double y;
   DPoint (double x, double y) {
      this.x = x;
      this.y = y;
   }
   DPoint () {
      x = 0;
      y = -1;
   }
   public void show () {
      System.out.println("("+x+","+y+")");
   }
}

class Test14 {
   public static void main(String args[]) {
      DPoint p1;
      DPoint p2;
      p1 = new DPoint(3.5, 4.7);
      p1.show();
      p2 = new DPoint();
      p2.show();
   }
}

java Test14
(3.5,4.7)
(0.0,-1.0)

The class Point3D extends the class DPoint.

class Point3D extends DPoint {
   protected double z;
   Point3D(double x, double y, double z) {
      this.x = x;
      this.y = y;
      this.z = z;
   }
   public void show() {
      System.out.println("("+x+","+y+","+z+")");
   }
}

class Test15 {
   public static void main(String args[]) {
      Point3D p1;
      p1 = new Point3D(3, 4, 9);
      p1.show();
   }
}
java Test15
(3.0,4.0,9.0)

Exceptions

In C, a fatal error can cause a core dump.

This cannot happen in Java.

In C under Unix, system errors are handled using errno.

Java handles these with exceptions.

A method which can cause an exception either handles it itself or can throw the exception to the calling method.

Examples of exceptions:

NullPointerException
ArrayIndexOutOfBoundsException
IOException
NumberFormatException

Methods that can throw an exception should be wrapped in a try --- catch

try {
   c = a/b;
} catch(ArithmeticException e) {
   System.out.println("Divide by 0");
}

Packages, Classes, Interfaces, Methods, Fields, and Objects

An object groups data and code.

The code in an object is made up of methods.

The data is contained in fields.

A class is a description of the methods and fields of a collection of objects.

A constructor is a special method in a class which is used to initialize an object when the object is created.

An interface contains constants and method prototypes.

A class can extend another class and implement any number of interfaces.

A package is a collection of classes.

Packages and Name Space Resolution

Packages allow a collection of classes to grouped together.

In addition to allowing for better organization it allows you to use the same class names as someone else.
In Java programs, objects in other packages can be specified using the package name: packagename.objectname.
Java compilers and interpreters look for classes in directories based on the package name and class name.
The directory name is formed by taking packagename.classname and replacing all periods with '/'.
The packagename may contain embedded periods but the classname may not.
You specify the package name in the source file by having the first line: package packagename;
The java compiler, interpreter, and appletviewer use the CLASSPATH environment variable to determine the base directory for class searches.

Class Modifiers

abstract

Contains abstract methods (methods without implementation), or is otherwise not directly implemented. final

final

public

private

Can only be used within a file < empty >

public

private

Method and Field Modifiers

public

Can be accessed by methods from outside of its class. protected

Can only be accessed by a subclass. private

Can only be accessed by methods in this class < empty >

public

private

protected

final

The method cannot be overridden or the variable cannot be changed. static

< Class >.method

synchronized

The method will lock the object on entry and unlock it on exit. If the object was already locked, the method waits until the lock is released before executing. native

The method is implemented by a stub written in another language.

CS 4773 Object Oriented Systems The Java Language

Primitive Types:

Type Promotion in Expressions

Assignment Conversion:

Type Casting

Java Types and Default Values

Arrays

Arrays of non-primitive type only contain references

Arrays with Multiple Dimensions

Strings

A String Example

A Command Line Arguments Example

Arrays of Characters

String Buffers

When to use String, StringBuffer, char[]

Operators

Flow Control

Classes

Exceptions

Packages, Classes, Interfaces, Methods, Fields, and Objects

Packages and Name Space Resolution

Class Modifiers

Method and Field Modifiers

Next topic: Object Oriented Programming Concepts

CS 4773 Object Oriented Systems
The Java Language