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Basic Java Syntax
INE2720
Web Application Software Development
Essential Materials
Outline
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Primitive types
Arithmetic, logical and relational operators
Conditional expressions and loops
Reference types
Building arrays
Using wrappers to convert primitive data
types to objects
Handling exceptions
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Primitive Types
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Java has two fundamental kinds of data
types: Primitive and Reference
– Primitive are those types not “objects”
– Boolean: true / false
– char: 16-bit unsigned integer representing a
Unicode character.
– byte: 8-bit, signed, two’s complement integer.
– short: 16-bit, signed, two’s complement integer.
– int: 32-bit, signed, two’s complement integer.
– long: 64-bit, signed, two’s complement.
– float: 32-bit floating-point, double: 64-bit fp.
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Primitive-type conversion
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Type2 type2Var = (Type2) type1Var;
int i = 3;
byte b = (byte) i;
// cast i to a byte
long x = 123456L;
short s = (short) x;
// cast x to a short, lossy
double d = 3.1416;
float f = (float) d;
// cast from 64 to 32 bits
short s = (short) f;
// cast a float to a short
int i = s;
// upward conversion, no cast is needed
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Operators
Operators
Meaning
Example
+, -
Add and subtract
x = y + 5;
*, /, %
Multiply, divide, remainder
z = x / y;
++, --
Prefix/postfix increment and decrement
x = i++; y = --j;
<<, >>, >>> Signed and unsigned shifts
y = x << 2;
~
Bitwise complement
x = ~127; // -128
&, |, ^
Bitwise AND, OR, XOR
x = 127 & 2;
==, !=
Equality, inequality
x = (1 == 1);
<, <=, >, >=
Numeric less than, less than or equal to, …
x = (2 > 3);
&&, ||
Logical AND, OR
(2 > 3) && (3 ==3)
!
Logical negation
!(2 > 3)
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Conditionals
Operators
Standard Forms
if
if (Boolean-expression) {
statement;
}
if (Boolean-expression) {
statement1;
} else {
statement2;
}
?:
Boolean-expression ? thenVal1 : elseVal2;
Switch
Switch (someInt) {
case val1: statement1; break;
case val2: statement2; break;
default: statementN;
}
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Iterations / Loops
Construct
Standard forms
while
while (continueTest) {
body;
}
do
do {
body;
} while (continueTest);
for
for (init; continueTest; updateOp) {
body;
}
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Reference Types
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Values that are objects are known as
reference values or references.
Any non-primitive variables are known
as objects and can be treated as
pointers.
Java forbids dereferencing pointers.
– Given a referenced object.
– A method cannot modify a reference so as
to refer to another object.
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Arrays
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Accessing arrays
– Access arrays by supplying the index in square brackets after the
variable name, variableName[index]
– The first index is 0, not 1
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Example
– Here, the argument to main is an array of Strings called args
public class Test {
public static void main(String[] args) {
System.out.println("First argument: " + args[0]);
}
}
> javac Test.java
> java Test Hello There
First argument is Hello
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The Array length Field
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Arrays have a built-in field called length that stores the size of
the array
– The length is one bigger than the biggest index, due to the fact
that the index starts at 0
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Example
public class Test2 {
public static void main(String[] args) {
System.out.println("Number of args is " + args.length);
}
}
> javac Test2.java
> java Test2
Number of args is 0
> java Test2 Hello There
Number of args is 2
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Building Arrays
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Arrays can be built in a one-step or two-step
process
1. The one-step process is of the following form:
type[] var = { val1, val2, ... , valN };
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For example:
int[] values = { 10, 100, 1000 };
Point[] points = { new Point(0, 0),
new Point(1, 2), ... };
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Building Arrays, cont.
2.
With the two-step process, first allocate an array of references:
type[] var = new type[size];
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For example:
int[] values = new int[7];
Point[] points = new Point[someArray.length];
–
–
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For primitive data types, each array cell is assigned a default value
For object data types, each array cell is a reference (initially set to null)
Second, populate the array
points[0] = new Point(...);
points[1] = new Point(...);
...
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Multidimensional Arrays
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Multidimensional arrays are implemented as
an array of arrays
int[][] twoD = new int[64][32];
String[][] cats = { { "Caesar", "blue-point" },
{ "Heather", "seal-point" },
{ "Ted",
"red-point" } };
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Note: the number of elements in each row (dimension)
need not be equal
int[][] irregular = { { 1 }, { 2, 3, 4},
{ 5 }, { 6, 7 } };
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TriangleArray, Example
public class TriangleArray {
public static void main(String[] args) {
int[][] triangle = new int[10][];
for(int i=0; i<triangle.length; i++) {
triangle[i] = new int[i+1];
}
for (int i=0; i<triangle.length; i++) {
for(int j=0; j<triangle[i].length; j++) {
System.out.print(triangle[i][j]);
}
System.out.println();
}
}
}
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> java TriangleArray
0
00
000
0000
00000
000000
0000000
00000000
000000000
0000000000
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Wrapper Classes
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Each primitive data type has a corresponding
object (wrapper class)
Primitive
Data Type
byte
short
int
long
float
double
char
boolean
Corresponding
Object Class
Byte
Short
Integer
Long
Float
Double
Character
Boolean
– The data is stored as an immutable field of the object
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Wrapper Uses
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Defines useful constants for each data type
– For example,
Integer.MAX_VALUE
Float.NEGATIVE_INFINITY
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Convert between data types
– Use parseXxx method to convert a String to the corresponding
primitive data type
try {
String value = "3.14e6";
double d = Double.parseDouble(value);
} catch (NumberFormatException nfe) {
System.out.println("Can't convert: " + value);
}
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Wrappers: Converting
Strings
Data Type
byte
new
short
new
int
new
long
new
float
new
double
new
Convert String using either …
Byte.parseByte(string )
Byte(string ).byteValue()
Short.parseShort(string )
Short(string ).shortValue()
Integer.parseInteger(string )
Integer(string ).intValue()
Long.parseLong(string )
Long(string ).longValue()
Float.parseFloat(string )
Float(string ).floatValue()
Double.parseDouble(string )
Double(string ).doubleValue()
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Error Handling: Exceptions
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In Java, the error-handling system is based on
exceptions
– Exceptions must be handed in a try/catch block
– When an exception occurs, process flow is immediately
transferred to the catch block
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Basic Form
try {
statement1;
statement2;
...
} catch(SomeException someVar) {
handleTheException(someVar);
}
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Exception Hierarchy
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Simplified Diagram of Exception Hierarchy
Throwable
Exception
…
IOException
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Error
RuntimeException
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Throwable Types
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Error
– A non-recoverable problem that should not be
caught (OutOfMemoryError, StackOverflowError,
…)
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Exception
– An abnormal condition that should be caught and
handled by the programmer
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RuntimeException
– Special case; does not have to be caught
– Usually the result of a poorly written program
(integer division by zero, array out-of-bounds, etc.)
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A RuntimeException is considered a bug
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Multiple Catch Clauses
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A single try can have more that one catch clause
try {
...
} catch (ExceptionType1 var1) {
// Do something
} catch (ExceptionType2 var2) {
// Do something else
}
– If multiple catch clauses are used, order them from the
most specific to the most general
– If no appropriate catch is found, the exception is handed to
any outer try blocks
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If no catch clause is found within the method, then the
exception is thrown by the method
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Try-Catch, Example
...
BufferedReader in = null;
String lineIn;
try {
in = new BufferedReader(new FileReader("book.txt"));
while((lineIn = in.readLine()) != null) {
System.out.println(lineIn);
}
in.close();
} catch (FileNotFoundException fnfe ) {
System.out.println("File not found.");
} catch (EOFException eofe) {
System.out.println("Unexpected End of File.");
} catch (IOException ioe) {
System.out.println("IOError reading input: " + ioe);
ioe.printStackTrace(); // Show stack dump
} – Web Application Software Development
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The finally Clause
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After the final catch clause, an optional finally
clause may be defined
The finally clause is always executed, even if the
try or catch blocks are exited through a break,
continue, or return
try {
...
} catch (SomeException someVar) {
// Do something
} finally {
// Always executed
}
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Thrown Exceptions
If a potential exception is not handled in the method,
then the method must declare that the exception can
be thrown
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public SomeType someMethod(...) throws SomeException {
// Unhandled potential exception
...
}
– Note: Multiple exception types (comma separated) can be
declared in the throws clause
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Explicitly generating an exception
throw new IOException("Blocked by firewall.");
throw new MalformedURLException("Invalid protocol");
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Summary
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Discuss Primitive and Object types
Address the basic Java syntax
Arrays have a public length data field
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Use the wrapper classes to:
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– Convert primitive data types to objects
– Convert string to primitive data types
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Code that may give rise to an exception must be in a
try/catch block or the method must throw the
exception
– The finally clause is always executed regardless how the
try block was exited
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References
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CWP2: Chapter 8
http://java.sun.com/docs/books/jls/
The End.
Thank you for patience!
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