Lecture Notes: Java Basics
By Jack G. Zheng, CIS@Georgia State University
Last updated 8/16/2006
This lecture assumes you have basic programming language experience with
C/C++, C#. For those who have taken C# course, here is a good 
syntax
comparison of C# and Java (J2SE 5.0). You can also download
examples in a ZIP file.
Topics
- Data types and variables
- Operators
- Control flow statements
- Conditional statements
- Iteration statements
Data types
Java is a strong type language; everything belongs to a type.
- Primitive type: simple, built-in data types provided by the programming language
- Reference type: or composite data type, its value (content) is usaully stored in a separate memory location
Primitive data types
Keyword |
Description |
Size (value range) |
(integers)
|
||
byte |
Byte-length integer |
8-bit (-128 to 127) |
short |
Short integer |
16-bit (-32768 to 32767) |
int |
Integer |
32-bit (-2^32/2 to 2^32/2-1) - about 2
billion |
long |
Long integer |
64-bit (…) |
(real numbers)
|
||
float |
Single-precision floating point |
32-bit |
double |
Double-precision floating point |
64-bit |
(other types)
|
||
char |
A single character |
16-bit Unicode character: 'a', '?' |
boolean |
A boolean value |
true, false |
Note: String is reference type
- For value ranges, see code example: PrimitiveDataType.java
Variables
Naming rules
- Starting with a letter, “_”, or “$”
- Consists of only letters, numbers, “_” or “$”
- Cannot be a Java keyword (http://java.sun.com/docs/books/tutorial/java/nutsandbolts/_keywords.html), such as class, default, try ...
- Naming conventions: what convention do you follow?
Variable declaration and assignment
- The value assigned to a variable has to be compatible with its data type.
- See code example: VariableAssignment.java
public static void main(String[] args)
{
// integer
int int1; //declaration
int1=1; //assignment
long long1=2;
System.out.println("int1: "+int1);
System.out.println("long1: "+long1);
// decimal
double d1=1.0;
double d2=1; //integers can be assigned to a decimal type variable
System.out.println("d1: "+d1);
System.out.println("d2: "+d2);
// char data type
char c1='A';
char c2=65; //ASCII code value can be used
System.out.println("c1: "+c1);
System.out.println("c2: "+c2);
// boolean
boolean b1=true;
boolean b2=false;
System.out.println("b1: "+b1);
System.out.println("b2: "+b2);
//String
String s1="hello";
System.out.println("s1: "+s1);
//constants, using the keyword "final"
final double PI=3.1415926; // trying to assign another value later will generate a compiler error
System.out.println("PI: "+PI);
}
Operators
- Arithmetic operators
- Relational and conditional operators
- Assignment operators
- Other operators
Arithmetic operator
- +, -, *, /, %
- What is the result of 5+2, 5-2, 5*2, 5/2, 5%2?
- ++ (increment), -- (decrement)
i + + vs. + + i - int i=1;
int y= i++; // y = =1 and i = =2;
int x= ++i; // x = =2 and i = =2; - See code example: ArithmeticOperator.java
Relational and conditional operators
- >, <, >=, <=, = =, !=
Conditional (Logical) Operators
- Logical “and”
- & (always evaluates both sides)
- && (conditionally evaluates the right side)
- Logical “or”
- | (always evaluates both sides)
- || (conditionally evaluates the right side)
- Not: !
- See code example: ConditionalOperator.java
Assignment operator
- =
- Shorthand assignments
- +=, -=, *=, /=
- i+=j; i=i+j;
Other operator
- Concatenation using +
System.out.print("Hello " + "World"); - Shortcut condition ? :
expression ? value1 : value2 - Casting type ()
i=(int)1.8; ( i= =1)
i=(double)5; (i= =5.0)
Operator Precedence
For complete precedence table, see http://java.sun.com/docs/books/tutorial/java/nutsandbolts/operators.html
Control flow statements
- Conditional statements
if … else
switch - Iteration statements
for
while
do … while
Conditional statement if ... else
- if (expression)
{
statement(s);
} - if (expression) //expression evaluates to true
{
statement(s);
}
else //expression evaluates to false
{
statement(s);
} - Expression can be anything (variable, method, logical comparison) that returns a boolean value (true/false)
- Shortcut condition operator ? :
var = expression ? value1 : value2;
// equivalents to
if (expression)
{ var = value1; }
else
{ var = value2; }
Multiple conditions, use else if
income=41000;
// use "if ... else if"
if (income >= 50000)taxRate=0.4; // note: curly braces are optional for a single statement
else if (income >= 40000)
taxRate=0.35;
else if (income >= 30000)
taxRate=0.3;
else if (income >= 20000)
taxRate=0.25;
else
taxRate=0.2;
System.out.println("Income: "+income+"\tTax rate: "+taxRate);
- see example MultiConditions.java
Switch
- Conditions based an expression/variable of integer, byte, short int or char data type
char k='C';
switch (k)
{
case 'A': System.out.println("1. "+k); break;
case 'B': System.out.println("2. "+k); break;
case 'C': System.out.println("3. "+k); break;
default: System.out.println("Default. "+k);
}
- break is important
- compare switch and a set of "if ... else if" statements
Loop
while loop
int i=0;
while (i<10)
{
System.out.print("*");
i=i+1;
}
do while loop
int i=0;
do
{
System.out.print("*");
i=i+1;
} while (i<10);
for loop
for (initialization; termination/continuation; increment/decrement)
{ statement(s); }
int i;
for (i=0;i<10;i++)
{
System.out.print("*");
}
for (int i=0;i<10;i=i+2)
System.out.print( "*"); // note: curly braces are optional for a single statement }
Loop summary
- Any type of loop can be written in the form of other two types, achieving
the same result
- See code example: Loops.java
- Loop applications
- Read the value of every elements in an array
- Dynamically generate a grid/table pattern (nested loops, see example NestedLoop.java)
- Read every record retrieved from the database
Dead (Infinity) Loops
int m, n;
for (m=1; m < 10; n ++) { ... }
int i=0;
while (i<10)
{
System.out.println("endless");
}
Summary
- Java language basic syntax resembles C/C++/C#
- This session give you a quick introduction / review of Java
basic syntax on
- Data types and variables
- Operators
- Control flow statements