A review of the Java programming language.

1. Trinity College
Java Review
An Review
of the Java Programming Language
Timothy Richards
Trinity College, Hartford CT • Department of Computer Science • CPSC 225

2. What is Java?
• Java is an object-oriented PL
• It is statically typed
• Types are checked at compile-time instead of run-time
• Eliminates several errors before executing the code
• It is compiled into bytecode
• For execution on the Java Virtual Machine
• It is popular
• It has LOTS of libraries
Trinity College, Hartford CT • Department of Computer Science • CPSC 225

3. Smallest Java Program
public class Hello {
public static void main(String[] args) {
System.out.println(“Hello, World”);
}
}
Everything in Java is contained
within a class declaration.
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4. Smallest Java Program
public class Hello {
public static void main(String[] args) {
System.out.println(“Hello, World”);
}
}
To make a class executable, you
need a special method called main
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5. Smallest Java Program
public class Hello {
public static void main(String[] args) {
System.out.println(“Hello, World”);
}
}
The main method is also a static method.
A static method is associated with a class
not an instance.
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6. Smallest Java Program
public class Hello {
public static void main(String[] args) {
System.out.println(“Hello, World”);
}
}
Java has arrays. Java has Strings. In Java,
you must specify the type of things.
String[] => an array of String objects
Trinity College, Hartford CT • Department of Computer Science • CPSC 225 6

7. Smallest Java Program
public class Hello {
public static void main(String[] args) {
System.out.println(“Hello, World”);
}
}
void is also a type. It represents the
absence of a type. It can only be used to
indicate that a method returns no value.
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8. Smallest Java Program
public class Hello {
public static void main(String[] args) {
System.out.println(“Hello, World”);
}
}
So, how do we “run” this program?
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9. Smallest Java Program
public class Hello {
public static void main(String[] args) {
System.out.println(“Hello, World”);
}
}
So, how do we “run” this program?
It is a 2-step process:
1. Compile the program (javac)
2. Run the program (java)
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10. Smallest Java Program
Hello.java
public class Hello {
public static void main(String[] args) {
System.out.println(“Hello, World”);
}
}
javac
Hello.class java output
Trinity College, Hartford CT • Department of Computer Science • CPSC 225 10

11. Smallest Java Program
Hello.java
public class Hello {
public static void main(String[] args) {
System.out.println(“Hello, World”);
}
}
javac
Let’s Give it
A Try!
Hello.class java output
Trinity College, Hartford CT • Department of Computer Science • CPSC 225 11

12. Java Types
• Primitive Types
• boolean, byte, char, short, int, long, double
• Reference Types
• Boolean, Integer, Character, ...
• String
• User-defined Classes
• Arrays
• Multi-dimensional
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13. Java Classes
• Have a name
• Have visibility
• default, public, private, protected
• Have instance variables
• Have methods
• Have class variables
• Have class methods
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14. Java Classes
public class Hello {
}
This class has a name
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15. Java Classes
public class Hello {
}
It is a “public” class.
It has package-level visibility.
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16. Java Classes
public class Hello {
private String name;
private int age;
}
These are instance variables.
Note, they are “private”.
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17. Java Classes
public class Hello {
private String name;
private int age;
public String name() {
return name;
}
public int age() {
return int;
}
}
These are instance methods.
Note, they are “public”.
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18. Java Classes
public class Hello {
private String name;
private int age;
public Hello(String name, int age) {
this.name = name;
this.age = age;
}
public String name() {
return name;
}
public int age() {
return int;
}
}
This is a constructor for class Hello
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19. Java Classes
public class Hello { public class Main {
public static void main(String[] args)
private String name; {
private int age;
}
public Hello(String name, int age) { }
this.name = name;
this.age = age;
}
public String name() {
return name;
}
public int age() {
return int;
}
}
Let’s create a new class called Main...
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20. Java Classes
public class Hello { public class Main {
public static void main(String[] args)
private String name; {
private int age; Hello h1 = new Hello(“tim”, 34);
Hello h2 = new Hello(“sue”, 79);
public Hello(String name, int age) { }
}
this.name = name;
this.age = age;
}
public String name() {
return name;
}
public int age() {
return int;
}
}
This is how you “create” new objects...
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21. Java Classes
public class Hello { public class Main {
public static void main(String[] args)
private String name; {
private int age; Hello h1 = new Hello(“tim”, 34);
Hello h2 = new Hello(“sue”, 79);
public Hello(String name, int age) { }
}
this.name = name;
this.age = age;
}
h1 h2
public String name() {
return name;
}
“tim” 34 “sue” 79
public int age() {
return int;
}
}
This is how you “create” new objects...
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22. Java Classes
public class Hello { public class Main {
public static void main(String[] args)
private String name; {
private int age; Hello h1 = new Hello(“tim”, 34);
Hello h2 = new Hello(“sue”, 79);
public Hello(String name, int age) {
System.out.println(h1.name());
this.name = name; System.out.println(h1.age());
this.age = age; System.out.println(h2.name());
} System.out.println(h2.age());
}
}
public String name() {
return name;
}
public int age() {
return int;
}
}
We can print their contents by calling
their public methods
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23. Java Classes
public class Hello { public class Main {
public static void main(String[] args)
private String name; {
private int age; Hello h1 = new Hello(“tim”, 34);
Hello h2 = new Hello(“sue”, 79);
public Hello(String name, int age) {
System.out.println(h1.name());
this.name = name; System.out.println(h1.age());
this.age = age; System.out.println(h2.name());
} System.out.println(h2.age());
}
}
...
public toString() {
return “Hello(” + name + “ “ +
age + “)”
}
}
Or, we could include a toString method...
Trinity College, Hartford CT • Department of Computer Science • CPSC 225 23

24. Java Classes
public class Hello { public class Main {
public static void main(String[] args)
private String name; {
private int age; Hello h1 = new Hello(“tim”, 34);
Hello h2 = new Hello(“sue”, 79);
public Hello(String name, int age) {
System.out.println(h1);
this.name = name; System.out.println(h2);
this.age = age; }
} }
...
public toString() {
return “Hello(” + name + “ “ +
age + “)”
}
}
And simply print the object
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25. Java Classes
public class Hello { public class Main {
public static void main(String[] args)
private String name; {
private int age; Hello h1 = new Hello(“tim”, 34);
Hello h2 = new Hello(“sue”, 79);
public Hello(String name, int age) {
System.out.println(h1);
this.name = name; System.out.println(h2);
this.age = age; }
} }
...
public toString() {
return “Hello(” + name + “ “ +
age + “)”
}
}
Let’s Try It!
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26. Java Control
• Making choices
• if-then-else
• switch
• Looping
• for loop
• while loop
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27. Java Control - if-else
public class Main {
public static void main(String[] args)
{
// Here is a comment
/* Here is also a comment
on several lines */
// declare a variable
int val = 5;
// check its value
if (val == 5)
System.out.println(“five”);
}
}
Trinity College, Hartford CT • Department of Computer Science • CPSC 225 27

28. Java Control - if-else
public class Main {
public static void main(String[] args)
{
// Here is a comment
/* Here is also a comment
on several lines */
// declare a variable
int val = 5;
// check its value
if (val == 5)
System.out.println(“five”);
else
System.out.println(“not sure”);
}
}
Trinity College, Hartford CT • Department of Computer Science • CPSC 225 28

29. Java Control - if-else
public class Main {
public static void main(String[] args)
{
// Here is a comment
/* Here is also a comment
on several lines */
// declare a variable
int val = 5;
// check its value
if (val == 5) {
System.out.println(“five”);
}
else {
System.out.println(“not sure”);
}
}
}
Trinity College, Hartford CT • Department of Computer Science • CPSC 225 29

30. Java Control - switch
public class Main {
public static void main(String[] args)
{
// Here is a comment
/* Here is also a comment
on several lines */
// declare a variable
int val = 5;
// check its value
switch (val) {
case 3:
System.out.println(“three”);
break;
case 4:
System.out.println(“four”);
break;
case 5:
System.out.println(“five”);
break;
default:
system.out.println(“not sure”);
}
}
}
Trinity College, Hartford CT • Department of Computer Science • CPSC 225 30

31. Java Control - for loop
public class Main {
public static void main(String[] args)
{
// Here is a comment
/* Here is also a comment
on several lines */
// declare a variable
int val = 5;
for (int i = 0; i < val; i++) {
System.out.println(i);
}
}
}
Trinity College, Hartford CT • Department of Computer Science • CPSC 225 31

32. Java Control - while loop
public class Main {
public static void main(String[] args)
{
// Here is a comment
/* Here is also a comment
on several lines */
// declare a variable
int val = 5;
while (val != 0) {
System.out.println(val);
val--;
Let’s Try It!
}
}
}
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33. Java IO
• Input/Output
• Java Provides Streams
• Streams can be layered
• Each layer provides a “service”
• Three Standard Streams (static)
• System.out
• System.in
• System.err
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34. Java IO
public class Main {
public static void main(String[] args) System.out is a object of
{ type PrintStream.
// We have seen System.out:
System.out.println(“hello”);
} It provides many
} convenient methods for
printing data values
conveniently
Trinity College, Hartford CT • Department of Computer Science • CPSC 225 34

35. Java IO
public class Main {
System.in is an object of public static void main(String[] args) System.out is a object of
type InputStream. It { type PrintStream.
// We have seen System.out:
provides methods for System.out.println(“hello”);
reading bytes. It provides many
// Reading input: convenient methods for
int val = System.in.read();
} printing data values
} conveniently
Trinity College, Hartford CT • Department of Computer Science • CPSC 225 35

36. Java IO
public class Main {
System.in is an object of public static void main(String[] args) System.out is a object of
type InputStream. It { type PrintStream.
// We have seen System.out:
provides methods for System.out.println(“hello”);
reading bytes. It provides many
// Reading input: convenient methods for
int val = System.in.read();
How do I read a } printing data values
line of input? } conveniently
Trinity College, Hartford CT • Department of Computer Science • CPSC 225 36

37. Java IO
public class Main {
System.in is an object of public static void main(String[] args) System.out is a object of
type InputStream. It { type PrintStream.
// We have seen System.out:
provides methods for System.out.println(“hello”);
reading bytes. It provides many
// Reading input: convenient methods for
int val = System.in.read();
How do I read a printing data values
line of input? InputStreamReader is = conveniently
new InputStreamReader(System.in);
BufferedReader br =
You need to wrap new BufferedReader(is);
System.in with an String line = br.readLine();
InputStreamReader
System.out.println(line);
and then a }
BufferedReader! }
Trinity College, Hartford CT • Department of Computer Science • CPSC 225 37

38. Java IO
public class Main {
System.in is an object of public static void main(String[] args) System.out is a object of
type InputStream. It { type PrintStream.
// We have seen System.out:
provides methods for System.out.println(“hello”);
reading bytes. It provides many
// Reading input: convenient methods for
int val = System.in.read();
How do I read a printing data values
line of input? InputStreamReader is = conveniently
new InputStreamReader(System.in);
BufferedReader br =
You need to wrap new BufferedReader(is);
System.in with an String line = br.readLine();
InputStreamReader
System.out.println(line);
and then a }
BufferedReader! }
Turns out this is not
quite right...
Let’s give it a try
and see...
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39. Exceptions
• Represent exceptional conditions
• A class can throw exceptions
• Indicated as part of a method definition
• Exceptions can be caught
• Using a try-catch block
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40. Exceptions
import java.io.InputStreamReader;
Easy way: import java.io.BufferedReader;
have someone else
public class Main {
handle the problem! public static void main(String[] args) throws IOException
{
// We have seen System.out:
System.out.println(“hello”);
// Reading input:
int val = System.in.read();
InputStreamReader is =
new InputStreamReader(System.in);
BufferedReader br =
new BufferedReader(is);
String line = br.readLine();
System.out.println(line);
}
}
Trinity College, Hartford CT • Department of Computer Science • CPSC 225 40

41. Exceptions
import java.io.InputStreamReader;
Easy way: import java.io.BufferedReader;
have someone else
public class Main {
handle the problem! public static void main(String[] args)
{
Hard way: // We have seen System.out:
System.out.println(“hello”);
Deal with the problem
using a try-catch block try {
// Reading input:
int val = System.in.read();
InputStreamReader is =
new InputStreamReader(System.in);
BufferedReader br =
new BufferedReader(is);
String line = br.readLine();
System.out.println(line);
}
catch (IOException e) {
System.out.println(e);
}
}
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42. Packages
• Convenient mechanism to group related
classes
• A package is defined in two parts:
• A java file begins with a package declaration
• That same java file exists in a directory structure that
mirrors the package name
• Example
• package: cpsc225.example
• directory: cpsc225/example/Hello.java
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43. Packages
cpsc225/example cpsc225/main cpsc225/util
Foo.java Bar.java Baz.java
public class Foo { import cpsc225.example.Foo; public class Baz {
... import cpsc225.util.Baz ...
} }
public class Bar {
public static void main
(String[] args) {
Foo f1 = new Foo();
Baz b1 = new Baz();
cpsc225.example.Foo f2 =
new cpsc225.example.Foo();
cpsc225.util.Baz b2 =
new cpsc225.util.Baz();
}
}
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44. Packages
cpsc225/example cpsc225/main cpsc225/util
Foo.java Bar.java Baz.java
package cpsc225.example; package cpsc225.main; package cpsc225.util;
public class Foo { import cpsc225.example.Foo; public class Baz {
... import cpsc225.util.Baz ...
} }
public class Bar {
public static void main
(String[] args) {
Foo f1 = new Foo();
Baz b1 = new Baz();
Let’s Try It!
cpsc225.example.Foo f2 =
new cpsc225.example.Foo();
cpsc225.util.Baz b2 =
new cpsc225.util.Baz();
}
}
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45. Hands-On Activity
$ git clone git://github.com/timdrichards/java-review.git
cd lab-01
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