Java Course 3: OOP

Java course - IAG0040

More Java Basics
and OOP

Anton Keks 2011

Construction
●
All objects are constructed using the new keyword
– MyClass fooBar = new MyClass();
●
Object creation always executes a constructor
– Constructors may take parameters as any other
methods
● Default constructor (without parameters) exists if
there are no other constructors defined
● Constructor is a method without a return type (it
returns the object's instance), the name of a
constructor is the same as of the class.
Java course – IAG0040 Lecture 3
Anton Keks Slide 2

Destruction
●
No need to destroy objects manually
●
Garbage Collector (GC) does the job of destruction
– Executed in background when memory gets low
– Can actually be faster than explicit deallocation
– Can be invoked manually with System.gc();
● An object is destroyed if there are no references left to
it in the program, this almost eliminates memory leaks
● Out of scope local variables are also candidates
● finalize() method may be used instead of a destructor,
however, its execution is not guaranteed
Anton Keks Slide 3

Conditions
●
if-else
– if (boolean-expression) a boolean-expression
should return a boolean
statement or Boolean value
else
statement
●
switch
an expression can return an
– switch (expression) { int, short, char, byte, their
case X: respective wrapper classes
statement (which are unboxed) or an
break; enum type
default:
statement a statement is a single Java
break; statement or multiple
} statements in curly braces { }

Anton Keks Slide 4

While loops
●
while
– while (boolean-expression)
statement
– Executes the statement while boolean-expression evaluates to true
● do-while
– do
statement
while (boolean-expression);
– Same, but boolean-expression is evaluated after each iteration, not
before

Anton Keks Slide 5

For loops
●
for loop
– for(initialization; boolean-expression; step)
statement
– Example: for(int i=0, j=1; i < 5; i++, j*=2) {}
●
for each loop (arrays and Iterables)
– for(variable : iterable)
statement
– for (int a : new int[] {1,2,3}) {}
– iterates over all elements of an iterable, executing the
statement for each of its elements, assigning the element itself
to the variable
Anton Keks Slide 6

break and continue
●
break and continue keywords can be used in any loop
– break interrupts the innermost loop
– continue skips to the next iteration of the innermost loop
●
goto keyword is forbidden in Java, but break and
continue can be used with labels
– outer: while(true) {
inner: while(true) {
break outer;
}
}
– Labels can be specified only right before a loop. Usage of break or
continue with a label affects the labeled loop, not the innermost
one.
Anton Keks Slide 7

Arrays
●
Array is a special type of Object
– Arrays are actually arrays of references
– Elements are automatically initialized with zeros or nulls
– Automatic range checking is performed, always zero based
● ArrayIndexOutOfBoundsException is thrown otherwise
– Special property is accessible: length
● Definition: type followed by [ ]: byte[] a; String s[];
● Creation using the new keyword: byte[] a = new byte[12];
● Access: byte b = a[0]; byte b2 = a[a.length – 1];
● Static initialization: byte[] a = new byte[] {1,2,3};
● Multidimensional: int[][] matrix = new int[3][3];
Anton Keks Slide 8

Strings
● An immutable char array with additional features
● Literals: “!” same as new String(new char[] {'!'})
● Any object can be converted toString()
● All primitive wrappers understand Strings: new Integer(“3”)
● Equality is checked with equals() method
● String pooling
– “ab” != new String(“ab”)
– “a1” == “a” + 1
● StringBuilder is used behind the scenes for concatenation

Anton Keks Slide 9

Classpath
●
Classpath tells the JVM where to look for
packages and classes
– java -cp or java -classpath
– $CLASSPATH environment variable
● Works like $PATH environment variable
● Every used class must be present on classpath
during both compilation and runtime

Anton Keks Slide 10

Classpath example
●
Your class net.azib.Hello
● Uses org.log4j.Logger and com.sun.World
● In filesystem:
– ~/myclasses/net/azib/Hello.class
– ~/log4j/org/log4j/Logger.class
– ~/lib/sun.jar, which contains com/sun/World.class
●
Should be run as following: (separated with “;” on Windows)

– java -cp myclasses:log4j:lib/sun.jar net.azib.Hello
--- classpath ---

Anton Keks Slide 11

Java API documentation

Let's examine the API documentation a bit:
http://java.sun.com/javase/6/docs/api/
(http://java.azib.net -> Lirerature & Links -> API documentation)

Anton Keks Slide 12

Javadoc comments
●
Javadoc allows to document the code in place
– Helps with synchronization of code and documentation
● Syntax: /** a javadoc comment */
– Documents the following source code element
– Tags: start with '@', have special meaning
● Examples: @author, @version, @param
● @deprecated – processed by the compiler to issue warnings
– Links: {@link java.lang.Object}, use '#' for fields or methods
● {@link Object#equals(Object)}
● javadoc program generates HTML documentation

Anton Keks Slide 13

Varargs
●
A way to pass unbounded number of parameters to methods
●
Is merely a convenient syntax for passing arrays
● void process(String ... a)
– In the method body, a can be used as an array of Strings
– There can be only one varargs-type parameter, and it should
always be the last one
● Caller can use two variants:
– process(new String[] {“a”, “b”}) - the old way
– process(“a”, “b”) - the varargs (new) way
● System.out.printf() uses varargs
Anton Keks Slide 14

Autoboxing
●
Automatically wraps/unwraps primitive types into
corresponding wrapper classes
●
Boxing conversion, e.g. boolean -> Boolean, int ->
Integer
●
Unboxing conversion, e.g. Character -> char, etc
● These values are cached: true, false, all bytes, chars
from u0000 to u007F, ints and shorts from -128 to
127
● Examples:
Integer n = 5; // boxing
char c = new Character('z'); // unboxing
Anton Keks Slide 15

Extending classes
●
Root class: java.lang.Object (used implicitly)
● Unlike C++, only single inheritance is supported
● class A {} // extends Object
class B extends A {}
– new B() instanceof A == true
– new B() instanceof Object == true
● Access current class/instance with this keyword
● Access base class with super keyword

Anton Keks Slide 16

Extending classes
● All methods are polymorphic (aka virtual in C++)
– final methods cannot be overridden
– super.method() calls the super implementation (optional)
● Constructors always call super constructors
– if no constructors are defined, default one is assumed
● class A { } // implies public A() {}
– default constructor is called implicitly if possible
● class B extends A {
public B(int i) { } // super() is called
public B(byte b) { super(); } // same here
}

Anton Keks Slide 17

Abstract classes
● Defined with the abstract keyword
● Not a concrete class, cannot be instantiated
●
Can contain abstract methods without implementation
● abstract class Animal { // new Animal() is not allowed
private String name;
String getName() { return name; }
abstract void makeSound();
}
● class Dog extends Animal {
void makeSound() {
System.out.println(“Woof!”);
}
}

Anton Keks Slide 18

Interfaces
●
Substitute for Java's lack of multiple inheritance
– a class can implement many interfaces
●
Interface are extreme abstract classes without
implementation at all
– all methods are public and abstract by default
– can contain static final constants
– marker interfaces don't define any methods at all, eg Cloneable
● public interface Eatable {
Taste getTaste();
void eat();
}
● public class Apple implements Eatable, Comparable
{ /* implementation */ }

Anton Keks Slide 19

Java Course 3: OOP

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