Initialization and Clean up
Advanced Programming
SUT • Spring 2019
Outline
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Init methods
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Constructors
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No Destructor
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Initialization
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Cleanup
Initialization
Initialization
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An instantiated object, is not a ready object
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It may be and invalid object
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Person p = new Person();
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p is an object without name, id and, …
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p is an invalid object
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It should be initialized
Example
public class Student {
//Mandatory
private String name;
private long id;
//Optional
private String homepage;
public void setName(String name) {
if (name != null && !"".equals(name.trim()) &&
name.matches("[a-zA-Z ]+"))
this.name = name;
}
public void setId(long id) {
if (id > 10000000 && id < 100000000)
this.id = id;
}
public void setHomepage(String homepage) {
homepage = homepage;
}
}
Initialization Method
public void init(String name, long id) {
setName(name);
setId(id);
}
Using the Object
public static void main(String[] args) {
Student st = new Student();
// st is an invalid object now
st.init("Hossein Alizadeh", 45205068);
// st is initialized now. ready to be used
System.out.println(st.getName());
System.out.println(st.getId());
}
Circle c = new Circle();
c.init(12);
Book b1 = new Book();
b1.init(“من او”, “رضا اميرخانی”);
Book b2 = new Book();
b2.init(“شاهنامه”, “ابوالقاسم فردوسی”);
init() Method
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What are the disadvantages of init() method?
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Init method is invoked manually
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There is no guarantee for init invocation
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Before calling init method, the object has an invalid state
Constructors
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Constructor is a special method
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With the same name as the class
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Without any return type
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A constructor is called when an object is instantiated
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No invalid object
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Constructor example
public class Circle {
private double radius;
public Circle(double r) {
radius = r;
}
public double getArea() {
return this.radius * this.radius * 3.14;
}
public static void main(String[] args) {
Circle c = new Circle(2);
System.out.println(c.getArea());
}
}Default Constructor
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Constructors may have parameters
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Default constructor: no parameter
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Is implicitly implemented
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You can write your own default-constructor
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If you write any constructor, default implicit constructor is vanished.
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Array Samples
public class Circle {
private double radius;
public Circle(double r) {
radius = r;
}
public Circle() {
System.out.println("Default Constructor");
}
public double getArea() {
return this.radius * this.radius * 3.14;
}
public static void main(String[] args) {
Circle c = new Circle(2);
System.out.println(c.getArea());
}
}Default Constructor
Constructor
Destructor
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Java needs no destructor
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Destructor method in C++
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Java has a finalize() method
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You can implement it for your class
Finalize method
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Java has no destructor
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Java has a special method: finalize
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finilize() is called when the object is garbage-collected
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If garbage collector is not invoked
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finalize() method is not called
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Why we may need finalize?
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Garbage collection is only about memory
Example
public class Circle {
private double radius;
public Circle(double r) {
radius = r;
}
public String toString() {
return "Circle [radius=" + radius + "]";
}
public void finalize() throws Throwable {
System.out.println("Finalize: " + toString());
}
public static void main(String[] args) {
f();
System.gc();
}
private static void f() {
Circle c = new Circle(2);
System.out.println(c);
}
}Title Text
