Transcript sina.sharif.edu

Chapter 4: Initialization and
cleanup
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●
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How to initializa an object
Method overloading
Cleanup: Finalization and gaurbage collection
Gauranteed initialization with constructor
//: c04:SimpleConstructor.java
// Demonstration of a simple constructor.
import com.bruceeckel.simpletest.*;
class Rock {
Rock() { // This is the constructor
System.out.println("Creating Rock");
}
}
public class SimpleConstructor {
static Test monitor = new Test();
public static void main(String[] args) {
for(int i = 0; i < 10; i++)
new Rock();
monitor.expect(new String[] {
"Creating Rock",
"Creating Rock",
"Creating Rock",
"Creating Rock",
"Creating Rock",
"Creating Rock",
"Creating Rock",
"Creating Rock",
"Creating Rock",
"Creating Rock"
});
}
} ///:~
Constructor with arguments
//: c04:SimpleConstructor2.java
// Constructors can have arguments.
import com.bruceeckel.simpletest.*;
class Rock2 {
Rock2(int i) {
System.out.println("Creating Rock number " + i);
}
}
public class SimpleConstructor2 {
static Test monitor = new Test();
public static void main(String[] args) {
for(int i = 0; i < 10; i++)
new Rock2(i);
monitor.expect(new String[] {
"Creating Rock number 0",
"Creating Rock number 1",
"Creating Rock number 2",
"Creating Rock number 3",
"Creating Rock number 4",
"Creating Rock number 5",
"Creating Rock number 6",
"Creating Rock number 7",
"Creating Rock number 8",
"Creating Rock number 9"
});
}
} ///:~
Method overloading
//: c04:Overloading.java
// Demonstration of both constructor
// and ordinary method overloading.
import com.bruceeckel.simpletest.*;
import java.util.*;
class Tree { int height;
Tree() {
System.out.println("Planting a seedling");
height = 0;
}
Tree(int i) {
System.out.println("Creating new Tree that is " + i + " feet tall");
height = i;
}
void info() {
System.out.println("Tree is " + height + " feet tall");
}
void info(String s) {
System.out.println(s + ": Tree is " + height + " feet tall");
}
}
Method overloading
public class Overloading {
static Test monitor = new Test();
public static void main(String[] args) {
for(int i = 0; i < 5; i++) {
Tree t = new Tree(i);
t.info();
t.info("overloaded method");
}
// Overloaded constructor:
new Tree();
monitor.expect(new String[] {
"Creating new Tree that is 0 feet tall",
"Tree is 0 feet tall",
"overloaded method: Tree is 0 feet tall",
"Creating new Tree that is 1 feet tall",
"Tree is 1 feet tall",
"overloaded method: Tree is 1 feet tall",
"Creating new Tree that is 2 feet tall",
"Tree is 2 feet tall",
"overloaded method: Tree is 2 feet tall",
"Creating new Tree that is 3 feet tall",
"Tree is 3 feet tall",
"overloaded method: Tree is 3 feet tall",
"Creating new Tree that is 4 feet tall",
"Tree is 4 feet tall",
"overloaded method: Tree is 4 feet tall",
"Planting a seedling"
});
}
Overloading with order of arguments
//: c04:OverloadingOrder.java
// Overloading based on the order of the arguments.
import com.bruceeckel.simpletest.*;
public class OverloadingOrder {
static Test monitor = new Test();
static void print(String s, int i) {
System.out.println("String: " + s + ", int: " + i);
}
static void print(int i, String s) {
System.out.println("int: " + i + ", String: " + s);
}
public static void main(String[] args) {
print("String first", 11);
print(99, "Int first");
monitor.expect(new String[] {
"String: String first, int: 11",
"int: 99, String: Int first"
});
}
Overloading on return values
void f() {}
int f() {}
if we call like this:
f();
Which one should be called?
Because of this confusion Java does not allow such overloading
Default constructor
If no constructor is defined compiler makes one:
//: c04:DefaultConstructor.java
class Bird {
int i;
}
public class DefaultConstructor {
public static void main(String[] args) {
Bird nc = new Bird(); // Default!
}
} ///:~
This keyword
//: c04:Leaf.java
// Simple use of the "this" keyword.
import com.bruceeckel.simpletest.*;
public class Leaf {
static Test monitor = new Test();
int i = 0;
Leaf increment() {
i++;
return this;
}
void print() {
System.out.println("i = " + i);
}
public static void main(String[] args) {
Leaf x = new Leaf();
x.increment().increment().increment().print();
monitor.expect(new String[] {
"i = 3"
});
}
} ///:~
Cleanup: finalization and garbage
collection
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Unlike C++, JAVA has no destructor concept
But a JAVA class may have a “finalize()” function
that is called when an object is garbage collected:
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Objects may not gaurbage collected -> finalize() may
not be called at all
Garbage collection is not destruction
Garbage collection is about memroy
finalize() method usage?
–
Special casses where the memory allocation is done
using non-standard JAVA ways. For example using
native method calls.
A sample usage of finalize()
//: c04:TerminationCondition.java
// Using finalize() to detect an object
that
// hasn't been properly cleaned up.
import com.bruceeckel.simpletest.*;
class Book {
boolean checkedOut = false;
Book(boolean checkOut) {
checkedOut = checkOut;
}
void checkIn() {
checkedOut = false;
}
public void finalize() {
if(checkedOut)
System.out.println("Error:
checked out");
}
}
public class TerminationCondition {
static Test monitor = new Test();
public static void main(String[] args) {
Book novel = new Book(true);
// Proper cleanup:
novel.checkIn();
// Drop the reference, forget to clean up:
new Book(true);
// Force garbage collection &
finalization:
System.gc();
monitor.expect(new String[] {
"Error: checked out"}, Test.WAIT);
}
} ///:~
How gaurbage collector work
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Stop and Copy
O1
O2
O3
rc=0
rc=3
rc=1
O2
●
O3
O4
heap
rc=0
New heap
Problems
– Two heaps is needed
– If program generate less garbage copy is very wasteful
How gaurbage collector work
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Mark and sweep
O1
O2
O3
rc=0
rc=3
rc=1
O2
O3
O4
heap
rc=0
Problems
– If many garbage is created it is very slow
– Compacting free space is very time consuming
heap
Member initialization
Local variables must be initialized by programmer:
void f() {
int i;
i++; // Error -- i not initialized
}
Member initialization
Class data members are initialized by java system:
//: c04:InitialValues.java
// Shows default initial values.
import com.bruceeckel.simpletest.*;
public class InitialValues {
static Test monitor = new Test();
boolean t;
char c;
byte b;
short s;
int i;
long l;
float f;
double d;
void print(String s) { System.out.println(s); }
void printInitialValues() {
print("Data type Initial value");
print("boolean
" + t);
print("char
[" + c + "]");
print("byte
" + b);
print("short
" + s);
print("int
" + i);
print("long
" + l);
print("float
" + f);
print("double
" + d);
}
public static void main(String[] args) {
InitialValues iv = new InitialValues();
iv.printInitialValues();
/* You could also say:
new InitialValues().printInitialValues();
*/
monitor.expect(new String[] {
"Data type Initial value",
"boolean
false",
"char
[" + (char)0 + "]",
"byte
0",
"short
0",
"int
0",
"long
0",
"float
0.0",
"double
0.0"
});
}
} ///:~
Specifying initialization
class InitialValues {
boolean b = true;
char c = 'x';
byte B = 47;
short s = 0xff;
int i = 999;
long l = 1;
float f = 3.14f;
double d = 3.14159;
//. . .
Order of initialization
//: c04:OrderOfInitialization.java
// Demonstrates initialization order.
import com.bruceeckel.simpletest.*;
// When the constructor is called to create a
// Tag object, you'll see a message:
class Tag {
Tag(int marker) {
System.out.println("Tag(" + marker + ")");
}
}
class Card {
Tag t1 = new Tag(1); // Before constructor
Card() {
// Indicate we're in the constructor:
System.out.println("Card()");
t3 = new Tag(33); // Reinitialize t3
}
Tag t2 = new Tag(2); // After constructor
void f() {
System.out.println("f()");
}
Tag t3 = new Tag(3); // At end
}
public class OrderOfInitialization {
static Test monitor = new Test();
public static void main(String[] args) {
Card t = new Card();
t.f(); // Shows that construction is done
monitor.expect(new String[] {
"Tag(1)",
"Tag(2)",
"Tag(3)",
"Card()",
"Tag(33)",
"f()"
});
}
} ///:~
Static data initialization
//: c04:StaticInitialization.java
// Specifying initial values in a class definition.
import com.bruceeckel.simpletest.*;
class Bowl {
Bowl(int marker) {
System.out.println("Bowl(" + marker + ")");
}
void f(int marker) {
System.out.println("f(" + marker + ")");
}
}
class Table {
static Bowl b1 = new Bowl(1);
Table() {
System.out.println("Table()");
b2.f(1);
}
void f2(int marker) {
System.out.println("f2(" + marker + ")");
}
static Bowl b2 = new Bowl(2);
}
Static data initialization (continued)
class Cupboard {
Bowl b3 = new Bowl(3);
static Bowl b4 = new Bowl(4);
Cupboard() {
System.out.println("Cupboard()");
b4.f(2);
}
void f3(int marker) {
System.out.println("f3(" + marker + ")");
}
static Bowl b5 = new Bowl(5);
}
Static data initialization (continued)
public class StaticInitialization {
static Test monitor = new Test();
public static void main(String[] args) {
System.out.println("Creating new Cupboard() in main");
new Cupboard();
System.out.println("Creating new Cupboard() in main");
new Cupboard();
t2.f2(1);
t3.f3(1);
monitor.expect(new String[] {
"Bowl(1)",
"Bowl(2)",
"Table()",
"f(1)",
"Bowl(4)",
"Bowl(5)",
"Bowl(3)",
"Cupboard()",
"f(2)",
Static data initialization (continued)
"Creating new Cupboard() in main",
"Bowl(3)",
"Cupboard()",
"f(2)",
"Creating new Cupboard() in main",
"Bowl(3)",
"Cupboard()",
"f(2)",
"f2(1)",
"f3(1)"
});
}
static Table t2 = new Table();
static Cupboard t3 = new Cupboard();
} ///:~
Process of object creation
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The first time an object of type Dog is created (the constructor is
actually a static method), or the first time a static method or static
field of class Dog is accessed, the Java interpreter must locate
Dog.class, which it does by searching through the classpath.
As Dog.class is loaded (creating a Class object, which you’ll learn
about later), all of its static initializers are run. Thus, static
initialization takes place only once, as the Class object is loaded for
the first time.
When you create a new Dog( ), the construction process for a Dog
object first allocates enough storage for a Dog object on the heap.
This storage is wiped to zero, automatically setting all the primitives
in that Dog object to their default values (zero for numbers and the
equivalent for boolean and char) and the references to null.
Any initializations that occur at the point of field definition are
executed.
Constructors are executed. As you shall see in Chapter 6, this might
actually involve a fair amount of activity, especially when inheritance
is involved.
Explicit static initialization
//: c04:ExplicitStatic.java
// Explicit static initialization with the "static"
clause.
import com.bruceeckel.simpletest.*;
class Cup {
Cup(int marker) {
System.out.println("Cup(" + marker + ")");
}
void f(int marker) {
System.out.println("f(" + marker + ")");
}
}
class Cups {
static Cup c1;
static Cup c2;
static {
c1 = new Cup(1);
c2 = new Cup(2);
}
Cups() {
System.out.println("Cups()");
}
public class ExplicitStatic {
static Test monitor = new Test();
public static void main(String[] args) {
System.out.println("Inside main()");
Cups.c1.f(99); // (1)
monitor.expect(new String[] {
"Inside main()",
"Cup(1)",
"Cup(2)",
"f(99)"
});
}
// static Cups x = new Cups(); // (2)
// static Cups y = new Cups(); // (2)
} ///:~
Non-static instance initialization
//: c04:Mugs.java
// Java "Instance Initialization."
import com.bruceeckel.simpletest.*;
class Mug {
Mug(int marker) {
System.out.println("Mug(" + marker +
")");
}
void f(int marker) {
System.out.println("f(" + marker + ")");
}
}
public class Mugs {
static Test monitor = new Test();
Mug c1;
Mug c2;
{
c1 = new Mug(1);
c2 = new Mug(2);
System.out.println("c1 & c2 initialized");
}
Mugs() {
System.out.println("Mugs()");
}
}
public static void main(String[] args) {
System.out.println("Inside main()");
Mugs x = new Mugs();
monitor.expect(new String[] {
"Inside main()",
"Mug(1)",
"Mug(2)",
"c1 & c2 initialized",
"Mugs()"
});
}
} ///:~
Array initialization
//: c04:Arrays.java
// Arrays of primitives.
import com.bruceeckel.simpletest.*;
public class Arrays {
static Test monitor = new Test();
public static void main(String[] args) {
int[] a1 = { 1, 2, 3, 4, 5 };
int[] a2;
a2 = a1;
for(int i = 0; i < a2.length; i++)
a2[i]++;
for(int i = 0; i < a1.length; i++)
System.out.println(
"a1[" + i + "] = " + a1[i]);
monitor.expect(new String[] {
"a1[0] = 2",
"a1[1] = 3",
"a1[2] = 4",
"a1[3] = 5",
"a1[4] = 6"
});
}
} ///:~
Using new for arrays
//: c04:ArrayNew.java
// Creating arrays with new.
import com.bruceeckel.simpletest.*;
import java.util.*;
public class ArrayNew {
static Test monitor = new Test();
static Random rand = new Random();
public static void main(String[] args) {
int[] a;
a = new int[rand.nextInt(20)];
System.out.println("length of a = " + a.length);
for(int i = 0; i < a.length; i++)
System.out.println("a[" + i + "] = " + a[i]);
monitor.expect(new Object[] {
"%% length of a = \\d+",
new TestExpression("%% a\\[\\d+\\] = 0", a.length)
});
}
} ///:~
Array of objects
//: c04:ArrayClassObj.java
// Creating an array of nonprimitive objects.
import com.bruceeckel.simpletest.*;
import java.util.*;
public class ArrayClassObj {
static Test monitor = new Test();
static Random rand = new Random();
public static void main(String[] args) {
Integer[] a = new Integer[rand.nextInt(20)];
System.out.println("length of a = " + a.length);
for(int i = 0; i < a.length; i++) {
a[i] = new Integer(rand.nextInt(500));
System.out.println("a[" + i + "] = " + a[i]);
}
monitor.expect(new Object[] {
"%% length of a = \\d+",
new TestExpression("%% a\\[\\d+\\] = \\d+", a.length)
});
}
} ///:~
Array of objects (alternate
initialization)
//: c04:ArrayInit.java
// Array initialization.
public class ArrayInit {
public static void main(String[] args) {
Integer[] a = {
new Integer(1),
new Integer(2),
new Integer(3),
};
Integer[] b = new Integer[] {
new Integer(1),
new Integer(2),
new Integer(3),
};
}
} ///:~
Array of objects
//: c04:VarArgs.java
// Using array syntax to create variable argument lists.
import com.bruceeckel.simpletest.*;
class A { int i; }
public class VarArgs {
static Test monitor = new Test();
static void print(Object[] x) {
for(int i = 0; i < x.length; i++)
System.out.println(x[i]);
}
public static void main(String[] args) {
print(new Object[] {
new Integer(47), new VarArgs(),
new Float(3.14), new Double(11.11)
});
print(new Object[] {"one", "two", "three" });
print(new Object[] {new A(), new A(), new A()});
monitor.expect(new Object[] {
"47",
"%% VarArgs@\\p{XDigit}+",
"3.14",
"11.11",
"one",
"two",
"three",
new TestExpression("%% A@\\p{XDigit}+", 3)
});
}
} ///:~
Multi-dimentional arrays
int[][] a1 = {
{ 1, 2, 3, },
{ 4, 5, 6, },
};
int[][][] a2 = new int[2][2][4];
int[][][] a3 = new int[rand.nextInt(7)][][];
for(int i = 0; i < a3.length; i++) {
a3[i] = new int[rand.nextInt(5)][];
for(int j = 0; j < a3[i].length; j++)
a3[i][j] = new int[rand.nextInt(5)];
}
Some excersises in class
1- Create a class with a default constructor (one that takes no
arguments) that prints a message. Create an object of this class.
2- Add an overloaded constructor to Exercise 1 that takes a String
argument and prints it along with your message.
5- Create an array of String objects and assign a string to each
element. Print the array by using a for loop.