Object in Memory

Advanced Programming

SUT • Spring 2019

Outline

  • Object Creation
  • Object Storage
  • More on Arrays
  • Parameter Passing
  • For Each
  • Var Args

Object Creation

Example

public class Dog {
    private String name;

    public void setName(String name) {
        this.name = name;
    }

    public void bark() {
        System.out.println("Hop! Hop!");
    }
}
public class Main {
    public static void main(String[] args) {
        // Dog
        Dog dog = new Dog();
        dog.setName("Fido");
        dog.bark();
    }
}
  •  Object Creation (instantiation)
  • changing the object’s state
  • passing message to object
  • dog is an object
  • dog is a reference to an object

Object Memory

  • Remember : an object has state, behaviour and identity

  • Each object is stored in memory

  • Memory address ≈ object identity

  • Memory content = object state

  • The behaviour of an object is declared in its class

  • Class declaration is also stored in memory

    • But class declaration is stored once for each class

    • For each object a separate piece of memory is needed

      • To store its state

new Operator

  • new creates a new object from specified type
    • new String();
    • new Book();
    • new int();
  • Primitive types are not referenced

new

  • new operator creates a new object from the specified type

  • Returns the reference to the created object

String string = new String();

Dog dog = new Dog();

Rectangle rectangle = new Rectangle();

Object References

  • Remember C++ pointers

  • When you declare an object, you declare its reference

  • Exception: ?

    • Primitive types

  • Primitive types are not actually objects

    • They can not have references

  • Java references are different from C++ pointers and references

String s;
Rectangle rectangle;

Create Objects

  • This code will not create an object:

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  • It just creates a reference

  • This is a key difference between Java and C++

  • You can not use “s” variable

  • “s” is null

  • null value in java

  • You should connect references to real objects

  • How to create objects?

    • new

String s;
Rectangle rectangle;

new

  • new creates a piece of memory

  • Returns its reference

  • Where is the piece of memory?

    • In Heap

  • Where is the Heap?

    • Later…

Array in java

  • Array elements are stored in heap
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  • Array elements are references not objects
    • Exception : primitives
public class Main {
    public static void main(String[] args)
        // Array in Java
        Integer[] integers;
        Dog[] dogs = new Dog[5];

        int n = 10;
        float[] float_numbers = new float[n];
    }
}

Primitive-Type Array Sample

public class Main {
    public static void main(String[] args) {
        // Primitive-Type Array Sample
        int[] array = new int[10];
        int[][] twoDimArray = new int[5][];
        twoDimArray[0] = new int[7];
        twoDimArray[1] = new int[12];

        int[][] matrix = new int[5][7];
    }
}

Array Samples

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        // Array Samples
        Scanner scanner = new Scanner(System.in);
        int arrayLength = scanner.nextInt();
        String[] names;
        names = new String[arrayLength];
        for (int i = 0; i < names.length; i++) {
            names[i] = scanner.next();
        }
        System.out.println(names[names.length - 1]);
    }
}

Array References

  • There is three type of variable in this code

    • array: reference

    • array[i]: references

      • Initial value: null

    • array[i]: objects

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        scanner = new Scanner(System.in);

        int arrayLength = scanner.nextInt();

        String[] names;
        names = new String[arrayLength]

        for (int i = 0; i < names.length; i++) {
            names[i] = scanner.next();
        }
    }
}

More Example

public class Student {
    private String name;
    private Long id;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }
}

What Does Happen to Students After Method Invocation?

static void studentCreation() {
    Student[] students = new Student[10];
    for (int i = 0; i < students.length; i++) {
        students[i] = new Student();
        students[i].setId((long) i);
    }
}

Object Destruction

  • Allocated memory should be released

  • delete operator in C++

  • Problems with delete in C++

    • Error-Prone

      • Segmentation Fault!

    • Sometimes causes memory leak

      • a program consumes memory but is unable to release it

    • Complicated in many situations

  • You don’t need it in java

  • Garbage Collection

What is the output of this code?

public class Student {
    private String name;
    private long ID;

    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public long getID() { return ID; }
    public void setID(long id) { ID = id; }

    public static void main(String[] args){
        Student[] s = new Student[10];
        for(int i = 0; i < 10; i++){
            s[i].setID(i + 10000);
        }
        System.out.println(s[5].getID());
    }
}

Example

  • Object Abstraction

    • Abstract Data Type

    • Object Declaration (Class Declaration)

public class Person {
	private String name;
	private int age;
	public void run(){...}
	public void talk(){...}
}

Example

  • Object Instantiation

    • new

public static void main(String[] args){
    Person JafarAgha = new Person();
    JafarAgha.setAge(50);
    JafarAgha.setName("Jafar");
    JafarAgha.talk();
    Person AzamKhanoom = new Person();
}

Objects in Memory

5

...

j

a

f

a

r

Parameter Passing Styles

  • Call by value

  • Call by reference

  • Call by pointer

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  • Java style: Call by passing value of references!

  • Let’s see!

What happens in a method call

public static void main(String[] args){
    X variable = new X();
    f(variable);
}

static void f(X parameter){
    ...
}

/*
*    static void f(){
*        X parameter = variable;     
*    }
*
*/

C++ Parameter Passing

  • Call by value

  • Call by pointer

  • Call by reference

C++ Example

void cppMethod(Person byValue, Person* byPointer, 
               Person& byReference){
    byValue.name = "ali";
    byPointer->name = "ali";
    byReference.name = "ali";
}

Person p1, p3; Person* p2;
p2 = new Person(…);
cppMethod(p1, p2, p3);

  • Does p1.name change?

    • no

  • Does p2->name change?

    • yes

  • Does p3.name change?

    • yes

C++ Example

void cppMethod(Person byValue, Person* byPointer, 
               Person& byReference){
    Person* newP = new Person;
    byValue = *newP;
    byPointer = newP;
    byReference = *newP;
}

cppMethod(p1, p2, p3);

  • Does p1 change?

    • no

  • Does p2 change?

    • no

  • Does p3 change?

    • yes

Java Parameter Passing

  • Java has no pointer

  • Java references are different from C++ references

  • Java references are more like C++ pointers

    • than C++ references

  • A Java reference is something like a limited pointer

Java Example

public void javaMethod(Person first, Person second, int number){
	first.age = 12;
	number = 5;

	Person newP = new Person();
	second = newP; 
}

javaMethod(p1, p2, myInt);

  • Does p1.age change?

  • yes

  • Does myInt change?

  • no

  • Does p2 change?

  • no

  • In java, primitive variables are passed to methods by their values

  • Reference values are passed by their reference values.

Swap

public class Swap {
    public static void main(String[] args) {
        int a = 4, b = 5;
        badswap(a, b);
        System.out.println(a + " " + b);
    }

    static void badswap(int a, int b) {
        int tmp = a;
        a = b;
        b = tmp;
    }
}

Swap

public class Swap {
    public static void main(String[] args) {
        String a = "4", b = "5";
        badswap(a, b);
        System.out.println(a + " " + b);
    }

    static void badswap(String a, String b) {
        String tmp = a;
        a = b;
        b = tmp;
    }
}

Call by reference in C++

void cpp_swap(int& var1, int& var2){
    int temp = var1;
    var1 = var2;
    var2 = temp;
}

int main() {
    int a = 5;
    int b = 4;
    cpp_swap(a, b)
}

In java

  • Everything is passed by value

    • Primitive-types are passed by value

    • References are passed by value

  • But not the value of the object

    • the value of the reference

  • If you want to pass something by reference…

    • Wrap it in an object

    • And make it mutable

Java Swap

public class Swap {
    public static void main(String[] args) {
        IntWrapper a = new IntWrapper(4), b = new IntWrapper(5);
        swap(a, b);
        System.out.println(a.value + " " + b.value);
    }

    static void swap(IntWrapper a, IntWrapper b) {
        int tmp = a.value;
        a.value = b.value;
        b.value = tmp;
    }
}


class IntWrapper {
    public int value;

    public IntWrapper(int value) {
        this.value = value;
    }
}

For Each

public class ForEach {
    public static void main(String[] args) {
        int[] array = new int[4];
        for (int i = 0; i < array.length; i++) {
            array[i] = i;
        }
        print(array);
    }

    static void print(int[] array) {
        for (int item : array) {
            System.out.println(item);
        }
    }
}

For Each (2)

  • In for each expression, each element is assigned to another variable

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  • If X is a primitive type, element values are copied into item variable

public class ForEach {
    public static void main(String[] args) {
        X[] array = new X[4];
        
        for (int i = 0; i < array.length; i++) {
            X item = array[i];
        }

        for (X item: array) {

        }
    }
}

Variable argument lists

package com.company;

public class VarArgs {
    static void print(String... params) {
        String[] array = params;
        System.out.println(array.length);
        for (String s : params) {
            System.out.println(s);
        }
    }

    public static void main(String[] args) {
        print("sallam", "taghai");
        print("Sallam");
        print();
    }
}

  • Sometimes they are called vararg

  • Varargs are actually arrays

Practice

public static void main(String[] args){
    int[] array = new int[4];
    
    for (int i = 0; i < 4; i++){
        array[i] = i;
    }
    
    f(array);
    System.out.println(array[2]);
}

private static void f(int[] a){
    a[2] = 0;
    for (int i: a){
        i = 5;
    }

    a = new int[10];
    a[2] = 1;
}

Storage

Where storage lives

  • Registers

  • Stack

  • Heap

  • Constants

  • Non-RAM

Memory Hierarchy

Registers

  • Fastest

  • Inside the CPU

  • Number of registers are limited

  • You don’t have direct control over registers

  • In assembly you have direct access to registers

  • C and C++ have access to this storage to some extent

The Stack

  • In RAM

  • Slower than register but less limited

  • Mechanism of function call in CPU

    • Stack pointer (cp)

    • Support of CPU

  • Java references are (usually) placed on stack

  • Primitive data types are also (usually) located in stack

  • Java compiler must know the lifetime and size of all the items on the stack

  • Java objects themselves are not placed on the stack

The stack (cont.)

  • C++ allows allocation of objects on the stack

  • E.g. this code creates an object on the stack

    • Person p;

  • In C++ it creates an object on the stack

  • In Java it creates only a reference on the stack

  • The actual object will be on Heap

  • C++ allows arrays of known size on stack

  • Java does not!

Compile time vs. Run time

  • Some information are available at compile time

  • Stack elements should be specified in compile time

  • So C++ allows these variables on stack:

    • int array[10];

    • Person p;

  • Some information are not available at compile time

  • So variable length variables can not be on stack

    • If n is a variable “int array[n] “ is not allowed in C++

  • Java is simple! No object on stack!

The Heap

  • This is a general-purpose pool of memory

  • Also in the RAM area

  • All Java objects live here

  • The compiler doesn’t need to know the length of the variables

  • new operator = the storage is allocated on the heap

  • The objects may become garbage

    • Garbage collection

Heap Generations

  • The heap is split up into generations

  • The young generation

    • stores short-lived objects that are created and immediately garbage collected

  • The Old generation

    • Objects that persist longer are moved to the old generation

    • also called the tenured generation

  • The permanent generation (or permgen)

    • is used for class definitions and associated metadata

Primitive Types

  • new is not efficient for these small variables

    • int a;

    • char ch;

  • In these cases, automatic variable is created

    • that is not a reference

  • The variable holds the value directly

  • It’s placed on the stack

    • Much more efficient

  • When these primitives are not stored on stack?

    • When they are inside an object

Primitive Wrapper Classes

  • Used to represent primitive values when an Object is required

  • All of them are immutable

Primitive Type Wrapper Class
byte Byte
short Short
int Integer
long Long
float Float
double Double
chat Character
boolean Boolean

Sample

public class Wrapper {
    public static void main(String[] args) {
        Integer i = new Integer(2);
        Integer j = new Integer(2);
        System.out.println(i == j);
        //Prints false. Why?
        
        i = j;        //Reference Assignment
        i = 2;        //OK. A new shortcut in Java5+
        Long l = 2;   //Syntax Error. Why?
        Long l = 2L;  //OK
        l = i;        //Syntax Error. Why?
    }
}

Title Text

Object in Memory

By Behnam Hatami

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Object in Memory

Object in Memory / Advanced Programming Course @ SUT, Spring 2019

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