Course Notes Set 8: Linear Lists: Simulated Pointer Implementation

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Course Notes Set 8: Linear Lists: Simulated Pointer Implementation COMP 2210 Dr. Hendrix Computer Science and Software Engineering Auburn University Limitations Of Java Pointers May be used for internal data structures only. Data structure backup requires serialization and deserialization.

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### Course Notes Set 8:Linear Lists:Simulated PointerImplementation

COMP 2210

Dr. Hendrix

Computer Science and Software Engineering

Auburn University

### Limitations Of Java Pointers

May be used for internal data structures only.

Data structure backup requires serialization and deserialization.

No arithmetic.

### Memory Layout

c

a

e

d

b

Data structure memory is an array, and each array position has an element field (type Object) and a next field (type int).

### Node Representation

class SimulatedNode

{

// package visible data members

Object element;

int next;

// constructors not shown

}

c

a

e

d

b

14

firstNode = 4

next

11

14

-1

8

0

c

a

e

d

b

element

0

1

2

3

4

5

8

11

14

### Still Drawn The Same

firstNode

-1

a

b

c

d

e

Exercise 1 p 204 Sahni with Simulated Pointers

Initial state

firstNode = 4

next

• 14

11

14

-1

8

0

c

a

e

d

b

element

0

1

2

3

4

5

8

11

14

firstNode = 1

next

• 14

4

11

14

-1

8

0

f

c

a

e

d

b

element

0

1

2

3

4

5

8

11

14

Exercise 1 p 204 Sahni with Simulated Pointers

firstNode = 1

next

• 14

4

11

0

14

-1

8

2

f

g

c

a

e

d

b

element

0

1

2

3

4

5

8

11

14

firstNode = 1

next

• 14

4

11

0

14

3

8

2

-1

f

g

c

a

e

d

b

h

element

0

1

2

3

4

5

8

11

14

Exercise 1 p 204 Sahni with Simulated Pointers

remove(0)

firstNode = 4

next

• 14

11

0

14

3

8

2

-1

g

c

a

e

d

b

h

element

0

1

2

3

4

5

8

11

14

remove(4)

firstNode = 4

next

• 14

0

14

3

2

8

-1

g

c

a

e

b

h

element

0

1

2

3

4

5

8

11

14

### Memory Management

Linked system (Java or simulated pointer) requires:

a way to keep track of the memory that is not in use (i.e., a storage pool)

way to allocate a node

Java has the method new

way to free a node that is no longer in use

Java uses garbage collection

### Garbage Collection

The system determines which nodes/memory are not in use and returns these nodes (this memory) to the pool of free storage.

This is done in two or three steps:

Mark nodes in use.

Compact free space (optional).

Move free nodes to storage pool.

### Marking

c

a

e

d

b

firstNode

Unmark all nodes.

Start at each program variable that contains a reference, follow all pointers, mark nodes that are reached.

### Marking

c

c

a

a

e

e

d

d

b

e

firstNode

Start at firstNode and mark all nodes reachable from firstNode.

Repeat for all reference variables.

### Compaction

Free Memory

c

a

e

d

b

e

d

b

firstNode

Move all marked nodes (I.e., nodes in use) to one end of memory, updating all pointers as necessary.

### Put Free Memory In Storage Pool

Scan memory for unmarked nodes (if no compaction done), otherwise put single free block (unless no free memory) into pool.

Programmer doesn’t have to worry about freeing nodes as they become free.

However, for garbage collection to be effective, we must set reference variables to null when the object being referenced is no longer needed.

Applications may run faster when run on computers that have more memory.

Garbage collection time is linear in memory size (not in amount of free memory).

### Alternative To Garbage Collection

Provide a method to free/deallocate a node.

e.g., delete method of C++

Now free nodes are always in storage pool.

Time to free nodes is proportional to number of nodes being freed and not to total memory size.

User must write methods to free data structure nodes.

Time is spent freeing nodes that may not be reused.

Application run time does not improve with increase in memory size.

### Storage Pool Organization When All Nodes Have Same Size

firstNode

null

a

b

c

d

e

• Maintain a chain of free nodes
• Allocate from front of chain
• Add node that is freed to chain front

### Simulated Pointer Memory Management

/** memory management for simulated pointer classes */

package dataStructures;

import utilities.*;

public class SimulatedSpace1

{

// data members

private int firstNode;

SimulatedNode [] node; // package visible

// constructor and other methods come here

}

### Constructor

public SimulatedSpace1(int numberOfNodes)

{

node = new SimulatedNode [numberOfNodes];

// create nodes and link into a chain

for (int i = 0; i < numberOfNodes - 1; i++)

node[i] = new SimulatedNode(i + 1);

// last node of array and chain

node[numberOfNodes - 1] = new SimulatedNode(-1);

// firstNode has the default initial value 0

}

### Allocate A Node

public int allocateNode(Object element, int next)

{// Allocate a free node and set its fields.

if (firstNode == -1)

{ // double number of nodes, code omitted

}

int i = firstNode; // allocate first node

firstNode = node[i].next; // firstNode points to next free node

node[i].element = element;

node[i].next = next;

return i;

}

### Free A Node

public void deallocateNode(int i)

{// Free node i.

// make i first node on free space list

node[i].next = firstNode;

firstNode = i;

// remove element reference so that space can be garbage

// collected

node[i].element = null;

}

### Simulated Pointers

Can allocate a chain of nodes without having to relink.

Can free a chain of nodes in O(1) time when first and last nodes of chain are known.

Don’t use unless you see a clear advantage to using simulated pointers over Java references.