Solving specific problems which fit within the following categories. managing layers of software creating pipelines for data processing dealing with user input maintaining complex business rules dealing with class hierarchies Most widely used Design Patterns discussed Builder Command Composite Decorator Event-Observer Factory Specification Strategy Visitor Speaker: Gokul Subramanian

2. Design is an art
Art requires creativity
Creativity is a product of the brain
The brain can be understood scientifically
So, design is a science

4. Fragile base-class problem
class File {
private String contents = “”;
void write(String more) {
contents += more;
}
void writeMany(String[] mores) {
for (String more : mores) {
write(more);
}
}
}
class InstrumentedFile extends File {
private int writeCount;
@Override
void write(String more) {
writeCount++;
super.write(more);
}
int getWriteCount() {
return writeCount;
}
}
contents += more;

5. Solution
interface File {
void write(String more);
void writeMany(String[] mores);
}
interface InstrumentedFile extends File {
int getWriteCount();
}
abstract class AbstractFile implements File {
private String contents = “”;
@Override
public void write(String more) {
contents += more;
}
@Override
public void writeMany(String[] mores) {
for (String more : mores) {
write(more);
}
}
}
class InstrumentedFileImpl implements InstrumentedFile {
private int writeCount;
private File base;
InstrumentedFileImpl(File base) {
this.base = base;
}
@Override
public void write(String more) {
writeCount++;
base.write(more);
}
@Override
public void writeMany(String[] mores) {
writeCount++;
base.writeMany(mores);
}
@Override
public int getWriteCount() { return writeCount; }
}

6. Identify the differences (be nice)

7. The Decorator pattern

8. Proxy, Adapter, Facade

9. Is a square a rectangle?
class Rectangle {
private int len;
private int wid;
Rectangle(int l, int w) { len = l; wid = w; }
int getLength() { return len; }
void setLength(int l) { len = l; }
int getWidth() { return wid; }
void setWidth(int w) { wid = w; }
}
class Square extends Rectangle {
Square(int s) { super(s, s); }
int getSide() { return getLength(); }
void setSide(int s) {
setLength(s);
setWidth(s);
}
}
Rectangle sq = new Square(100);
sq.setLength(40);

10. Solution
class Rectangle {
private int len;
private int wid;
Rectangle(int l, int w) { len = l; wid = w; }
int getLength() { return len; }
void setLength(int l) { len = l; }
int getWidth() { return wid; }
void setWidth(int w) { wid = w; }
}
class Square {
private Rectangle rect;
Square(int s) {
rect = new Rectangle(s, s);
}
int getSide() { return rect.getLength();}
void setSide(int s) {
rect.setLength(s);
rect.setWidth(s);
}
}

11. But I want polymorphism to compute areas
interface Areable {
int getArea();
}
class Rectangle implements Areable {
…
@Override
public int getArea() {
return len * wid;
}
}
class Square implements Areable {
…
@Override
public int getArea() {
return rect.getArea();
}
}
List<Areable> areables = …
for (Areable ar : areables) {
System.out.println(ar.getArea());
}
This is what OOP is all
about.

12. Euclid’s GCD algorithm

13. The Composite pattern

15. Which car do you want?

16. The Strategy pattern

18. The Builder pattern

19. There shall be no
other king!

20. Code: Deep dive

21. Prescription
Define contracts by creating interfaces
Keep interfaces small and mutually exclusive
Add behavior by implementing these interfaces
Depend only on interfaces, not implementations
Depend only on direct dependencies

22. Further reading + coding
https://github.com/gokul2411s/patternsrepo
Read the Gang of Four book
Read books by Martin Fowler and Robert Martin (Uncle Bob)
Experience

23. Duh where’s the brand?
Google
Susquehanna International Group
UPenn GRASP robotics
Schlumberger
NIT Surathkal