Sorting. Sorting. Selection sort

Transcription

1 Sorting 1 Sorting Given a linear list of comparable objects of the same class (or values of the same type), we wish to sort (or reärrange) the objects in the increasing order. For simplicity, let s just assume that we are given an array of n integers. 2 Selection sort Find the least value in the array. Swap it with the value in the index 0. Repeat the steps above for the remainder of the array, starting from the index 1 and advancing each time. 3 1

2 Selection sort Suppose we are given an array A of n integers. for (int i = 0; i < n - 1; i++) { int minindex = i; for (int j = i + 1; j < n; j++) if (A[j] < A[minIndex]) minindex = j; if (minindex!= i) swap A[i] with A[minIndex]; 4 Bubble sort By comparing and (swapping if necessary) adjacent pairs of numbers, bubble up the largest value in the array. Repeat the above step for the remainder of the array, ending at the second-to-the last index and advancing each time. 5 Bubble sort Suppose we are given an array A of n integers. for (int i = 1; i < n; i++) for (int j = 0; j < n - i; j++) if (A[j] > A[j + 1]) swap A[j] with A[j + 1]; 6 2

3 Object-oriented paradigm: goals and principles 7 Goals The object-oriented paradigm has three main goals to achieve: Robustness capable of handling expected and unexpected events correctly. Adaptability capable of responding to changing environments over time. Reüsability being reüsable in different systems by different users. 8 Principles To achieve these goals, the object-oriented paradigm has three main principles: Abstraction distilling a complicated system down to its most fundamental parts. Encapsulation concealing messy details and revealing just what is necessary. Modularity dividing different tasks into separate functional units. 9 3

4 10 is the notion of one class inheriting the fields and methods from another class. In Java, inheritance happens when one class is defined as a subclass of another. allows us to organize classes in hierarchical structures. prevents redundancy. 11 Example. Consider creating classes for vehicles, cars, trucks, sedans, wagons, etc. We could create all these classes independently, but they are closely related: A car is a special kind of a vehicle, so is a truck. A sedan is a special kind of a car, so is a wagon. 12 4

5 Example. A car is a special kind of a vehicle. A car should have all the attributes and behaviors of a vehicle. Can we define a car as some kind of a vehicle s child? Yes, we can create cars as a subclass of vehicles. 13 Example. Car as a subclass of Vehicle. public class Vehicle { protected int numofwheels; protected int weight; public void drive() { 14 public class Car extends Vehicle { // Everything from Vehicle plus the following: protected int numofdoors; protected String bodycolor; public void headlightson() { 15 5

6 How about constructors? A subclass s constructor must always first call its superclass s constructor. public Car() { super(); numofdoors = 4; bodycolor = Silver ; 16 How about constructors? A subclass s constructor must always first call its superclass s constructor. public Car() { numofdoors = 4; bodycolor = Silver ; If omitted, super(); will be inserted. 17 public Car() { super(); numofdoors = 4; bodycolor = Silver ; This is an example of method overriding refinement: additional code is added to the constructor of its superclass. 18 6

7 What happens if the method drive(), already defined in Vehicle, is redefined in its subclass Car? public class Car extends Vehicle { public void drive() { 19 What happens if the method drive(), already defined in Vehicle, is redefined in its subclass Car? In this case, the entire code of Vehicle s method drive() is replaced by the code of Car s method drive(). This is an example of method overriding replacement: the superclass s entire code is replaced by a subclass s new code. 20 There are two types of method overriding: Refinement: additional code is added to the superclass s code. This applies to constructors. Replacement: the superclass s code is entirely replaced by a subclass s code. This applies to regular methods. 21 7

8 Abstract classes 22 Abstract classes In biology, to categorize species effectively, we often introduce abstract species (for example, mammals). Java supports a mechanism for such abstraction as well. In Java, an abstract class is a class designed to group similar classes together, without implementing methods. 23 Abstract classes An abstract class has methods with no bodies. Such methods are called abstract methods. An abstract class all by itself is not useful; in particular, we cannot instantiate an object of an abstract class. An abstract class becomes useful when used with its subclasses. 24 8

9 Abstract class Mammal public abstract class Mammal { protected char gender; public Mammal() { public String tostring() { return Gender: + gender; public abstract void breathe(); The method breathe() is abstract and has no body (i.e., implementation). 25 Abstract class Mammal public abstract class Mammal { protected char gender; public Mammal() { public String tostring() { return Gender: + gender; public abstract void breathe(); This class all by itself is useless. It becomes useful only when used with its subclasses. 26 Dog as a subclass of Mammal public class Dog extends Mammal { // Dog inherits everything from Mammal protected String breed; // This implements Mammal s abstract method breathe(). public void breathe() { System.out.println( This dog is breathing. ); In a subclass, we implement the abstract method breathe() defined in Mammal. 27 9

10 Cat as a subclass of Mammal public class Cat extends Mammal { // Cat inherits everything from Mammal protected String coatpattern; // This implements Mammal s abstract method breathe(). public void breathe() { System.out.println( This cat is breathing. ); In a subclass, we implement the abstract method breathe() defined in Mammal. 28 Abstract classes Let A be an abstract class and B be a subclass of A. B inherits both fields and regular methods defined in A. In addition, in B, we must implement all abstract methods defined in A as regular methods. B can also have its own methods not defined in A