1 G54PRG Programming Lecture 1 Amadeo Ascó Adam Moore 17 Inheritance.
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1 G54PRG Programming Lecture 1 Amadeo Ascó Adam Moore 17 Inheritance
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Transcript of 1 G54PRG Programming Lecture 1 Amadeo Ascó Adam Moore 17 Inheritance.
1
G54PRG ProgrammingLecture
1
Amadeo Ascó Adam Moore
17
Inheritance
2Amadeo Ascó , Adam Moore
Previously
• Java String– String as an Object– String as an Array of Characters– Equalities– Length– Index– Substring– Split
3Amadeo Ascó , Adam Moore
Overview
• Defining Classes
• Inheritance of Properties
• Inheritance of Methods
• Sub and super classes
4Amadeo Ascó , Adam Moore
Defining Classes
• Classes may be defined in terms of other classes • For example:
– Tigers, cheetahs, leopards & jaguars are all types of cats • Class tiger is a subclass of class cat
– Ball point pens, fountain pens & marker pens are all types of pens
• Ball point pen is a subclass of class pen
• Subclasses inherit properties from their parent – All cats are furry and have four feet - therefore tigers are
furry and have four feet– All pens contain ink - therefore ball point pens contain ink
5Amadeo Ascó , Adam Moore
HierarchiesThing
Mineral Alive
Animal Vegetal
Mammals
Monotremes Marsupials Placentals
Each level has• own defining features and• defining features from previous - inherited
6Amadeo Ascó , Adam Moore
Class hierarchies
• Classes are arranged into hierarchies • Subclasses provide specialised behaviour, whereas
superclasses are more general. • Inheritance is one-way (i.e. downwards) • All Java classes are ultimately inherited from class
Object • Methods are inherited down a hierarchy • They may be left unchanged • They may be modified (i.e. overridden)
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Inheritance - of properties • Animals
– Invertebrates – Vertebrates Backbone
• Fish Scales • Amphibians • Reptiles • Birds Feathers • Mammals Females with mammary glands • Bats Wings • Cattle Hooves • Carnivore Big Teeth
– Dogs – Cats
Tigers are vertebratesThus they have a backbone
Tigers are carnivoresThus they have big teeth
Tigers are not birdsThus they do not have feathers
8Amadeo Ascó , Adam Moore
Inheritance of behaviour (methods)
• Writing Implements Method:Draw Line – Pencil Method:sharpen – Pen Property:Ink Colour
• Ball-point pen • Fountain pen Method:fill with ink • Felt-tip pen Method:remove cap
– Permanent Marker pen – Dry Wipe pen
9Amadeo Ascó , Adam Moore
Sub and Super Classes
• Consider the following classes, relative to “Felt-tip pen”
• Writing Implements Ancestor Class – Pencil – Pen Superclass
• Ball-point pen • Fountain pen • Felt-tip pen Class
– Permanent Marker pen Subclass – Dry Wipe pen Subclass
10Amadeo Ascó , Adam Moore
The “extends”reserved word
• Class modifier • Declares one class to be a subclass of another • For example:
class Tiger extends Cat {
…} // end class Tiger
11Amadeo Ascó , Adam Moore
The super reserved word
• The super reserved word refers to the immediate superclass of a class.
• The superclass constructor may be invoked by calling super.
• On its own super invokes the constructor of the immediate superclass.
12Amadeo Ascó , Adam Moore
class Thing {private String mstrName;
public Thing(String strName) {mstrName = strName;
} // Constructor ()
public String getName() {return mstrName;
} // getName()
public boolean isLiving() {return false;
} // isLiving()} // end class Thing
class Thing {private String mstrName;
public Thing(String strName) {mstrName = strName;
} // Constructor ()
public String getName() {return mstrName;
} // getName()
public boolean isLiving() {return false;
} // isLiving()} // end class Thing
Superclass
13Amadeo Ascó , Adam Moore
class Alive extends Thing {public Alive(String strName) {
super(strName);} // Constructor ()
public String group() {return "Alive";
} // group()
public boolean isLiving() {return true;
} // isLiving()
} // end class Alive
class Alive extends Thing {public Alive(String strName) {
super(strName);} // Constructor ()
public String group() {return "Alive";
} // group()
public boolean isLiving() {return true;
} // isLiving()
} // end class Alive
class Mineral extends Thing {public Mineral(String strName) {
super(strName);} // Constructor ()
public String group() {return "Mineral";
} // group()
} // end class Mineral
class Mineral extends Thing {public Mineral(String strName) {
super(strName);} // Constructor ()
public String group() {return "Mineral";
} // group()
} // end class Mineral
Subclasses
14Amadeo Ascó , Adam Moore
class Animal extends Alive {public Animal(String strName) {
super(strName);} // Constructor ()
public String subgroup() {return "Animal";
} // subgroup()
} // end class Animal
class Animal extends Alive {public Animal(String strName) {
super(strName);} // Constructor ()
public String subgroup() {return "Animal";
} // subgroup()
} // end class Animal
Subclasses
class Vegetal extends Alive {public Vegetal(String strName) {
super(strName);} // Constructor ()
public String subgroup() {return "Vegetal";
} // subgroup()
} // end class Vegetal
class Vegetal extends Alive {public Vegetal(String strName) {
super(strName);} // Constructor ()
public String subgroup() {return "Vegetal";
} // subgroup()
} // end class Vegetal
15Amadeo Ascó , Adam Moore
class Mammal extends Animal {private int miNumLegs;
public Mammal(String strName, int iNumLegs) {super(strName);
miNumLegs = iNumLegs;} // Constructor ()
public String type() {return "Mammal";
} // type()public int getNumLegs() {
return miNumLegs;} // getNumLegs()
} // end class Mammal
class Mammal extends Animal {private int miNumLegs;
public Mammal(String strName, int iNumLegs) {super(strName);
miNumLegs = iNumLegs;} // Constructor ()
public String type() {return "Mammal";
} // type()public int getNumLegs() {
return miNumLegs;} // getNumLegs()
} // end class Mammal
16Amadeo Ascó , Adam Moore
Mammal elephant = new Mammal("Elephant", 4);
System.out.println("Name: " + elephant.getName());System.out.println("Is living been? " + elephant. isLiving());System.out.println("Group: " + elephant.group());System.out.println("Subgroup: " + elephant.subgroup());System.out.println("Type: " + elephant.type());System.out.println("Num legs: " + elephant.getNumLegs());
Mammal elephant = new Mammal("Elephant", 4);
System.out.println("Name: " + elephant.getName());System.out.println("Is living been? " + elephant. isLiving());System.out.println("Group: " + elephant.group());System.out.println("Subgroup: " + elephant.subgroup());System.out.println("Type: " + elephant.type());System.out.println("Num legs: " + elephant.getNumLegs());
Fromclass
ThingAliveAliveAnimalMammalMammal
Fromclass
ThingAliveAliveAnimalMammalMammal
Name: ElephantIs living been? trueGroup: AliveSubgroup: AnimalType: MammalNum legs: 4
17Amadeo Ascó , Adam Moore
Sub and Super Classes
• In Java if no constructor has been defined– Java defines a default constructor for the class– Constructor without parameters
• All subclass constructors call their previous class constructor– If parent class does not have defined implicitly a
constructor then the default one is called– Otherwise you must call the parents constructor in
your constructor
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class Alive extends Thing {public Alive(String strName) {
super(strName);} // Constructor ()
public String group() {return "Alive";
} // group()
...} // end class Alive
class Alive extends Thing {public Alive(String strName) {
super(strName);} // Constructor ()
public String group() {return "Alive";
} // group()
...} // end class Alive
Sub and Super Classes
class Thing {private String mstrName;
public Thing(String strName) {mstrName = strName;
} // Constructor ()
public String getName() {return mstrName;
} // getName()...
} // end class Thing
class Thing {private String mstrName;
public Thing(String strName) {mstrName = strName;
} // Constructor ()
public String getName() {return mstrName;
} // getName()...
} // end class Thing
Example
