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Inheritance in Java
CS 3331
Two Programming Languages
Words
• Overloading
• Overriding
Overloading of Methods and
Constructors
public class Point { public Point() { /* … / } public Point(int x, int y) { / … / } public double distance(Point other) { / … / } public double distance(int x, int y) { / … / } public double distance() { / … */ } // … }
Why use overloading?
Why use overloading?
public class StringBuffer { public StringBuffer append(String str) { /* … / } public StringBuffer append(boolean b) { / … / } public StringBuffer append(char c) { / … / } public StringBuffer append(int i) { / … / } public StringBuffer append(long l) { / … */ } // … }
When to Overload? (Cont.)
public class String { public String substring(int i, int j) { // base method: return substring from index i to j - 1. } public String substring(int i) { // provide default argument return substring(i, length()); } // … }
How does Java figure out which
method to use?
- new StringBuffer sb = append( …); Overloading 1: Point() Point() 2: Point(int x,int y) 3: double distance(Point other) Point(int,int)distance(Point) 4: double distance(int x,int y) distance(int,int) 5: double distance() Point p1 = new Point(); (^) // which constructor?distance() Point p2 = new Point(10,20); p2.distance(p1); // which method? p2.distance(20,30); p2.distance(); Overriding Methods
- Def. Overriding
- Consequences Overriding Methods (Cont.) public class T { public void m() { … } } public class S extends T { public void m() { … } } T t = new T(); S s = new S(); t.m(); // invoke m of class T s.m(); // invoke m of class S What value is returned? class Student { public int maxCredits() { return 15; } … } class GraduateStudent extends Student { public int maxCredits() { return 12; } … } Student s; // … s.getMaxCredits(); // which maxCredits method?
Implementation of Dynamic
Binding
- Storage structure of instance variables
- Dynamic bindings of messages to methods
Another Example class Point { public String description = “Point”; } class CPoint extends Point { public String description = “CPoint”; } CPoint c1 = new CPoint(); Point c2 = c1; system.out.println (c1.description); system.out.println (c2.description); Inheritance
- Inheritance models the is-a relationship.
- If class S extends class T, then all objects
of S can act-like an object of T.
- In Java, only single inheritance is allowed
among classes.
- All public and protected members of a
superclass are accessible in the
subclasses.*
*All protected members are also accessible within the package.
Constructors of Subclasses
- Can invoke a constructor of the direct
superclass.
- super(…) must be the first statement.
- If the super constructor call is missing,
by default the no-arg super() is invoked
implicitly.
- Can also invoke another constructor
of the same class.
- this(…) must be the first statement. Example of “this” Calls public class Point { private int x, y; public Point(int x, int y) { this.x = x; this.y = y; } public Point() { // default constructor this.x = 0; this.y = 0; } } Example of “super” Calls public class ColoredPoint extends Point { private Color color; public ColoredPoint(int x, int y, Color color) { this.x = x; this.y = y; this.color = color; } public ColoredPoint(int x, int y) { this(x, y, Color.BLACK); // point with default value } public ColoredPoint() { color = Color.BLACK; // what will be the values of x and y? } } Default Constructor public ClassName() { super(); } Q. What would be the use of default constructor?
Execution Order of Constructors public class T { int x = 10; // first public T() { x = 20; // second } // ... } public class S extends T { int y = 30; // third public S() { super(); y = 40; // fourth } // ... } Types
- What is a “type”? Subtyping
- What’s subtyping? Substitution Property
- Def. Substitution property
- A value of subtype can appear where a value of its supertype is expected, e.g., in arguments, results,
receivers, and assignments.
Substitution Property (Cont.)
- Rules of (polymorphic) assignment
- The type of expression at the right-hand side of an
assignment must be a subtype of the type of the variable
at the left-hand side of the assignment.
class Student { … } class Undergraduate extends Student { … } class Graduate extends Student { … } Student s1, s2; s1 = new Undergradute(); // polymorphic assignment s2 = new Graduate(); // polymorphic assignment Graduate s3 = s2; // is this OK? Graduate s3 = (Graduate) s2; // explicit casting Widening and Narrowing
- Def. widening and narrowing
- The conversion of a subtype to one of its supertypes is
called widening , and the conversion of a supertype to
one of its subtype is called narrowing (or downcasting ).
// s is a stack of strings Stack s = new Stack(); s.push(“Hello”); … // Stack defines a method top, i.e., “public Object top()”. s.top().size(); // okay? ((String) s.top()).size(); // downcasting
