程序代写代做代考 Java Bounded Generics in Java Programming

Bounded Generics in Java Programming

Ritwik Banerjee

rbanerjee@cs.stonybrook.edu

c© Ritwik Banerjee Bounded Generics in Java Programming 1/11

Type Parameters

I Generics allow us to use “types” (i.e., classes and interfaces) as
parameters when defining new classes, interfaces or methods.

I When we talk of parameters, there are two broad categories:
I Formal parameters used in method declaration.
I Type parameters so that we can re-use code with different types

of inputs.

I Inputs to formal parameters are values.
I Inputs to type parameters are types.

c© Ritwik Banerjee Bounded Generics in Java Programming 2/11

Bounded Type Parameters

I Sometimes, we may want to restrict the types that can be used
as type arguments in a parameterized type.

I An operation on numbers should only accept inputs that are
instances of java.lang.Number or its subclasses.

I This is where a bounded type parameter comes in.
I A type parameter that is restricted to a certain part of the Java

class hierarchy.

Expressing Restrictions . . . in terms of the Java class hierarchy

I A descendant (i.e., subclass) of Number
I C extends java.lang.Number

I An ancestor (i.e., superclass) of Number
I C super java.lang.Number

c© Ritwik Banerjee Bounded Generics in Java Programming 3/11

An example of type parameter restriction

public class Box {
private T t;
public void set(T t) { this.t = t; }
public T get() { return t; }

// Number is the upper bound of the parameter U
public void inspect(U u){
System.out.println(“T: ” + t.getClass().getName());
System.out.println(“U: ” + u.getClass().getName());

}

public static void main(String[] args) {
Box integerBox = new Box();
integerBox.set(new Integer(10));
integerBox.inspect(“some text”); // Error!

}
}

c© Ritwik Banerjee Bounded Generics in Java Programming 4/11

Methods defined within bounds

public class NaturalNumber {
private T n;
public NaturalNumber(T n) { this.n = n; }
public boolean isEven() { return n.intValue() % 2 == 0; }

}

I The isEven method invokes the intValue method defined in
the superclass Integer.

c© Ritwik Banerjee Bounded Generics in Java Programming 5/11

Multiple Bounds

I A type parameter can have multiple bounds
I In such cases, the type variable is a subtype of all the types

listed in the bound.

Class A { … }
interface B { … }
interface C { … }

I Then, we may write a bounded type of the form
class D

I If one of the bounds is a class, it must be specified first.
Otherwise there will be a compile-time error.
class D ←− Compile-time error

c© Ritwik Banerjee Bounded Generics in Java Programming 6/11

Generic Methods with Bounded Type Parameters

A generic method that counts the number of elements in a set that
are greater than a specified element:

static int countGreater(Set aSet, T t) {
int count = 0;
for (T item : aSet)

if (item > t)
++count;

return count;
}

This code will fail to compile. WHY?

I The operator > applies only to primitive types (short, int, double,
long, float, byte, char). It cannot to be used to compare objects.

I To fix this, use a type parameter bounded by the Comparable
interface.

c© Ritwik Banerjee Bounded Generics in Java Programming 7/11

Generic Methods with Bounded Type Parameters

A generic method that counts the number of elements in a set that
are greater than a specified element:

static int countGreater(Set aSet, T t) {
int count = 0;
for (T item : aSet)

if (item > t)
++count;

return count;
}

This code will fail to compile. WHY?

I The operator > applies only to primitive types (short, int, double,
long, float, byte, char). It cannot to be used to compare objects.

I To fix this, use a type parameter bounded by the Comparable
interface.

c© Ritwik Banerjee Bounded Generics in Java Programming 7/11

Generic Methods with Bounded Type Parameters

A generic method that counts the number of elements in a set that
are greater than a specified element:

static int countGreater(Set aSet, T t) {
int count = 0;
for (T item : aSet)

if (item > t)
++count;

return count;
}

This code will fail to compile. WHY?

I The operator > applies only to primitive types (short, int, double,
long, float, byte, char). It cannot to be used to compare objects.

I To fix this, use a type parameter bounded by the Comparable
interface.

Generic Methods with Bounded Type Parameters

A generic method that counts the number of elements in a set that
are greater than a specified element:

static >
int countGreater(Set aSet, T t) {

int count = 0;
for (T item : aSet)

if (item.compareTo(t) > 0)
++count;

return count;
}

Wildcards: ?

In Java, if a type parameter is unknown, it can represented by the
wildcard “?”. We can use wildcards to represent

I Unbounded type parameters,
I Type parameters with an upper bound, and
I Type parameters with a lower bound.

Upper Bounded Wildcards

I Say you want to write a method that works on List,
List, and List.

I Clearly, you want to write a piece of code that works for Number
and its subclasses.

I This is where a wildcard should be used, but with an upper
bound:

static double sum(List numbers) {
double sum = 0d;
for (Number aNumber : numbers)

sum += aNumber.doubleValue();
return sum;

}

Lower Bounded Wildcards

I In a different scenario, suppose you want to add integers to a list.
I And you want to write your code such that you can add your

integers to a list of Integers, or a list of Doubles, or a list of
Numbers.

I In this case, you want to work with anything that is a “supertype”
of Integer.

I Here, we want a wildcard with a lower bound:

static void addTo(int i,
List numbers) {

numbers.add(i);
}

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