Name: Java #19 - インターフェース (Java #19 - Interfaces)
Uploaded: 2021-01-14T04:26:39.000Z
Duration: 4 min 23 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

Here’s a picture of a club in New York City.

基礎受動態

attend their events and  use their fitness center.

which allows them to prove  they are a member.

Nonmembers, well, they can  admire the view of the exterior.

As you might guess,  membership in this club isn’t free—

For Java interfaces, the dues are  a list of methods

Here we see an interface Vehicle with  three methods,

I’ve explicitly labeled these  methods as public.

All interface methods are both  public and abstract

whether or not you specify  those keywords.

a class must implement  these three methods—

The Java keyword for being a  member of an interface is “implements”.

Here we see a class Car, that  implements the interface Vehicle.

It must have the three methods,  “start”, “move” and “stop”,

Once it has “paid its dues” by  implementing these three methods,

it gets all of the privileges of  being a Vehicle.

Anything that takes a Vehicle  will accept a Car.

For example, I can pass car1 to the  method travel.

I can also cast a variable of type Car  to a variable of type Vehicle.

And, if I ask if car1 is an instance of  Vehicle, the answer will be true.

Cars are card-carrying members of the  club vehicle,

with all the rights and privileges  pertaining thereto.

An interface might remind you  of an abstract class.

can have abstract methods that  other classes are forced to implement.

But, while a class can only have one  parent class,

it can implement as many  interfaces as it wants.

an abstract class can define methods  and attributes

Interfaces may only contain constants  and abstract methods—

One of the most useful interfaces  built into Java is “Comparable”.

To be comparable, there must be an  ordering between objects.

we can determine if the first is less than,  equal to, or greater than the second.

The compareTo method returns 0 if the  objects are equal,

a negative number if the first is smaller  and a positive number if the first is larger.

The reason for specifying just positive or  negative

instead of -1 and  1 is that this allows us to simply do a subtraction

rather than an if statement in  many cases.

The class OrderedPoint gives an  example of implementing Comparable.

so it couldn’t also be a child class of  a Comparable class.

OrderedPoint can be a child class of  Point and implement Comparable.

is present because Comparable is a  generic class,

compare OrderedPoints to other  OrderedPoints.

For our ordering, we’ll use the  distance to the origin.

Since we only interested in  comparing distances,

we actually use the square of the  distance

so we don’t have to bother  with the square root.

if the two points are equidistant  from the origin,

a negative number if “this” is  closer to the origin

we see that the big win for implementing Comparable

is that we can use the method  Arrays.sort.

If we pass an array of Comparable  objects to Arrays.sort,

it will sort them in increasing order for us

using a tuned quicksort algorithm that  should be very fast for most purposes.

There’s also a static binarySearch method in the Arrays class

that can be used to find items quickly  in a sorted array.

Sorting and searching arrays are  very common tasks,

and Comparable allows us to avoid  having to rewrite those algorithms

Running the program, it generates  10 random points in the first quadrant

sorts them in order of increasing  distance from the origin,

You can download the source code for  OrderedPoint and PointsInOrder

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Java #19 - インターフェース (Java #19 - Interfaces)

sort

negative

positive

increase