Java 适配器模式讲解和代码示例

适配器是一种结构型设计模式，它能使不兼容的对象能够相互合作。

适配器可担任两个对象间的封装器，它会接收对于一个对象的调用，并将其转换为另一个对象可识别的格式和接口。

进一步了解适配器模式

复杂度：

流行度：

使用示例： 适配器模式在 Java 代码中很常见。基于一些遗留代码的系统常常会使用该模式。在这种情况下，适配器让遗留代码与现代的类得以相互合作。

Java 核心程序库中有一些标准的适配器：

java.util.Arrays#asList()
java.util.Collections#list()
java.util.Collections#enumeration()
java.io.InputStreamReader(InputStream) （返回 Reader对象）
java.io.OutputStreamWriter(OutputStream) （返回 Writer对象）
javax.xml.bind.annotation.adapters.XmlAdapter#marshal() 和 #unmarshal()

识别方法： 适配器可以通过以不同抽象或接口类型实例为参数的构造函数来识别。当适配器的任何方法被调用时，它会将参数转换为合适的格式，然后将调用定向到其封装对象中的一个或多个方法。

让方钉适配圆孔

这个简单的例子展示了适配器如何让不兼容的对象相互合作。

round

round/RoundHole.java: 圆孔

package refactoring_guru.adapter.example.round;

/**
 * RoundHoles are compatible with RoundPegs.
 */
public class RoundHole {
    private double radius;

    public RoundHole(double radius) {
        this.radius = radius;
    }

    public double getRadius() {
        return radius;
    }

    public boolean fits(RoundPeg peg) {
        boolean result;
        result = (this.getRadius() >= peg.getRadius());
        return result;
    }
}

round/RoundPeg.java: 圆钉

package refactoring_guru.adapter.example.round;

/**
 * RoundPegs are compatible with RoundHoles.
 */
public class RoundPeg {
    private double radius;

    public RoundPeg() {}

    public RoundPeg(double radius) {
        this.radius = radius;
    }

    public double getRadius() {
        return radius;
    }
}

square

square/SquarePeg.java: 方钉

package refactoring_guru.adapter.example.square;

/**
 * SquarePegs are not compatible with RoundHoles (they were implemented by
 * previous development team). But we have to integrate them into our program.
 */
public class SquarePeg {
    private double width;

    public SquarePeg(double width) {
        this.width = width;
    }

    public double getWidth() {
        return width;
    }

    public double getSquare() {
        double result;
        result = Math.pow(this.width, 2);
        return result;
    }
}

adapters

adapters/SquarePegAdapter.java: 方钉到圆孔的适配器

package refactoring_guru.adapter.example.adapters;

import refactoring_guru.adapter.example.round.RoundPeg;
import refactoring_guru.adapter.example.square.SquarePeg;

/**
 * Adapter allows fitting square pegs into round holes.
 */
public class SquarePegAdapter extends RoundPeg {
    private SquarePeg peg;

    public SquarePegAdapter(SquarePeg peg) {
        this.peg = peg;
    }

    @Override
    public double getRadius() {
        double result;
        // Calculate a minimum circle radius, which can fit this peg.
        result = (Math.sqrt(Math.pow((peg.getWidth() / 2), 2) * 2));
        return result;
    }
}

Demo.java: 客户端代码

package refactoring_guru.adapter.example;

import refactoring_guru.adapter.example.adapters.SquarePegAdapter;
import refactoring_guru.adapter.example.round.RoundHole;
import refactoring_guru.adapter.example.round.RoundPeg;
import refactoring_guru.adapter.example.square.SquarePeg;

/**
 * Somewhere in client code...
 */
public class Demo {
    public static void main(String[] args) {
        // Round fits round, no surprise.
        RoundHole hole = new RoundHole(5);
        RoundPeg rpeg = new RoundPeg(5);
        if (hole.fits(rpeg)) {
            System.out.println("Round peg r5 fits round hole r5.");
        }

        SquarePeg smallSqPeg = new SquarePeg(2);
        SquarePeg largeSqPeg = new SquarePeg(20);
        // hole.fits(smallSqPeg); // Won't compile.

        // Adapter solves the problem.
        SquarePegAdapter smallSqPegAdapter = new SquarePegAdapter(smallSqPeg);
        SquarePegAdapter largeSqPegAdapter = new SquarePegAdapter(largeSqPeg);
        if (hole.fits(smallSqPegAdapter)) {
            System.out.println("Square peg w2 fits round hole r5.");
        }
        if (!hole.fits(largeSqPegAdapter)) {
            System.out.println("Square peg w20 does not fit into round hole r5.");
        }
    }
}

OutputDemo.txt: 执行结果

Round peg r5 fits round hole r5.
Square peg w2 fits round hole r5.
Square peg w20 does not fit into round hole r5.