Java代码中如何交换两个对象

原文地址：How to swap or exchange objects in Java?

Java代码如何交换对象

假设我们有一个叫Car的类，这个类中也有一些属性。我们创建Car类的两个对象，一个叫car1，另一个叫car2，那如何交换car1与car2的数据呢？

交换成员：例如，如果这个Car类只有一个整型的车号，我们可以通过交换两个车的号码来交换两个车。

// A Java program to demonstrate that we can swap two// objects be swapping members.// A car with number class Carclass Car{    int no;    Car(int no) { this.no = no; }}// A class that uses Car objectsclass Main{    // To swap c1 and c2    public static void swap(Car c1, Car c2)    {        int temp = c1.no;        c1.no = c2.no;        c2.no = temp;    }    // Driver method    public static void main(String[] args)    {        Car c1 = new Car(1);        Car c2 = new Car(2);        swap(c1, c2);        System.out.println("c1.no = " + c1.no);        System.out.println("c2.no = " + c2.no);    }}

输出：

c1.no = 2c2.no = 1

如果我们不知道类的成员呢？

以上方法只能是在我们直到车号这个成员的时候才能使用，如果我们不知道Car的成员或者说成员列表太多了，那咋办？这是一个十分常见的情况，一个类用到其他的类，但是又不能访问这个类的成员。看看下面这段代码能不能搞定？

// A Java program to demonstrate that simply swapping// object references doesn't work// A car with number and nameclass Car{    int model, no;    // Constructor    Car(int model, int no)    {        this.model = model;        this.no = no;    }    // Utility method to print Car    void print()    {        System.out.println("no = " + no +                           ", model = " + model);    }}// A class that uses Carclass Main{    // swap() doesn't swap c1 and c2    public static void swap(Car c1, Car c2)    {        Car temp = c1;        c1 = c2;        c2 = temp;    }    // Driver method    public static void main(String[] args)    {        Car c1 = new Car(101, 1);        Car c2 = new Car(202, 2);        swap(c1, c2);        c1.print();        c2.print();    }}

输出：

no = 1, model = 101no = 2, model = 202

从上面的输出可以看出，对象没有交换成功。我们在前面一章讨论过在Java中参数的传递。所以当我们传给swap方法c1与c2的时候，这个方法就创建了这两个引用的副本。

用封装类（Wrapper Class）解决，如果我们建立一个封装类，这个类包含Car的引用，我们可以通过交换封装类的引用来交换这两个car。

// A Java program to demonstrate that we can use wrapper// classes to swap to objects// A car with model and no.class Car{    int model, no;    // Constructor    Car(int model, int no)    {        this.model = model;        this.no = no;    }    // Utility method to print object details    void print()    {        System.out.println("no = " + no +                            ", model = " + model);    }}// A Wrapper over class that is used for swappingclass CarWrapper{   Car c;   // Constructor   CarWrapper(Car c)   {this.c = c;}}// A Class that use Car and swaps objects of Car// using CarWrapperclass Main{    // This method swaps car objects in wrappers    // cw1 and cw2    public static void swap(CarWrapper cw1,                             CarWrapper cw2)    {        Car temp = cw1.c;        cw1.c = cw2.c;        cw2.c = temp;    }    // Driver method    public static void main(String[] args)    {        Car c1 = new Car(101, 1);        Car c2 = new Car(202, 2);        CarWrapper cw1 = new CarWrapper(c1);        CarWrapper cw2 = new CarWrapper(c2);        swap(cw1, cw2);        cw1.c.print();        cw2.c.print();    }}

输出：

no = 2, model = 202no = 1, model = 101

所以如果用户类不能访问到被交换的类的成员的时候，封装类可以解决这个问题。