Java參數(shù)傳遞

Java支持兩種數(shù)據(jù)類型：基本數(shù)據(jù)類型和引用數(shù)據(jù)類型。
原始數(shù)據(jù)類型是一個(gè)簡單的數(shù)據(jù)結(jié)構(gòu)，它只有一個(gè)與之相關(guān)的值。 引用數(shù)據(jù)類型是一個(gè)復(fù)雜的數(shù)據(jù)結(jié)構(gòu)，它表示一個(gè)對象。

原始數(shù)據(jù)類型的變量將該值直接存儲(chǔ)在其存儲(chǔ)器地址處。使用對象和引用變量時(shí)會(huì)有所不同。Java中的所有參數(shù)都通過值傳遞。

當(dāng)參數(shù)是原始數(shù)據(jù)類型時(shí)，實(shí)際參數(shù)的值將復(fù)制到參數(shù)。
在方法主體中對參數(shù)值進(jìn)行的任何更改只會(huì)更改形式參數(shù)的副本，而不會(huì)更改實(shí)際參數(shù)的值。當(dāng)參數(shù)通過參考值傳遞時(shí)，存儲(chǔ)在實(shí)際參數(shù)中的參考被復(fù)制到形式參數(shù)。 實(shí)際參數(shù)和形式參數(shù)都指向內(nèi)存中的相同對象。

可以將另一個(gè)對象的引用分配給方法主體中的形式參數(shù)。

示例

下面的代碼演示了Java中的引用傳遞機(jī)制。

class Phone {
  public String model = "Unknown";
  public int year = 2016;
  public double price = 0.0;
}

public class Main {
  public static void main(String[] args) {
    Phone myPhone = new Phone();

    myPhone.model = "iPhone";
    myPhone.year = 2018;
    myPhone.price = 16000.0;

    System.out.println("#1: model  = " + myPhone.model + ", year   = "
        + myPhone.year + ", price = " + myPhone.price);

    Main.test(myPhone);

    System.out.println("#4: model  = " + myPhone.model + ", year   = "
        + myPhone.year + ", price = " + myPhone.price);
  }

  public static void test(Phone xPhone) {
    System.out.println("#2: model  = " + xPhone.model + ", year   = "
        + xPhone.year + ",  price = " + xPhone.price);

    // Let's make xyCar refer to a new object
    xPhone = new Phone();

    System.out.println("#3: model  = " + xPhone.model + ", year   = "
        + xPhone.year + ", price = " + xPhone.price);
  }
}

上面的代碼生成以下結(jié)果。

#1: model  = iPhone, year   = 2018, price = 16000.0
#2: model  = iPhone, year   = 2018,  price = 16000.0
#3: model  = Unknown, year   = 2016, price = 0.0
#4: model  = iPhone, year   = 2018, price = 16000.0

注意
當(dāng)引用類型參數(shù)傳遞給方法時(shí)，形式參數(shù)可以訪問對象，實(shí)際參數(shù)也可以訪問該對象。

形式參數(shù)可以通過直接更改實(shí)例變量的值或通過調(diào)用對象上的方法來修改對象。通過形式參數(shù)對對象進(jìn)行的任何修改都可以通過實(shí)際參數(shù)立即可見，因?yàn)樗鼈兌急４鎸?nèi)存中同一對象的引用。形式參數(shù)本身可以被修改以引用方法內(nèi)的另一個(gè)對象。

要禁用將引用類型形式參數(shù)更改為引用不同對象的方法，請?jiān)谝妙愋托问絽?shù)聲明中使用關(guān)鍵字final。

public class Main {
  public static void main(String[] args) {
    Phone myPhone = new Phone();

    myPhone.model = "iPhone";
    myPhone.year = 2017;
    myPhone.price = 16000.0;

    Main.test(myPhone);
  }

  public static void test(final Phone xPhone) {
    System.out.println("#2: model  = " + xPhone.model + ", year   = "
        + xPhone.year + ",  price = " + xPhone.price);

    // Let's make xyCar refer to a new object
    //xPhone = new Phone();
  }
}
class Phone {
  public String model = "Unknown";
  public int year = 2014;
  public double price = 0.0;
}

上面的代碼生成以下結(jié)果。

#2: model  = iPhone, year   = 2017,  price = 16000.0