Java Method Parameters

2017-03-28

在 Java 中,方法有两种参数类型:

  • Primitive types(numbers, boolean types)
  • Object reference

对于这两种类型,Java 都是采用 call by value 的方式进行传递。

一个例子如下:

public class ParamTest
{
   public static void main(String[] args)
   {
      /*
       * Test 1: Methods can't modify numeric parameters
       */
      System.out.println("Testing tripleValue:");
      double percent = 10;
      System.out.println("Before: percent=" + percent);
      tripleValue(percent);
      System.out.println("After: percent=" + percent);

      /*
       * Test 2: Methods can change the state of object parameters
       */
      System.out.println("\nTesting tripleSalary:");
      Employee harry = new Employee("Harry", 50000);
      System.out.println("Before: salary=" + harry.getSalary());
      tripleSalary(harry);
      System.out.println("After: salary=" + harry.getSalary());

      /*
       * Test 3: Methods can't attach new objects to object parameters
       */
      System.out.println("\nTesting swap:");
      Employee a = new Employee("Alice", 70000);
      Employee b = new Employee("Bob", 60000);
      System.out.println("Before: a=" + a.getName());
      System.out.println("Before: b=" + b.getName());
      swap(a, b);
      System.out.println("After: a=" + a.getName());
      System.out.println("After: b=" + b.getName());
   }

   public static void tripleValue(double x) // doesn't work
   {
      x = 3 * x;
      System.out.println("End of method: x=" + x);
   }

   public static void tripleSalary(Employee x) // works
   {
      x.raiseSalary(200);
      System.out.println("End of method: salary=" + x.getSalary());
   }

   public static void swap(Employee x, Employee y)
   {
      Employee temp = x;
      x = y;
      y = temp;
      System.out.println("End of method: x=" + x.getName());
      System.out.println("End of method: y=" + y.getName());
   }
}

class Employee // simplified Employee class
{
   private String name;
   private double salary;

   public Employee(String n, double s)
   {
      name = n;
      salary = s;
   }

   public String getName()
   {
      return name;
   }

   public double getSalary()
   {
      return salary;
   }

   public void raiseSalary(double byPercent)
   {
      double raise = salary * byPercent / 100;
      salary += raise;
   }
}

对于 Test 1,输出结果为:

Testing tripleValue:
Before: percent=10.0
End of method: x=30.0
After: percent=10.0

对于 Test 2,输出结果为:

Testing tripleSalary:
Before: salary=50000.0
End of method: salary=150000.0
After: salary=150000.0

对于 Test 3,输出结果为:

Testing swap:
Before: a=Alice
Before: b=Bob
End of method: x=Bob
End of method: y=Alice
After: a=Alice
After: b=Bob

可以看到,在传入参数时,Java 对参数进行了复制。那么在方法中使用的是复制后的值。对于基本类型,复制过程相当于创建了新的值。对于对象引用类型,相当于创建了一个新的引用,并与参数指向了同一个实例变量。因为方法中使用的是复制后的值,所以 tripleValueswap 方法是无效的。但是对于引用来说,因为它与参数指向了同一个实例变量,并且实例变量存在于堆中,那么使用该引用调用实例的方法对实例状态进行修改后,外界也能看到这一修改。