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    javz笔记之:有趣的静态方法的使用

    2018-11-14 08:50:27 次阅读 稿源:互联网
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    代码如下:

    import java.util.*;

    public class welcome {

        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);
              percent = tripleValue(percent);
              System.out.println("After: percent =" + percent);  //这里输出为30了!正常的结果

              /*
               * 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 double tripleValue(double x) // doesn't work
           {
              return 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
        {
           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;
           }

           private String name;
           private double salary;
        }

    如果是以下代码:System.out.println("After: percent =" + percent);  //这里输出为10了!因为静态方法达不成你要的效果

    这是因为静态方法不能对对象产生效果,和静态域一样,它属于类,不属于任何对象
    代码如下:

    /**
     * This program demonstrates parameter passing in Java.
     * @version 1.00 2000-01-27
     * @author Cay Horstmann
     */
    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
    {
       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;
       }

       private String name;
       private double salary;
    }

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