Java程序设计语言总是采用按值调用。也就是说,方法得到的是所有参数值的一个拷贝,特别是,方法不能修改传递给它的任何参数变量的内容。
Java中方法参数使用情况总结:
- 一个方法不能修改一个基本数据类型的参数(即数值型或布尔型)。
- 一个方法可以改变一个对象参数的状态。
- 一个方法不能让对象参数引用一个新的对象。
- 首先试图将一个值参数的值提高两倍,但没有成功:
Testing tripleValue:
Before: percent=10.0
End of method: x=30.0
After: percent=10.0
- 随后,成功地将一个雇员的薪金提高了两倍:
Testing tripleSalary:
Before: salary=50000.0
End of method: salary=150000.0
After: salary=150000.0
方法结束之后,harry引用的对象状态发生了改变。这是因为这个方法可以通过对象引用的拷贝修改所引用的对象状态。
- 最后,程序演示了swap方法的失败效果:
Testing swap:
Before: a=Alic
Before: b=Bob
End of method: x=Bob
End of method: y=Alic
After: a=Alic
After: b=Bob
可以看出,参数变量x和y交换了,但是a和b没有受到影响。
完整代码如下:
/**
* 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("Alic", 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 {
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;
}
}
引用自《Java核心技术 卷I》