如果参数类型是原始类型，那么传过来的就是这个参数的一个副本，也就是这个原始参数的值，这个跟之前所谈的传值是一样的。如果在函数中改变了副本的 值不会改变原始的值.

    如果参数类型是引用类型，那么传过来的就是这个引用参数的副本，这个副本存放的是参数的地址。如果在函数中没有改变这个副本的地址，而是改变了地址中的值，那么在函数内的改变会影响到传入的参数。如果在函数中改变了副本的地址，如new一个，那么副本就指向了一个新的地址，此时传入的参数还是指向原来的地址，所以不会改变参数的值。

例子：

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());}private 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());}private static void tripleSalary(Employee x) {x.raiseSalary(200);System.out.println("End of method: salary=" + x.getSalary());}private static void tripleValue(double x) {x=3*x;System.out.println("End of Method X= "+x);}
}

输出：

Testing tripleValue:
Before: percent=10.0
End of Method X= 30.0
After: percent=10.0Testing tripleSalary:
Before: salary=50000.0
End of method: salary=150000.0
After: salary=150000.0Testing swap:
Before: a=Alice
Before: b=Bob
End of method: x=Bob  //可见引用的副本进行了交换
End of method: y=Alice
After: a=Alice  //引用本身没有交换
After: b=Bob