对象a作为参数argument在方法中使用时，如果argument在方法中赋予另一个对象的地址，则之后方法中对参数argument的操作，都不会影响到对象a.

方法中参数argument如果是对象，argument是源对象a的副本,方法执行结束，副本argument就被移出stack,在合适的时间就会被GC回收

/*2015-7-22*/public class ReferenceTypeDemo {    public static void main(String[] args) {        Person tom = new Person("Tom", 20);        System.out.println(tom);        modify(tom);        System.out.println(tom);    }    private static void modify(Person tom) {        System.out.println("modify():" + tom);        Person jack = new Person("Jack", 10);        tom = jack;        System.out.println(tom);        System.out.println("modify():" + tom);        tom.setName("Tom-modify");        System.out.println("modify():" + tom);    }}class Person {    private String name;    private int age;    public Person(String name, int age) {        super();        this.name = name;        this.age = age;    }    public void setName(String name) {        this.name = name;    }    @Override    public String toString() {        return Integer.toHexString(hashCode()) + ";Person [name=" + name + ", age=" + age + "]";    }}

Output:

14318bb;Person [name=Tom, age=20]modify():14318bb;Person [name=Tom, age=20]ca0b6;Person [name=Jack, age=10]modify():ca0b6;Person [name=Jack, age=10]modify():ca0b6;Person [name=Tom-modify, age=10]14318bb;Person [name=Tom, age=20]

 Tips:

hasCode()可以标识对象。Map中判断是否包含一个对象，就是使用hashCode()和Equals()来判断