假设有一个类：
class Book{
public int id=0;
public String name="itheima";
}
两个方法：
public static Book changeBook1(Book b){
b.id=2;b.name="itcast";
return b;
}
public static Book changeBook2(Book b){
b=new Book();
b.id=3;b.name="CSDN";
return b;
}
//测试：
Book b0=new Book();
System.out.println("book.id="+b0.id+"\tbook.name="+b0.name);//初始化后的Book对象
Book b1=changeBook1(b0);//更改Book对象的值
System.out.println("book.id="+b0.id+"\tbook.name="+b0.name);
System.out.println(b0==b1);//此处为何为true???
Book b2= changeBook2(b0);//更改Book对象的值
System.out.println("book.id="+b0.id+"\tbook.name="+b0.name);
System.out.println(b0==b2);//此处为何为false???