Java面向对象编程2.1.ppt

Create Object Declare： Point p;//只是声明引用，不在内存中为对象分配地址空间，而是分配一个引用空间； Instantiation： 运算符new为对象分配内存空间，它调用对象的构造方法，返回引用；一个类的不同对象分别占据不同的内存空间。 Create： 执行构造方法，进行初始化；根据参数不同调用相应的构造方法。 Creating objects of a class Creating objects of a class Automatic garbage collection The object does not have a reference and cannot be used in future. The object becomes a candidate for automatic garbage collection. Java automatically collects garbage periodically and releases the memory used to be used in the future. the compiler will complain because it cant find a method called getSalary in the class Person—at this point in the code, the compiler only knows the person is a Person. person is change the reference person into an Employee reference with a cast. person is Employee employee = (Employee) person; double salary = employee.getSalary(); // or double salary = ((Employee) person). getSalary(); is invoke the getSalary method We can cast the reference person to an Employee reference because person really points to an Employee object. If it did not and we ran the code, the cast would be illegal and the Java virtual machine (JVM) would throw a ClassCastException. Person person = new Person (Joe Smith, 100 Main Ave); person person 1 Name=“Joe” Address=“aa” = is is is double salary = ((Employee) person).getSalary();// illegal This code will compile, but when it is executed, the JVM will throw a ClassCastException because, in this example, the reference person does not point to an Employee object. The instanceof Operator two operands: an object reference , a class name object instanceof ClassX true: if the specified object is an instance of the specified class if object is an instance of a ClassX subclass if object is an instance of any class that has ClassX as an ancestor false:If the left-hand operand is null Note: the instanceof operator cannot be used to determine the type of a p