“我们在Java运行时更改任何对象的行为”的含义是什么？

Question

我正在阅读Orielly Design模式，并且有一行“ We can change the Behavior of any object at runtime ”以及如何使用getter和setter在运行时更改对象的行为。

Answer 1

The behavior is how the object works. Run time is the application life. So that statement means that during the run of program we are able to manipulate what object can do.

To simulate that please see following example:

public class MyObjectTest {

  public static final void main(String[] args) {

   MyObject myObject = new MyObject(); 

   String theMessage = "No message yet.";

   theMessage = myObject.readSecretMessage();

   myObject.setIsSafe(true);

   theMessage = myObject.readSecretMessage();

  }
}


public class MyObject {

 private boolean isSafe = false;


 public boolean isSafe() {
    return this.isSafe;
 } 

 public boolean setIsSafe(boolean isSafe) {
   return this.isSafe = isSafe;
 }

 public String readSecretMessage() {

   if(isSafe()) {
    return "We are safe, so we can share the secret";
   }
   return "They is no secret message here.";
 }
}

Analysis:

The program will return two different messages, the decision depend on the field isSafe . That can be modified during object life (object life start with operator new ) in run time.

And that what it means, that we can change the behavior of object.

Answer 2

Let me walk the cats on 2 and 4 legs alternately at the runtime :).. basically, this means that you can change the behavior of Cat at runtime using getters and setters.

class Cat
{
    int legs = 2;

    public void walk() {
       System.out.println("Walking on " + legs + " legs");
    }

    public int getLegs()
    {
       return legs;
    }

    public void setLegs(int legs)
    {
       this.legs = legs;
    }
}

public void static main()
{
     Cat c = new Cat();
     c.setLegs(4);
     c.walk();

     c.setLegs(2);
     c.walk();
}

Answer 3

On the face of it, it is meaningless, or at least incorrect. You can change an object's properties, but not its behaviour (except of course to the extent that its behaviour depends on the values of its properties). You would have to supply more context to make sense of this.