只要有一個方法操作的是類而非介面,那麼你就只能使用這個類及其子類,如果你想要將這個方法應用於不在此繼承結構中的某個類,

//像本例這樣,建立一個能夠根據所傳遞的引數物件的不同而具有不同行為的方法,被稱為策略設計
//策略就是傳遞進去的引數物件,它包含要執行的程式碼
//: interfaces/classprocessor/Apply.java
package object;
import java.util.*;
import static net.mindview.util.Print.*;

class Processor {
  public String name() {
    return getClass().getSimpleName();
  }
  Object process(Object input) { 
 
return input; }
}    

class Upcase extends Processor {
  String process(Object input) { // Covariant return
    return ((String)input).toUpperCase();
  }
}

class Downcase extends Processor {
  String process(Object input) {
    return ((String)input).toLowerCase();
  }
}

class Splitter extends Processor {
  String process(Object input) {
    
 
// The split() argument divides a String into pieces:
    return Arrays.toString(((String)input).split(" "));//split()返回String[]陣列
  }
}    

public class Apply {
  public static void process(Processor p, Object s) {
    print("Using Processor " + p.name());
    print(p.process(s));
  }
  public static String s =
    "Disagreement with beliefs is by definition incorrect";
  
 
public static void main(String[] args) {
    process(new Upcase(), s);
    process(new Downcase(), s);
    process(new Splitter(), s);
  }
} /* Output:
Using Processor Upcase
DISAGREEMENT WITH BELIEFS IS BY DEFINITION INCORRECT
Using Processor Downcase
disagreement with beliefs is by definition incorrect
Using Processor Splitter
[Disagreement, with, beliefs, is, by, definition, incorrect]
*///:~

那麼你就會觸黴頭,介面可以在很大程度上放寬這種限制,因此,我們可以編寫可服用性更好的程式碼