pitfall: "overriding" private methods

最新推荐文章于 2021-03-05 19:48:52 发布

原创最新推荐文章于 2021-03-05 19:48:52 发布 · 134 阅读

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探讨了Java中试图覆写私有方法的常见错误，解释了私有方法实际上是最终的，不能在派生类中被覆写。通过示例说明了即使在派生类中声明相同签名的方法也不会产生预期的覆写效果。

Error example

// polymorphism/PrivateOverride.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
// Trying to override a private method
// {java polymorphism.PrivateOverride}
package polymorphism;

public class PrivateOverride {
  private void f() {
    System.out.println("private f()");
  }

  public static void main(String[] args) {
    PrivateOverride po = new Derived();
    po.f();
  }
}

class Derived extends PrivateOverride {
  public void f() {
    System.out.println("public f()");
  }
}
/* Output:
private f()
*/

You might reasonably expect the output to be "public f()", but a private method is automatically final , and is also hidden from the derived class. So Derived's f() here is a brand new method; it’s not even overloaded, since the base-class version of f() isn’t visible in Derived .

The result of this is that only non- private methods can be overridden, but watch out for the appearance of overriding private methods, which generates no compiler warnings, but doesn’t do what you might expect.

Use the @Override annotation to detect the problem.

The error compiler message is :

polymorphism/PrivateOverride2.java:21: error: method does not override or implement a method from a supertype
  @Override
  ^
1 error

references:

1. On Java 8 - Bruce Eckel

2. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/polymorphism/PrivateOverride.java

3. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/polymorphism/PrivateOverride2.java