如何为变量分配超值？

Question

I'd like to do the following:

public class Sub extends Super {
  public Sub(Super underlying) {
    if (underlying == null) {
      underlying = super; // this line is illegal
    }

    this.underlying = underlying;
  }

  @Override
  public void method() {
    underlying.method();
  }
}

How can something like this be done?

Answer 1

You have not understood the super keyword in java correctly. Refer the javadoc for super

If your method overrides one of its superclass's methods, you can invoke the overridden method through the use of the keyword super. You can also use super to refer to a hidden field (although hiding fields is discouraged)

Also, super() is used to call the parent class constructors.

Answer 2

It looks like you want to implement the delegation pattern .

Simple extend Super, and let your IDE override all methods with the creation of super calls.

Then replace "super." with "underlying."

Error prone, but that's it.

public class Sub extends Super {
    Super underlying;
    public Sub(Super underlying) {
        this.underlying = underlying;
    }

    @Override
    public void f() {
        underlying.f();
    }

Answer 3

public class Sub extends Super {
  public Sub(Super underlying) {
    this.underlying = (underlying == null)
        ? new Super()
        : underlying;
  }

  @Override
  public void method() {
    underlying.method();
  }
}

Since another object is being created, it's not exactly the same, but it behaves as expected.

Answer 4

I think something like what you want can be done if you are willing to add, and call, subclass-specific method names for the methods involved:

public class Test {
  public static void main(String[] args) {
    Super sup = new Super();
    Sub sub1 = new Sub(null);
    Sub sub2 = new Sub(sup);
    sub1.subMethod();
    sub2.subMethod();
    sup.method();
  }
}

class Super {
  public void method(){
    System.out.println("Using method from "+this);
  }
}

class Sub extends Super {
  private Super underlying;
  public Sub(Super underlying) {
    if (underlying == null) {
      underlying = this;
    }

    this.underlying = underlying;
  }

  public void subMethod() {
    underlying.method();
  }
}