匿名内部 class：返回匿名内部 class 的实例

Question

I am self-learning Java with the book "Thinking in java 4th Edition".

When i come to trying one example from the book, it gives compiler error.

https://github.com/BruceEckel/TIJ4-code/blob/master/examples/innerclasses/Parcel7.java

//: innerclasses/Parcel7.java
// Returning an instance of an anonymous inner class.

public class Parcel7 {
  public Contents contents() {
    return new Contents() { // Insert a class definition
      private int i = 11;
      public int value() { return i; }
    }; // Semicolon required in this case
  }
  public static void main(String[] args) {
    Parcel7 p = new Parcel7();
    Contents c = p.contents();
  }
} ///:~

.\Parcel7.java:4: error: cannot find symbol
    public Contents contents() {
           ^
  location: class Parcel7
.\Parcel7.java:5: error: cannot find symbol
        return new Contents() {
                   ^
  symbol:   class Contents
  location: class Parcel7
.\Parcel7.java:13: error: cannot find symbol
        Contents c = p.contents();
        ^
  symbol:   class Contents
  location: class Parcel7
3 errors
PS S:\Java Learning\001. Hello> javac .\Parcel7.java
.\Parcel7.java:2: error: cannot find symbol
    public Contents contents() {
           ^
  symbol:   class Contents
  location: class Parcel7
.\Parcel7.java:3: error: cannot find symbol
      return new Contents() { // Insert a class definition
                 ^
  symbol:   class Contents
  location: class Parcel7
.\Parcel7.java:10: error: cannot find symbol
      Contents c = p.contents();
      ^
  symbol:   class Contents
  location: class Parcel7
3 errors

Do I miss anything?

Answer 1

This anonymous class declaration creates an anonymous class that is a sub-type of the class Contents .

Looking at the error, I assume that you do not have a named class called Contents defined anywhere. If you already have that class, then it may not have been imported properly.