Accessing Declarations in Enclosing Context

Member declarations and access rules for local classes (p. 512) also apply to anonymous classes. Example 9.15 is an adaptation of Example 9.9, Example 9.10, and Example 9.11. It illustrates what members can be declared in an anonymous class and what can be accessed in its enclosing context. The local classes from previous examples have been adapted to anonymous classes in Example 9.15.

The TLCWithAnonClasses class declares a non-static method at (3) in which a local variable (baseRef) is initialized with an instance of a non-static anonymous class at (9). The baseRef variable is used to invoke the printValue() method on this instance at (28).

The TLCWithAnonClasses class declares a static field baseField at (29) that is initialized with an instance of a static anonymous class. The field baseField is used to invoke the printValue() method on this instance at (36).

Accessing Local Declarations in the Enclosing Block

Being inner classes, there are many similarities between local classes and anonymous classes. An anonymous class can only access (effectively) final variables in its enclosing local context, shown both for static fields at (10), (11), and (12), and for instance fields at (13), (14), and (15). Local variables accessed in the anonymous class must be declared before use, and definitely assigned, as shown at (16) and (17), respectively. A field name in the anonymous class can shadow a local variable with the same name in the local context, as shown at (18). Note that the anonymous class declared at (29) does not have an enclosing local context, only an enclosing class, as it is defined in non-static context.

Accessing Members in the Enclosing Class

A member (either inherited or declared) in an anonymous class can hide a member with the same name in the enclosing object or class. The inherited field nsf1 hides the field by the same name in the enclosing class, as shown at (19), (20), and (21).

Non-static anonymous classes can access any non-hidden members in the enclosing class by their simple names and the qualified this, as shown at (22), (23), and (24). Static anonymous classes can only access non-hidden static members in the enclosing class by their simple names, as shown at (30), (31), and (32).

Example 9.15 Accessing Declarations in Enclosing Context (Anonymous Classes)

Click here to view code image

// File: AnonClient.java
abstract class Base {           // (1) Superclass
  protected int nsf1;
  abstract void printValue();
}
//_____________________________________________________________________________
class TLCWithAnonClasses {      // (2) Top level Class
  private int nsf1;             // Non-static field
  private int nsf2;             // Non-static field
  private static int sf = 5;    // Static field
  public void nonStaticMethod(final int fp) { // (3) Non-static Method
    // Local variables:
    int flv = 10;               // (4) Effectively final local variable
    final int hlv = 20;         // (5) Final local variable (constant variable)
    int nflv1 = 30;             // (6) Non-final local variable
    int nflv3;                  // (7) Non-final local variable declaration
    nflv1 = 40;                 // (8) Not effectively final local variable
    Base baseRef = new Base() { // (9) Non-static anonymous class
      // Static fields: Accessing local declarations in the enclosing block:
      static int sff1 = fp;   // (10) Final param from enclosing method
      static int sff2 = flv;  // (11) Effect. final variable from enclosing method
//    static int sf1 = nflv1; // (12) Not effect. final from enclosing method
      // Instance fields: Accessing local declarations in the enclosing block:
      int f1 = fp;       // (13) Final param from enclosing method
      int f2 = flv;      // (14) Effectively final variable from enclosing method
//    int f3 = nflv1;    // (15) Not effectively final from enclosing method

//    int f4 = nflv2;    // (16) nflv2 cannot be resolved: not decl-before-use
//    int f5 = nflv3;    // (17) Not definitely assigned: not initialized
      int hlv;           // (18) Shadows local variable at (5)
      // Accessing member declarations inherited from superclass:
      int f6 = nsf1;                   // (19) Inherited from superclass
      int f7 = this.nsf1;              // (20) Inherited from superclass
      int f8 = super.nsf1;             // (21) Inherited from superclass
      // Accessing (hidden) member declarations in the enclosing class:
      int f9 = TLCWithAnonClasses.this.nsf1;          // (22) In enclosing object
      int f10 = nsf2;                  // (23) Instance field in enclosing object
      int f11 = sf;                    // (24) Static field from enclosing class
      { nsf1 = fp; }                   // (25) Non-static initializer block
      @Override void printValue() {                        // (26) Instance method
        System.out.println(“Instance field nsf1: ” + nsf1);// (27)
      }
    };
int nflv2 = 70;
    nflv3 = 80;
    baseRef.printValue();              // (28) Invoke method on anonymous object
  }
  public static final Base baseField = new Base() { // (29) Static anonymous class
    // Accessing (hidden) member declarations in the enclosing class:
//  int f1 = TLCWithAnonClasses.this.nsf1; // (30) Not OK. No enclosing object
//  int f2 = nsf2;                         // (31) Not OK. No enclosing object
    { nsf1 = sf; }                         // (32) Non-static initializer block
    @Override void printValue() {                        // (33) Instance method
      System.out.println(“Instance field nsf1: ” + nsf1);// (34)
    }
  };
}
//_____________________________________________________________________________
public class AnonClient {
  public static void main(String[] args) {
    new TLCWithAnonClasses().nonStaticMethod(100);                // (35)
    TLCWithAnonClasses.baseField.printValue();                    // (36)
  }
}

Output from the program:

Click here to view code image Instance field nsf1: 100
Instance field nsf1: 5