Instance Initializers

You would typically initialize data members of a class with a constructor. * An anonymous class cannot provide constructors, since it has no class name.

Instance initializers allow an anonymous object to be properly initialized.

Refresher - Instance Initializers

• Any class can contain an instance initializer.
• A stand-alone code block inside a class definition is an instance initializer.

  // A top-level class
  public class InstanceInitializerExample {
    private String id;
  
    // instance initializer
    {
      id = "DEFAULT";
    }
  
    // ...
  }
  

• Multiple instance initializers are allowed; they are run from top to bottom.

• They run after the superclass constructor, and before the current class's constructor (if it isn't anonymous).

• The initialization of a data member can be performed immediately after the declaration of the variable.

• This is helpful if a simple assignment won't accomplish the task.

Examples

// Anonymous Inner Class with instance initializer
public class Outer4 {
  private int outerField;

  public void aMethod() {
    final int localVar = 0;

    Object in = new Object() {
      private int innerField;

      // instance initializer
      {
        innerField = 12;
      }

      public String toString() {
        return "o: " + outerField + " i: " + innerField
            + " l: " + localVar;
      }
    };

    System.out.println(in);
  }

  public static void main(String[] args) {
    Outer4 out = new Outer4();
    out.aMethod();
  }
}

The Runnable anonymous instance below records the time it was created, and prints the time between creation and the call to run().

Runnable rTimed = new Runnable() {
  private LocalDateTime time;

  // instance initializer
  {
    time = LocalDateTime.now(); // Gets the current moment in time
    System.out.println("Instance created at " + time);
  }

  public void run() {
    // Calculate time between
    Duration d = Duration.between(time, LocalDateTime.now());
    System.out.println("Running " + d + " later...");
  }
};

Drill

InnerClasses/com.example.innerclasses.drills.InstanceInitalizerDrill

• Paste the anonymous Runnable instantiation above into main.
• Call run on the instance to see what LocalDateTime and Duration print.
• Note: these classes are in the java.time package.

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