shapelist.java

JAVA 2入门经典 练习答案

// Chapter 6 Exercise 2

// CLASS TO DEFINE A LINKED LIST OF SHAPES. 
// This class has an inner class defining a node in a linked list.
// Because Shape is a super-class of the classes defining specific shapes,
// a variable of type Shape can store any kind of shape of a sub-class type.

public class ShapeList {
  private ShapeNode start;		// Reference to first Shape in List.
  private ShapeNode end;		// Reference to last Shape in List.

  // Construct a ShapeList from a single shape:
  public ShapeList(Shape shape) {
    // Create a one shape list:
    start = new ShapeNode(shape);
    end = start;
  }

  // Construct a ShapeList from an array of Shapes:
  public ShapeList(Shape[] shapes) {
    this(shapes[0]);            // Create a one shape list.
    
    // Now add the other shapes:
    for(int i=1 ; i<shapes.length ; i++)
      add(shapes[i]);
  }

  // Add a shape to the list:
  public void add(Shape data) {
    ShapeNode newEnd = new ShapeNode(data);	// Create a new list node
    if(start == null)                       // If the list is empty
      end = start = newEnd;			            // set the first & last
    else {				                          // references to refer to new shape
      end.setNext(newEnd);			            // Otherwise set next reference
      end = newEnd;				                  // to new shape's reference.
    }						                            // Update end reference.
  }


  // Output all the shapes in the list:
  public void show() {
    System.out.println("\nShapeList shapes are:");
    ShapeNode current = start;		// Set current to reference start.

    while(current != null) {		// If there is a current shape.					
      current.show();                   // Display it.
      current = current.getNext();	// Get the next element in the list.
    }
  }

  // Inner class defines a node in a linked list of shapes:
  class ShapeNode {
    ShapeNode next;	                // A reference to the next shape node in the list.
    Shape shape;		        // A reference to the shape object.


    // Constructor:
    public ShapeNode(Shape shape) {
      this.shape = shape;	        // Store the Shape object.
      next = null;                      // Initialize the next reference.
    }

    // Method to set the reference to the next shape:
    public void setNext(ShapeNode next) {
      this.next = next;
    }

    // Get the reference to the next shape:
    public ShapeNode getNext()  {
      return next;
    }

    // Overrides the method inherited from Object:
    public String toString() {
      return shape.toString();
    }

    // Output the shape contained in the ShapeNode:
    public void show() {
      System.out.println(toString());
    }
  }
}