shapelist.java

JAVA 2入门经典 练习答案

// Chapter 6 Exercise 3

// 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 references to refer to new shape.
    else {                     
      end.setNext(newEnd);			               // Otherwise set next reference to new shape's reference.
      end = newEnd;				                     // 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());
    }
  }
}