打印所有学生记录时，如何使我的程序恢复正常？

Question

 import java.util.Scanner; public class MainBinaryTreeArray { public static void main(String[] args) { int choice; Scanner scan= new Scanner(System.in); BinaryTreeArray data = new BinaryTreeArray(); Listing l1 = new Listing("Carol", 4354, 3.2); Listing l2 = new Listing("Timothy", 2353, 4.0); Listing l3 = new Listing("Dean", 4544, 2.4); Listing l4 = new Listing("Sue", 3445, 3.0); data.insert(l1); data.insert(l2); data.insert(l3); data.insert(l4); do { // Choose which operation by entering a number System.out.println("*****************(Menu Operations:)******************"); System.out.println(" (Press 1). Insert."); System.out.println(" (Press 2). Fetch."); System.out.println(" (Press 5). Output all student records."); System.out.println(" (Press 6). Exit the program."); System.out.println("Enter your choice: "); choice = scan.nextInt(); switch(choice) { case 1: System.out.println("Are students inserted: " + data.insert(l1)); break; case 2: System.out.println("The student's info that's fetched: "); System.out.print(data.fetch("Timothy")); break; case 5: System.out.print("Output all the student's records: "); data.showAll(); } }while(choice!=6); } } 

 public class BinaryTreeArray { private Listing[] data; private int size; public BinaryTreeArray() { size = 100; data = new Listing[size]; } public void showAll() { for(int i=0; i<size; i++) System.out.print(data[i] + " "); } public boolean insert(Listing newListing) { int i = 0; while(i < size && data[i]!= null) { if(data[i].getKey().compareTo(newListing.getKey()) > 0) i = 2 * i + 1; else i = 2 * i + 2; } if(i >= size) return false; else { data[i] = newListing.deepCopy(); return true; } } public Listing fetch(String targetKey) { int i= 0; while(i< size && data[i]!= null && data[i].getKey()!=targetKey) { if(data[i].getKey().compareTo(targetKey) > 0) i = 2 * i + 1; else i = 2 * i + 2; } if(i >= size || data[i] == null) return null; else return data[i].deepCopy(); } } 

 public class Listing implements Comparable<Listing> { private String name; // key field private int ID; private double GPA; public Listing(String n, int id, double gpa) { name = n; ID = id; GPA = gpa; } public String toString() { return("Name is " + " " + name + "\\nID is" + " " + ID + "\\nGPA is" + " " + GPA + "\\n"); } public Listing deepCopy() { Listing clone = new Listing(name, ID, GPA); return clone; } public int compareTo(Listing other) { return this.name.compareTo(other.getKey()); } public String getKey() { return name; } }// end of class Listing 

Hello All,

My java program compiles fine, but I am having a terrible and miserable time with getting my program to stop printing all those nulls when I output all student records in my BinaryTreeArray. Here is complete program. Any suggestions? Please do give any advice. So to make what I am saying clear, I need help with understanding why when I print out student records it includes a whole bunch of extra nulls that really have no purpose and just make my program look crazy. Any solutions to this problem?

Answer 1

When you initialize the BinaryTreeArray(), you set the field variable "size" to 100 and use this to initialize the data Listing[] array. When you print out the data, the loop uses the same "size" variable so you get all 100 entries, including the null entries. A simple solution would be to filter the data for null when you show all.

public class BinaryTreeArray
{ 

  ...

  public void showAll()
  { 
      for(int i=0; i<size; i++)
      {
          if (data[i] != null)
              System.out.print(data[i] + " "); 
      }
  } 

  ...

}

An alternate solution would be to change your use of size to maxSize and then create a variable size that is incremented as you insert listings.