Used of toString

[Armel Moukoss]

Hi,

I don't really understand how the toString method is used in the code below :

import java.util.*;
 
//
// This class implements a vector of characters, capable of sorting
// and reversing itself.
//
 

public class CleverVector extends Vector<Character>
{
  // Initializes this object with nChars unique characters.
  public CleverVector(int nChars) {
    while (size() < nChars) {
      Character c = randomChar();
      if (!contains(c))                          // Don't add any duplicates
        add(c);
    }
  }
 

  //
  // Returns a random character in the range A-Z, a-z.
  //
 
  private static Character randomChar() {
    boolean uppercase = (Math.random() > 0.5);
    char base = uppercase ? 'A' : 'a';
    int rand = (int)(26 * Math.random());         // 0 - 25
    char c = (char)(base + rand);                 // A-Z or a-z
    return c;                                     // Autoboxed 
  }
 

  // 
  // Sorts the contents of this vector, by copying the elements into
  // a TreeSet, then retrieving them in sorted order.
  //
 
  public void sort() {
    // Copy contents into sorting set.
    TreeSet<Character> sorter = new TreeSet<Character>();
    for (Character ch:this)
      sorter.add(ch);  
 
    // Remove contents of this vector.
    clear();
 
    // Retrive set contens in sorted order, and add them to this vector.
    for (Character ch:sorter)
      add(ch);
  }
 

  //
  // Reverses the contents of this vector, by copying the elements into
  // another Vector, then retrieving them in reverse order. Enhanced-for 
  // isn't helpful here, but copying the elements into the other Vector
  // is easy because there's a copying constructor.
  //
 
  public void reverse() {
    // Copy this vector.
    Vector<Character> otherVec = new Vector<Character>(this);  // Copying constructor
 
    // Remove contents of this vector.
    clear();
 
    // Retrieve from otherVec in reverse order.
    for (int i=otherVec.size()-1; i>=0; i--)
      add(otherVec.get(i));
  }
 

  public String toString() {
    String s = "";
    for (Character c:this)
      s += c.charValue();
    return s;
  }
    
 
  public static void main(String[] args) {
    int nChars = Integer.parseInt(args[0]);
    CleverVector vec = new CleverVector(nChars);
    System.out.println("Initial State: " + vec);
    vec.sort();
    System.out.println("       Sorted: " + vec);
    vec.reverse();
    System.out.println("     Reversed: " + vec);
  }
 
}

One of the possible result :


Initial State: [k, q, i, j, f, V, J, S, c, v, u', U, C, z, I, y, d, H, X, e]
Sorted: [C, H, I, J, S, U, V, X, c, d, e, f, i, j, k, q, u', v, y, z]
Reversed: [z, y, v, u', q, k, j, i, f, e, d, c, X, V, U, S, J, I, H, C]



I don't see any place where the toString method is called. But if i put it in commentary the result is not the same. Can someone help me, please ? thank you.

Ps : I take the code on the cd of the book "complete java 2 certification", in solution of the chapter 8.

[Kaydell Leavitt]

In this code example, I believe that the toString() method is called by the following code:

System.out.println("       Sorted: " + vec);

The first parameter to println() is a String, so the "+" sign means string concatenation (not addition) and so the vec variable will be converted to a String, using its toString() method, since it is in a String context. The Vector must, in turn, call toString() for each element in the Vector.

Kaydell
[ May 28, 2007: Message edited by: Kaydell Leavitt ]

[Nik Arora]

Hi,
Its called when System.out.println() is invoked. Even you can override to print in the format you want.




Regards
Nik
Scjp 1.5

[Deepak Bala]

Heres a simple example

  
Class Test
{
   public String toString()
   {
      return "This is overridden and a better representation of Test";
   }
}

Try doing this

System.out.println(new Test());

You should see the custom definition of the class on your console instead of its memory location.

[Armel Moukoss]

Hi,

Just to thank you very much guys for yours answers.

Armel