I can NOT compile first class below

[Leonidas Savvides]

I can NOT compile first class below I get the>>>.
//Chapter: 18-Generics, Problem - textbook:HTP7 JAVA DEITEL (c)2007, Exe.:18.4

C:\Users\User\Documents\TextBooksDataFiles\RexPractice\JAVA\MySolutions\18-4>javac SelectionSort.java
SelectionSort.java:53: incompatible types
found : T
required: T
T x = data[smallest];
^
SelectionSort.java:54: incompatible types
found : T
required: T
T y = data[index];
^
2 errors

  import java.util.Random;

public class SelectionSort< T >
{

   private T[] data; // array of values
   
   public SelectionSort( T[] data1 )
   {
        data = data1;
   }

   // sort array using selection sort
   public < T extends Comparable< T >> void sort()
   {
      int smallest; // index of smallest element
      //data[]=data1; 
      // loop over data.length - 1 elements
      for ( int i = 0; i < data.length - 1; i++ )
      {
         smallest = i; // first index of remaining array
         //T c=data[]; 
         // loop to find index of smallest element
         for ( int index = i + 1; index < data.length; index++ )
            {
			T x = data[smallest];
			T y = data[index];
			if ( x.compareTo(y)>0 )  //data[smallest].compareTo(data[index])>0
               smallest = index;
			}

         swap( i, smallest, data); // swap smallest element into position
         printPass( i + 1, smallest, data ); // output pass of algorithm
      } // end outer for
   } // end method sort

   // helper method to swap values in two elements
   public void swap( int first, int second, T[] data )
   {
      T temporary = data[ first ]; // store first in temporary
      data[ first ] = data[ second ]; // replace first with second
      data[ second ] = temporary; // put temporary in second
   } // end method swap

   // print a pass of the algorithm
   public void printPass( int pass, int index, T[] data )
   {
      System.out.print( String.format( "after pass %2d: ", pass ) );

      // output elements till selected item
      for ( int i = 0; i < index; i++ )
         System.out.print( data[ i ] + "  " );

      System.out.print( data[ index ] + "* " ); // indicate swap

      // finish outputting array
      for ( int i = index + 1; i < data.length; i++ )
         System.out.print( data[ i ] + "  " );
     
      System.out.print( "\n               " ); // for alignment

      // indicate amount of array that is sorted
      for( int j = 0; j < pass; j++ )
         System.out.print( "--  " );
      System.out.println( "\n" ); // add endline
   } // end method printPass

   // method to output values in array
   public String toString()
   {
      StringBuilder temporary = new StringBuilder();

      // iterate through array
      for ( T element : data )
         temporary.append( element + "  " );

      temporary.append( "\n" ); // add endline character
      return temporary.toString();
   } // end method toString
} // end class SelectionSort  

  
public class SelectionSortTest
{
   public static void main( String[] args )
   {
      // create object to perform selection sort
      int sortArray = new Array[ 10, 5, 78, 8, 65, 3, 38, 9009, 76, 56 ];
      float sortArray1 = new Array[ 10.7, 7.9,88.09,34.4, 67.8,12.6, 70.23, 45.6, 8.7, 8.09];

      SelectionSort sortArrayc = new SelectionSort( sortArray );
      SelectionSort sortArray1c = new SelectionSort( sortArray1 );
	  
	  
	  
      System.out.println( "Unsorted array:" );
      System.out.println( sortArray ); // print unsorted array
      System.out.println( "Unsorted array float:" );
      System.out.println( sortArray1 ); // print unsorted array
	  
      sortArrayc.sort(); // sort array
      sortArray1c.sort(); // sort array
	  
      System.out.println( "Sorted array:" );
      System.out.println( sortArray ); // print sorted array	  
      System.out.println( "Sorted array float:" );
      System.out.println( sortArray1 ); // print sorted array
   } // end main
} // end class SelectionSortTest  

[Tom Reilly]

I changed your code to compile. I did not test it. I also don't know whether it will do what you intended it to do. In answer to your original queston, you were defining T at the class level and T at the method level so you had two Ts defined. In your original code, try changing the method T to, for example S and and the compiler error will not be so confusing. BTW, You cannot use primitive types with generics.

public class SelectionSort<T extends Comparable<T>>
{
   private T[] data; // array of values
   
   public SelectionSort( T[] data1 )
   {
        data = data1;
   }

   // sort array using selection sort
   public  void sort()
   {
      int smallest; // index of smallest element
      //data[]=data1; 
      // loop over data.length - 1 elements
      for ( int i = 0; i < data.length - 1; i++ )
      {
         smallest = i; // first index of remaining array
         //T c=data[]; 
         // loop to find index of smallest element
         for ( int index = i + 1; index < data.length; index++ )
            {
			T x = data[smallest];
			T y = data[index];
			if ( x.compareTo(y)>0 )  //data[smallest].compareTo(data[index])>0
               smallest = index;
			}

         swap( i, smallest, data); // swap smallest element into position
         printPass( i + 1, smallest, data ); // output pass of algorithm
      } // end outer for
   } // end method sort

   // helper method to swap values in two elements
   public void swap( int first, int second, T[] data )
   {
      T temporary = data[ first ]; // store first in temporary
      data[ first ] = data[ second ]; // replace first with second
      data[ second ] = temporary; // put temporary in second
   } // end method swap

   // print a pass of the algorithm
   public void printPass( int pass, int index, T[] data )
   {
      System.out.print( String.format( "after pass %2d: ", pass ) );

      // output elements till selected item
      for ( int i = 0; i < index; i++ )
         System.out.print( data[ i ] + "  " );

      System.out.print( data[ index ] + "* " ); // indicate swap

      // finish outputting array
      for ( int i = index + 1; i < data.length; i++ )
         System.out.print( data[ i ] + "  " );
     
      System.out.print( "\n               " ); // for alignment

      // indicate amount of array that is sorted
      for( int j = 0; j < pass; j++ )
         System.out.print( "--  " );
      System.out.println( "\n" ); // add endline
   } // end method printPass

   // method to output values in array
   public String toString()
   {
      StringBuilder temporary = new StringBuilder();

      // iterate through array
      for ( T element : data )
         temporary.append( element + "  " );

      temporary.append( "\n" ); // add endline character
      return temporary.toString();
   } // end method toString

} // end class SelectionSort  

public class SelectionSortTest
{
   public static void main( String[] args )
   {
      // create object to perform selection sort
      Integer[] sortArray = { 10, 5, 78, 8, 65, 3, 38, 9009, 76, 56 };
      Float[] sortArray1 = { 10.7f, 7.9f, 88.09f ,34.4f, 67.8f, 12.6f, 70.23f, 45.6f, 8.7f, 8.09f};

      SelectionSort<Integer> sortArrayc = new SelectionSort<Integer>( sortArray );
      SelectionSort<Float> sortArray1c = new SelectionSort<Float>( sortArray1 );
	  
      System.out.println( "Unsorted array:" );
      System.out.println( sortArray ); // print unsorted array
      System.out.println( "Unsorted array float:" );
      System.out.println( sortArray1 ); // print unsorted array
	  
      sortArrayc.sort(); // sort array
      sortArray1c.sort(); // sort array
	  
      System.out.println( "Sorted array:" );
      System.out.println( sortArray ); // print sorted array	  
      System.out.println( "Sorted array float:" );
      System.out.println( sortArray1 ); // print sorted array
   } // end main
} // end class SelectionSortTest  

[Leonidas Savvides]

exe instr: given an example in another chapter in selection sort[Searching/Sorting chapter], problem was use method sort with <T extends Comparable<T>>, and NOT class with this, to sort 10 numbers (in the original problem numbers in array random generated), well?

you say "try changing the method T to, for example S and and the compiler error will not be so confusing" how then bind T to S?
may wrtite this without in class the T but only in method: public < T extends Comparable< T >> void sort(T[] data) ???

BTW your code compiles and run correct.

To appear the contents of an obj array like Integer or Float I must use absolutelly: ENHANCED FOR LOOP correct?

[Leonidas Savvides]

My code It compile/run success with below (is any error)?
can you comment on it (see and prior post)...

  public class SelectionSort3
{
   //private T[] data; // array of values
 /*  
   public SelectionSort3( T[] data1 )
   {
        data = data1;
   }
*/
   // sort array using selection sort
   public static < T extends Comparable< T >> void sort(T[] data)
   {
      int smallest; // index of smallest element
      //data[]=data1; 
      // loop over data.length - 1 elements
      for ( int i = 0; i < data.length - 1; i++ )
      {
         smallest = i; // first index of remaining array
         //T c=data[]; 
         // loop to find index of smallest element
         for ( int index = i + 1; index < data.length; index++ )
            {
			T x = data[smallest];
			T y = data[index];
			if ( x.compareTo(y)>0 )  //data[smallest].compareTo(data[index])>0
               smallest = index;
			}

         swap( i, smallest, data); // swap smallest element into position
         printPass( i + 1, smallest, data ); // output pass of algorithm
      } // end outer for
   } // end method sort

   // helper method to swap values in two elements
   public static < T > void swap( int first, int second, T[] data )
   {
      T temporary = data[ first ]; // store first in temporary
      data[ first ] = data[ second ]; // replace first with second
      data[ second ] = temporary; // put temporary in second
   } // end method swap

   // print a pass of the algorithm
   public static < T > void printPass( int pass, int index, T[] data )
   {
      System.out.print( String.format( "after pass %2d: ", pass ) );

      // output elements till selected item
      for ( int i = 0; i < index; i++ )
         System.out.print( data[ i ] + "  " );

      System.out.print( data[ index ] + "* " ); // indicate swap

      // finish outputting array
      for ( int i = index + 1; i < data.length; i++ )
         System.out.print( data[ i ] + "  " );
     
      System.out.print( "\n               " ); // for alignment

      // indicate amount of array that is sorted
      for( int j = 0; j < pass; j++ )
         System.out.print( "--  " );
      System.out.println( "\n" ); // add endline
   } // end method printPass

   // method to output values in array
   public static < T > String toString(T[] data)
   {
      StringBuilder temporary = new StringBuilder();

      // iterate through array
      for ( T element : data )
         temporary.append( element + "  " );

      temporary.append( "\n" ); // add endline character
      return temporary.toString();
   } // end method toString

} // end class SelectionSort    

  public class SelectionSortTest3
{
   public static void main( String[] args )
   {
      // create object to perform selection sort
      Integer[] sortArray = { 10, 5, 78, 8, 65, 3, 38, 9009, 76, 56 };
      Float[] sortArray1 = { 10.7f, 7.9f, 88.09f ,34.4f, 67.8f, 12.6f, 70.23f, 45.6f, 8.7f, 8.09f};

	  
      System.out.println( "Unsorted array:" );
      System.out.println( sortArray ); // print unsorted array
      System.out.println( "Unsorted array float:" );
      System.out.println( sortArray1 ); // print unsorted array
	  
      SelectionSort3.sort(sortArray); // sort array
      SelectionSort3.sort(sortArray1); // sort array
	  
      System.out.println( "Sorted array:" );
      System.out.println( sortArray ); // print sorted array	  
      System.out.println( "Sorted array float:" );
      System.out.println( sortArray1 ); // print sorted array
   } // end main
} // end class SelectionSortTest    

[Tom Reilly]

Leonidas Savvides wrote:you say "try changing the method T to, for example S and and the compiler error will not be so confusing" how then bind T to S?

You cannot. T and S are different class types. I only suggested that to clear up confusion about the compiler error - not to remove the compiler error.

Leonidas Savvides wrote:To appear the contents of an obj array like Integer or Float I must use absolutelly: ENHANCED FOR LOOP correct?

No. See this recent post: https://coderanch.com/t/500383/java/java/operator-works-Integer-class

[Leonidas Savvides]

well since My code It compile/run success with below (is any error/improvement)?
can you comment on it (see code in prior post)...

[Tom Reilly]

Leonidas Savvides wrote:well since My code It compile/run success with below (is any error/improvement)?
can you comment on it (see code in prior post)...

I think they just wanted to show that you have choices in code design.