Answer:

String singleA = "A" ;
String singleB = new String( "B" );

System.out.println( singleA.length() );
System.out.println( singleA.concat( singleB ) );

outputs:

1
AB

The following

char oneC = 'C' ;
char oneD = new char( 'B' );

System.out.println( oneC.length() );
System.out.println( oneD.concat( oneC ) );

will not even compile. Line two will not compile because a char is not an object, and so has no constructor. The remaining lines will not compile because a char primitive has no methods.

Column Checker

Here is a program that checks that every line of a text file has a space character in column 10. This might be used to verify the correct formatting of columnar data or of assembly language source programs.

import java.util.Scanner;
import java.io.*;

class ColumnCheck
{
  public static void main (String[] arg)
  {
    final int colNum = 10;
    int counter = 0;
    String line = null;
    Scanner scan = new Scanner( System.in );
   
    while ( scan.hasNext() )
    {    
      line = scan.nextLine() ;
      counter = counter +1;
      if ( line.length() > colNum && line.charAt( colNum ) != ' ' )
        System.out.println( counter + ":/t" + line );
    }    
  }
}

The program is used with redirection (see Chapter 21):

C:/>javac ColumnCheck.java

C:/>java ColumnCheck < datafile.txt

A better program would ask the user for several column numbers to check.

QUESTION 17:

Will the program crash if a line has fewer than 10 characters? Inspect the statement

      if ( line.length() > colNum && line.charAt( colNum ) != ' ' )
        System.out.println( counter + ":/t" + line );

How is the short-circuit nature of && used here?