Common Conversions in Java

A page of sample conversions is always a useful thing especially when you're arriving from a different language. The examples are encumbered a bit with some exception handling, but that's important in the real world too.

(The output from the test is below.)

package com.javahotchocolate.conversions;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

public class Conversions
{
  public static void main( String[] args )
  {
    try
    {
      // convert a string to integer...
      Integer x = Integer.parseInt( "46" );
      int     y = Integer.parseInt( "1573" );
      Integer z = Integer.parseInt( "10010011", 2 );
      //Long    bad = Long.parseLong( "This will throw an exception." );

      System.out.println( "String to number: x = " + x + ", y = " + y + ", z = " + z );
    }
    catch( NumberFormatException e )
    {
      System.out.println( "String was not a number!" );
    }

    // convert a number to a string...
    String s = "" + 1573;
    String t = "" + 10.2;
    String u = Integer.toString( 99 );
    System.out.println( "Number to string: s = " + s + ", t = " + t + ", u = " + u );

    // converting a string date to Date...
    DateFormat df = new SimpleDateFormat("dd/MM/yyyy");

    try
    {
      Date today = df.parse( "5/12/2010" );
      //Date tomorrow = df.parse( "This will throw an exception." );
      System.out.println( "The specified date is " + df.format( today ) );
    }
    catch( ParseException e )
    {
      System.out.println( "String date was not in correct dd/MM/yyyy format!" );
    }

    // convert from Unicode to UTF-8 and back...
    try
    {
      String unicode = "abc\u5639\u563b";
      byte[] utf8    = unicode.getBytes( "UTF-8" );

      String  U = new String( utf8, "UTF-8" );
      System.out.println( "Unicode string: " + U );
    }
    catch( UnsupportedEncodingException e )
    {
      System.out.println( "Unicode string was bad!" );
    }
  }
}

Output from the test above...

String to number: x = 46, y = 1573, z = 147
Number to string: s = 1573, t = 10.2, u = 99
The specified date is 05/12/2010
Unicode string: abc??

More conversions...

This is nonsense, but it's occasionally necessary to understand how to move from a complex Java type like ArrayList to an array of objects (or ints).

import java.util.ArrayList;

public class ArrayListToArray
{
  public static void main( String[] args )
  {
    int sum = 0;

    // create an array list...
    ArrayList al = new ArrayList();

    // add elements...
    al.add( new Integer( 1 ) );
    al.add( new Integer( 2 ) );
    al.add( new Integer( 3 ) );
    al.add( new Integer( 4 ) );
    al.add( new Integer( 5 ) );

    System.out.println( "contents of al : " + al );

    Object oa[] = al.toArray();

    // add up the array elements...
    for( int i = 0; i < oa.length; i++ )
      sum += ( ( Integer ) oa[ i ] ).intValue();

    System.out.println( "Sum is :" + sum );
  }
}