integer to String : 
    int i = 42;
    String str = Integer.toString(i);
or 
    String str = "" + i
double to String :
    String str = Double.toString(i);
long to String :
    String str = Long.toString(l);
float to String :
    String str = Float.toString(f);
String to integer :
    str = "25";
    int i = Integer.valueOf(str).intValue();
or 
    int i = Integer.parseInt(str);
String to double :
    double d = Double.valueOf(str).doubleValue();
String to long :
    long l = Long.valueOf(str).longValue();
or 
    long l = Long.parseLong(str);
String to float :
    float f = Float.valueOf(str).floatValue();
decimal to binary :
    int i = 42;
    String binstr = Integer.toBinaryString(i);
decimal to hexadecimal :
    int i = 42;
    String hexstr = Integer.toString(i, 16);
or 
    String hexstr = Integer.toHexString(i);
hexadecimal (String) to integer :
    int i = Integer.valueOf("B8DA3", 16).intValue();
or 
    int i = Integer.parseInt("B8DA3", 16);     
ASCII code to String :
    int i = 64;
    String aChar = new Character((char)i).toString();
integer to ASCII code (byte) :
    char c = 'A';
    int i = (int) c; // i will have the value 65 decimal
To extract Ascii codes from a String :
      String test = "ABCD";
      for ( int i = 0; i < test.length(); ++i ) {
        char c = test.charAt( i );
        int i = (int) c;
        System.out.println(i);
        }
integer to boolean :
    b = (i != 0);
boolean to integer :
    i = (b)?1:0;
note :To catch illegal number conversion, try using the try/catch mechanism. 
 try{
   i = Integer.parseInt(aString);
   }
 catch(NumberFormatException e)
   {
   }