Convert from type X to type Y

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 bin = Integer.toBinaryString(i);

decimal to hexadecimal :

int i = 42;
   String hexstr = Integer.toString(i, 16);

   or
   String hexstr = Integer.toHexString(i);

   or (with leading zeroes and uppercase)
   public class Hex {
     public static void main(String args[]){
       int i = 42;
       System.out.print
         (Integer.toHexString( 0x10000 | i).substring(1).toUpperCase());
     }
   }

Byte array to hexadecimal string:
See this How-to hexadecimal (String) to integer :

int i = Integer.valueOf("B8DA3", 16).intValue();
   or
int i = Integer.parseInt("B8DA3", 16);     

char to String

String s = String.valueOf('c');

integer to ASCII code (byte)

char c = 'A';
int i  = (int) c; // i ==  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 j = (int) c;
   System.out.println(j);
}

integer to boolean

b = (i != 0);
// ex : 42 != 0 --> true

boolean to integer

i = (b)?1:0;
// true --> 1

Note :To catch illegal number conversion, use the try/catch mechanism :

try{
  i = Integer.parseInt(aString);
}
catch(NumberFormatException e) {
  ...
}