Java data type conversion

Java auto boxing is a nice feature but it can sometimes cause a lot of problems, especially when going from Object to primitive. A NullPointerException can be very hard to spot, because it can be throw in strange places where you normally would not expect it. See this for some auto boxing errors.
This blog will list some nice data type conversion between different data type and from primitive and Object type and vise versa.

Integer/int conversion
See examples here. 

Data type from, to	Use	Comment
int to Integer	int i = 10;  Integer intObj = Integer.valueOf(i);
int to Long	int i = 10; Long longObj= Long.valueOf(i);
int to long	int i = 10; long l = (long) i;
int to String	int i = 10; String s = Integer.toString(i);
Integer to int	Integer intObj = new Integer(10); int i = intObj.intValue();	NumberFormatException will be thrown if the intObj is a null Integer.
Integer to String	Integer intObj = new Integer(10); String s = intObj.toString();	NumberFormatException will be thrown if the intObj is a null Integer.
Integer to long	Integer intObj = new Integer(10); long l = intObj.longValue();	NumberFormatException will be thrown if the intObj is a null Integer.
Integer to Long	Integer intObj = new Integer(10); Long longObj = Long.valueOf(intObj.longValue());	NumberFormatException will be thrown if the intObj is a null Integer.

Long/long conversion
Careful when converting from Long/long to Integer/int. No exception is thrown if the Long value is bigger than Integer.MAX_VALUE or smaller than Integer.MIN_VALUE. It will simple do an int overflow and the output will be unexpected.
See examples here. 

long to Long	long l = 10L; Long longObj = Long.valueOf(l);
long to Integer	long l = 10L; Integer intObj = Integer.valueOf(Long.valueOf(l).intValue());	No really good way of doing this. Overflow may occur.
long to String	long l = 10L; String s = Long.toString(l);
long to int	long l = 10L; int i = (int) l;	Use simple cast. Overflow may occur.
Long to long	Long longObj = new Long(10L); long l = longObj.longValue();	NumberFormatException will be thrown if the longObj is a null Long.
Long to String	Long longObj = new Long(10L); String s = longObj.toString();
Long to Integer	Long longObj = new Long(10L); Integer intObj = Integer.valueOf(longObj.intValue());	No really good way of doing this. Overflow may occur.
Long to int	Long longObj = new Long(10L); int i = longObj.intValue();	Overflow may occur.