强制转换为 String 和 String.valueOf 之间的区别

Casting means that the object needs to be of type String, while String.valueOf() can take other types as well.

Both generates same output in case of String.

Casting fails in case of provided object is Not a string.

Casting to string only works when the object actually is a string:

Object reallyAString = "foo";
String str = (String) reallyAString; // works.

It won't work when the object is something else:

Object notAString = new Integer(42);
String str = (String) notAString; // will throw a ClassCastException

String.valueOf() however will try to convert whatever you pass into it to a String. It handles both primitives (42) and objects (new Integer(42), using that object's toString()):

String str;
str = String.valueOf(new Integer(42)); // str will hold "42"
str = String.valueOf("foo"); // str will hold "foo"
Object nullValue = null;
str = String.valueOf(nullValue); // str will hold "null"

Note especially the last example: passing null to String.valueOf() will return the string "null".

The first one i.e, String.valueOf returns a string only if the object is a representable type which is a value type or a String.. Else it throws the exception.

In the latter one, you are directly casting which can fail if the object isn't a string.

Online example.

http://ideone.com/p7AGh5

String.valueOf(foo) invokes foo's .toString() method and assigns the result to the the bar. It is null and type safe operation.

Casting will just assign foo to the bar, if the types are matching. Otherwise, the expression will throw a ClassCastException.

String.valueOf method is used to get the String represenation of it's parameter object.

(String) value casts object value to string.

You can use the String.valueOf method to get the String representation of an object without worrying about null references. If you try to cast String on a null reference you would get a NullPointerException.

in String.valueOf(); string as work typecasting all the argument passed in valueof() method convert in String and just like integer.string() convert integer into string only

final Object obj = null;
final String strValOfObj = String.valueOf(obj);
final String strCastOfObj = (String) obj;
if (strValOfObj == null) System.out.println("strValOfObj is null");
if (strCastOfObj == null) System.out.println("strCastOfObj is null");

Output: strCastOfObj is null