parseXxx() and valueOf()

[Hama Kamal]

hello guys;

page 241 from the K&B book, it states:
The difference
between the two methods is
■ parseXxx() returns the named primitive. >>> it also returns object of the type that invoked
■ valueOf() returns a newly created wrapped object of the type that invoked >>> it also returns the named primitivethe method.
Here are some examples of these methods in action:

double d4 = Double.parseDouble("3.14"); // convert a String
// to a primitive
System.out.println("d4 = " + d4); // result is d4 = 3.14
Double d5 = Double.valueOf("3.14"); // create a Double obj
System.out.println(d5 instanceof Double); // result is "true"

the following is alos true

Double d4 = Double.parseDouble("3.14"); // convert a String to an Object
System.out.println("d4 = " + d4); // result is d4 = 3.14
double d5 = Double.valueOf("3.14"); // create a Double primitive

your comments please !


and one more thing

why this one runs well:

Float f = Float.valueOf("5.2f");

but this gives NFE at runtime?!

Long l = Long.valueOf("4l");

[Henry Wong]

Hama Kamal wrote:
page 241 from the K&B book, it states:
The difference
between the two methods is
■ parseXxx() returns the named primitive. >>> it also returns object of the type that invoked
■ valueOf() returns a newly created wrapped object of the type that invoked >>> it also returns the named primitivethe method.
Here are some examples of these methods in action:

double d4 = Double.parseDouble("3.14"); // convert a String
// to a primitive
System.out.println("d4 = " + d4); // result is d4 = 3.14
Double d5 = Double.valueOf("3.14"); // create a Double obj
System.out.println(d5 instanceof Double); // result is "true"

It doesn't also returns object of the type or vice versa -- as you can't define a method to return both an Object and a primitive value.

Hama Kamal wrote:
the following is alos true

Double d4 = Double.parseDouble("3.14"); // convert a String to an Object
System.out.println("d4 = " + d4); // result is d4 = 3.14
double d5 = Double.valueOf("3.14"); // create a Double primitive

your comments please !

While those statements do compile, it doesn't mean that both primitives and objects are returned. What is happening is that autoboxing is taking place. The compiler will generate code to convert primitives with its wrapper type. Specifically, it does this to the code -- which you don't see.

Double d4 = Double.valueOf(Double.parseDouble("3.14")); // convert a String to an Object
System.out.println(new StringBuilder("d4 = ").append(d4).toString()); // result is d4 = 3.14
double d5 = Double.valueOf("3.14").doubleValue(); // create a Double primitive

Henry

[Hama Kamal]

Henry Wong wrote:

Hama Kamal wrote:
page 241 from the K&B book, it states:
The difference
between the two methods is
■ parseXxx() returns the named primitive. >>> it also returns object of the type that invoked
■ valueOf() returns a newly created wrapped object of the type that invoked >>> it also returns the named primitivethe method.
Here are some examples of these methods in action:

double d4 = Double.parseDouble("3.14"); // convert a String
// to a primitive
System.out.println("d4 = " + d4); // result is d4 = 3.14
Double d5 = Double.valueOf("3.14"); // create a Double obj
System.out.println(d5 instanceof Double); // result is "true"

It doesn't also returns object of the type or vice versa -- as you can't define a method to return both an Object and a primitive value.

Hama Kamal wrote:
the following is alos true

Double d4 = Double.parseDouble("3.14"); // convert a String to an Object
System.out.println("d4 = " + d4); // result is d4 = 3.14
double d5 = Double.valueOf("3.14"); // create a Double primitive

your comments please !

While those statements do compile, it doesn't mean that both primitives and objects are returned. What is happening is that autoboxing is taking place. The compiler will generate code to convert primitives with its wrapper type. Specifically, it does this to the code -- which you don't see.

Double d4 = Double.valueOf(Double.parseDouble("3.14")); // convert a String to an Object
System.out.println(new StringBuilder("d4 = ").append(d4).toString()); // result is d4 = 3.14
double d5 = Double.valueOf("3.14").doubleValue(); // create a Double primitive

Henry

Hello Mr.Henry

thanks for your explanation,, what about the issue below

why the below statement runs well

Float f = Float.valueOf("5.2f");

but this one give NFE at runtime

Long l = Long.valueOf("4L");

[Henry Wong]

Hama Kamal wrote:
Hello Mr.Henry

thanks for your explanation,, what about the issue below

why the below statement runs well

Float f = Float.valueOf("5.2f");

but this one give NFE at runtime

Long l = Long.valueOf("4L");

Take a look at the javadoc for java.lang.Float and java.lang.Long, it will become clear why one works and the other doesn't. As for why the designers implemented the two parser inconsistently, that I do not know.

Henry