I come from Python background and taking a dive into Java world.
I am trying to convert Float to int in Java. Something we do like this in Python int_var = int(float_var)
public class p1 {
public static void main(String args[]) {
Integer a = new Integer(5);
Float b;
b = new Float(3.14);
a = (int)b;
System.out.println(a);
}
}
Yields the following error -
p1.java:7: error: inconvertible types
a = (int)b;
^
required: int
found: Float
1 error
You can do it like
a = b.intValue()
That's one of the annoying things in Java. Fix your problem with this:
a = (int)(float)b;
Unboxing will require that you cast from Float to float and then to int
Use primitives types, and you will be fine:
int a = 5;
float b = 3.14f; // 3.14 is by default double. 3.14f is float.
a = (int)b;
The reason it didn't work with Wrappers is those types are non-covariant, and thus are incompatible.
And if you are using Integer types (Which you really don't need here), then you should not create Wrapper type objects using new. Just make use of auto-boxing feature:
Integer a = 5; // instead of `new Integer(5);`
the above assignment works after Java 1.5, performing auto-boxing from int primitive to Integer wrapper. It also enables JVM to use cached Integer literals if available, thus preventing creation of unnecessary objects.
Since you're using boxed primitive, a = b.intValue(); should suit your needs.
float a =10.0f;
int k=(int)a;
Use this code
a = b.intValue();
Use Math.round method:-
int a = Math.round(b);
for the best always use:
Math.floor(d)
Math.round(d)
Math.abs(d)
These are meant for conversions.
You can simply use the Use Math.round() as,
int a = Math.round(14.32f)
Related
Can someone please explain how/why is this allowed in Java?
public class Test {
private int text;
public Integer getText() {
return text;
}
I am basically having the wrapper class as the return type, while I am infact returning a primitive.
Because Java supports Autoboxing and Unboxing in versions 5 and up. That is an example of the former, but the later is equally important (and the reverse conversion). Per the link,
Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes.
Consider the following code: (From: http://docs.oracle.com/javase/tutorial/java/data/autoboxing.html)
List<Integer> li = new ArrayList<>();
for (int i = 1; i < 50; i += 2)
li.add(i);
Although you add the int values as primitive types, rather than Integer objects, to li, the code compiles. Because li is a list of Integer objects, not a list of int values, you may wonder why the Java compiler does not issue a compile-time error. The compiler does not generate an error because it creates an Integer object from i and adds the object to li. Thus, the compiler converts the previous code to the following at runtime:
List<Integer> li = new ArrayList<>();
for (int i = 1; i < 50; i += 2)
li.add(Integer.valueOf(i));
Converting a primitive value (an int, for example) into an object of the corresponding wrapper class (Integer) is called autoboxing. The Java compiler applies autoboxing when a primitive value is:
Passed as a parameter to a method that expects an object of the corresponding wrapper class.
Assigned to a variable of the corresponding wrapper class.
From javadocs : Since java 5 Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes. For example, converting an int to an Integer, a double to a Double, and so on.
The java compiler applies autoboxing usually when -
Passed as a parameter to a method that expects an object of the
corresponding wrapper class.
Assigned to a variable of the corresponding wrapper class.
For the sake of performance, not everything in Java is an object. There are also primitives, such as int, long, float, double, etc.
For example java.lang.Integer class :-
It wraps an int.
Has two static final fields of type int: MIN_VALUE and MAX_VALUE.
MIN_VALUE contains the minimum possible value for an int (-2^31) and MAX_VALUE the maximum possible value for an int (2^31 - 1).
It also has two constructors - public Integer (int value) & public Integer (String value).
Integer has the no-arg byteValue, doubleValue, floatValue, intValue, longValue, and shortValue methods that convert the wrapped value to a byte, double, float, int, long, and short, respectively.
In addition, the toString method converts the value to a String.
Static methods parse a String to an int (parseInt) and convert an int to a String (toString).
class AutoBox {
public static void main(String args[]) {
// autobox an int
Integer a = 100;
// auto-unbox
int b = a;
System.out.println(b + " " + a); // displays 100 100
}
}
This feature has been added in Java 5.
text gets converted automatically into Integer by compiler. So basically its a syntactic sugar that can shorten your code (otherwise you would do conversions back and forth by yourself). Of course it has its price and if this happens a lot (I mean really a lot, big loops, frequent invocations and so forth), it can become a performance issue, so when using it, just remember the it happens under the hood and you'll be fine.
Integer.valueOf(text)
is called under the hood
The feature is called autoboxing btw
Hope this helps
Can anyone tell why the auto boxing is not working, and why with constructor it works fine:
int intValue = 12;
Double FirstDoubleValue = new Double(intValue);
Double SecondDoubleValue = intValue; // ==> Error
Thanks for advance :)
The constructor expects a double, a primitive type, in which case, through widening primitive conversion, an int can be used.
However, in
Double SecondDoubleValue = intValue; // ==> Error
you're trying to assign an int to a Double. Those are incompatible types.
Note that boxing conversion
converts expressions of primitive type to
corresponding expressions of reference type
so an int would become an Integer, but Integer is still not compatible with Double.
Try
Double SecondDoubleValue = (double)intValue;
Java cannot cast int to Double. It can cast int to double, which is what is happening on your second line.
Look here for some in depth answers about conversions.
http://docs.oracle.com/javase/specs/jls/se7/html/jls-5.html
Look at this... this evening I was trying to cast some primiteves to wrappers when I found that:
Integer i = (Integer)4;
Integer i = (Integer)4f; // Doesn't compile!
But if I rewrite the second line:
Integer i2 = (int)4f; // Then it compiles
What's the difference? Why should I be able to cast with a wrapper class in the first case, but not in the second?
here
Integer i = (Integer)4f;
youre casting a float primitive to an Integer wrapper - they dont fit. this has nothing to do with autoboxing either. even with explicit boxing, this still wont compile:
Integer i = (Integer)(Double.valueOf(4f));
while here:
Integer i2 = (int)4f;
you casting a float primitive to an int primitive (truncating in the process) and then java auto-boxes that into an Integer for you
class Test{
public static void main(String[] args){
int a = 1;
int b = 5;
Integer c = new Integer(1);
Integer d = 5; //autoboxing at work
System.out.println(c.compareTo(d));
System.out.println(a.compareTo(b));
}
}
Why doesn't a.compareTo(b) compile (int cannot be dereferenced)? I know that compareTo requires objects, but I thought autoboxing would automatically make an int an Integer when necessary. Why doesn't autoboxing occur in this case? And what other cases will it not occur?
From the Oracle tutorial on Autoboxing, the two cases where boxing will occur are, when primitives are:
Passed as a parameter to a method that expects an object of the corresponding wrapper class.
Assigned to a variable of the corresponding wrapper class.
The expression being evaluated in your example (a.compareTo(d)) does not fall in any one of those scenarios.
Theres some interesting additional information in the JCP proposal for autoboxing, describing the mechanics and rules for assignment conversion, method invocation conversion, and casting conversion.
What is the difference between these two lines?
int pInt = 500;
and
Integer wInt = new Integer(pInt);
Or
Integer wInt = new Integer(500);
None.
That's the exact same thing. In the first case you just have a supplementary variable.
Note that with autoboxing you rarely need to have both an int and an Integer variables. So for most cases this would be enough :
int pInt = 500;
The main case where the Integer would be useful is to distinguish the case where the variable is not known (ie null) :
Integer i = null; // possible
int i = null; // not possible because only Object variables can be null
But don't keep two variables, one is enough.
In Java, an instance of a primitve class holds the actual value of the instance, but instance of a wrapper class holds a reference to the object. i.e. The address of the place where the object would be found.
When you write a program with this line:
Integer integer = 500;
The compiler changes it to this:
Integer integer = new Integer(500);
This process is called autoboxing. That is automatically putting a primitive-instance in a "box" of Integer. Hence, output of the following program:
public class PrimitiveToObject {
public static void main(String[] args) {
printClassName(1);
printClassName(1L);
printClassName((char)1);
}
public static void printClassName(Object object){
System.out.println(object.getClass());
}
}
is this:
class java.lang.Integer
class java.lang.Long
class java.lang.Character
Also this:
int i = integer;
changes into this:
int i = integer.intValue();
This is called unboxing.
As you can see above, the dot operator(.) is used on the variable named integer but not on i. That is: a wrapper's object can be dereferenced, but not a primitive instance.
Boxing and unboxing may slow down the program a little bit. Hence, to a newbie, wrappers may look like added burden, but they are not so. Wrappers are used at places where the object needs to be a reference type. eg: Map<Integer,String>map=new HashMap<Integer,String>(); is a valid statement, but Map<int,String>map=new HashMap<int,String>(); is not a valid statement.
Another typical case where wrapper is very useful: In MySQL, NULL is a valid entry for a column of INT type. But in Java, int cannot have a null value, Integer can. This is because in SQL NULL symbolises Not Available. So if you are using JDBC to insert integer values in a MySQL table, a null in your java program will help in inserting NULL in the MySQL table.
A wrapper class can also be useful in a case similar or anologous to this:
Boolean decision; // Using wrapper for boolean.
if("YES".equalsIgnoreCase(consent))
decision = Boolean.TRUE; // In favour
else if("NO".equalsIgnoreCase(consent))
decision = Boolean.FALSE; // Not in favour
else if("CAN'T SAY".equalsIgnoreCase(consent))
decision = null; // Undecided
For starters
int pInt = 500; , here pInt is not an object whereas in
Integer wInt = new Integer(500);
wInt is an reference
This is also a reason why java is not pure Object Oriented Language. Because everything is not object with java.
Wrapper class will have a box in that box it will cover the primitive data types there are 8 primitive data types namely byte,int ,long ,double, float, short ,Boolean ,char these all covered in wrapper class .
to use primitive data types we use like int a;
but to use wrapper class we need to use like Integer a = new Integer(i);
The types of data are the same, but there are situations that manipulation of objects is more convenient than primitive types, like data structures, where you need more control of your data types.
For example, objects can be null and primitive types can't.
You also can't call methods in a primitive type (.compareTo(), .equals(), ...), but in wrapper classes you can.
The information below describe the types in primitive and wrapper class:
Primitive Types | Wrapper Class (Superclass = Object)
boolean - Boolean
char - Character
Primitive Types | Wrapper Class (Superclass = Number)
byte - Byte
short - Short
int - Integer
long - Long
float - Float
double - Double
To understand how the wapper classes works, consider the example below:
public final class IntWrapper {
private final int intVal;
IntWrapper(int intVal) {
this.intVal = intVal;
}
public int getInt() {
return intVal;
}
}
Now we can make an object out of our new IntWrapper class and 'box' the primitive int value 41:
int i = 41;
IntWrapper iw = new IntWrapper( i ); // box the primitive int type value into the object
i = iw.getInt(); // unbox the value from the wrapper object
My example IntWrapper class is immutable, immutable means that once its state has been initialized its state cannot be changed.
When the final keyword is applied to a class, the final class cannot be extended. In other words, a final class can never be the superclass of a subclass. A final class can be the subclass of superclass, not problem there. When a class is marked final, all of its methods are implicitly final as well.
It is important to note that when final is applied to a reference variable it does not prevent the members of the object instance from changing values.
This example is to better understanding how the wrapper classes works inside.
Next, to create Integer, Double and other wrapper classes, you can write:
Integer i = new Integer(4);
Double d = new Double(9.62);
Boolean b = new Boolean("true");
Character c = new Character('M');
To get the encapsulated number into wrapper objects, you can write:
long l = i.longValue();
double e = i.doubleValue();
float f = d.floatValue();
short s = d.shortValue();
Each wrapper class include special methods to convert between primitive type to wrapper objects, that represent not number values:
boolean bo = b.booleanValue();
char ch = c.charValue();
Up to Java 5 version, the creation of objects from wrapper classes had to be in the syntaxes like above, but to simplify these operations, mainly related to insertion of values in the data structures offered in Java collections (that only accept objects), now exists the autoboxing or boxing and autounboxing or unboxing options.
The autoboxing or boxing allows you to insert a primitive value to reference of equivalent wrapper types or Object type:
// Same result of Double d = new Double(-2.75);
Double objD = -2.75;
// Same result of Object objI = new Integer(13);
Object objI = 13;
The autounboxing or unboxing allows you to insert a wrapper object into a variable of primitive type, converting automatically between equivalent types:
// Same result of double vd = objD.doubleValue();
double vd = objD;
// Same result of int vi = objI.intValue();
int vi = objI;
The most important practical difference I've seen is Integer is way more slower to initialize and do calculations with, than int. I would avoid Integer unless necessary.
int x = 20_000_000;// 20 millions
for (int i = 0; i < x; i++) {
ix += 23;
}
it takes 138 ms(average over 50 trials) to complete the loop when ix is an Integer but only takes 10 ms when ix is an int