I have a program that creates lists and needs any assigned values to be 0. Its been running fine when I do with int[] humpty_dumpty = new int[20]; but to optimize the size of the lists I set them to Short[] and now my program is breaking because it takes the zero's as inputs(and Short[] humpty_dumpty = new Short[20]; is making the default value null).
Is there a way to set it to default zero without having to iterate through the entire list(I can do this via a for loop but was wondering if there was a way to make its behavior similar to int)?
There is a difference between a Short[] and a short[]. Elements of the latter will be initialized to 0 because short is a "primitive" type and cannot be null. The capitalized Short class will be initialized to null because it is really just an Object wrapping a short value.
You may create an array of primitive type instead of wrapper,
short []ar=new short[20];
Short[] doesn't 'optimize the size of the lists' at all, and it has a default value of null. short[] does, and it has a default value of zero.
Answers, in order:
1) Requirement first, optimization last. Don't use sparse arrays, or try to be 'smart', unless you specifically need to do this & deal with the extra code/ overhead.
2) Use common methods (possibly in an Instance or Static Helper class) to avoid repeating common code.
eg. short sparseGet (Short[] array, int i) {return (array[i] != null ? array[i] : 0);}
3) Perhaps use short[] rather than Short[]? Uppercase types are not primitives, but Value Wrapper classes -- and stored as object references (pointers) to instances, thus slower & more memory-intensive.
4) Uppercase 'Value Wrapper' types are appropriate where you may have null values, from a database. eg. Person.Age would ideally be Integer, if there's any possibility you/ the database might not have data for that field.
Cheers.
Related
I have an array list of integers and would like to delete an Integer value from it. To clarify: Let's say I have a function that takes in an ArrayList of Integers, and specified Integer value. I need to return the list with (value+1) deleted from it.
boolean deleteFromList(ArrayList<Integer> list, Integer value)
Now, if I do this:
return list.remove(value+1)
compiler will complain because it will try to invoke the delete method that takes int parameter and deletes an object from specified location, not the actual object.
So what is a proper way to deal with this? Is it better to do:
list.remove((Integer)(value+1))
or
int v = value.intValue();
v++;
list.remove(new Integer(v));
? In the second case, can I be sure the right value will be deleted?
list.remove(value+1) will remove at given index.
You can use list.remove(Integer.valueOf(value+1)) to remove the Integer.
Integer.valueOf(v) is recommanded instead of new Integer(v) because it allows reuse of Integer instances for special values (see javadoc).
EDIT :
In term of boxing/unboxing, it is possible to completely eliminate the problem using libraries like trove4j which define an dynamic array of integer storing the primitive values instead of numeric class (see TIntArrayList). No boxing, no unboxing, lower GC use and better performance.
In the second case, can I be sure the right value will be deleted?
Yes you can.
The remove method will use equals(Object) to identify the object to be removed from the list. Since Integer.equals compares by value (not by object identity), it doesn't matter how you created the Integer instances ... provided the wrapped int values are the same.
However ... your second version is inferior to the first version:
It cumbersome: 3 statements instead of 1.
You are using new Integer(...) rather than Integer.valueOf(...) which is always going to create a new instance. By contrast, autoboxing (or calling Integer.valueOf(...) explicitly) makes use of the Integer classes instance cache, and that will often avoid creating a new instance.
list.remove(Integer.valueOf(value+1));
should work fine.
Do you looking for this one.
Collections.sort(list);
int index = list.indexOf(5);
list = list.subList(0, index-1);
System.out.println(list);
You can use list.indexOf(value + 1) since that method only takes the object. Then use that index to remove the element.
int i = list.indexOf(value + 1);
list.remove(i);
If you need to remove all instances of value + 1 then continue until i is -1 (not found).
Or, just iterate over it and remove as you find them
for (Iterator<Integer> iterator = list.iterator(); iterator.hasNext(); ) {
Integer integer = iterator.next();
if (integer == value + 1) {
iterator.remove();
}
}
A more effective way might be to use a TIntArrayList from Trove4j library which wraps int[] array and doesn't use wrapper types. For this reason it takes 3x times less memory and is much faster.
In java is it possible and if not how to calculate in c?
if it's really array (not a pointer), you can do sizeof(arr)/sizeof(*arr)
In Java, the length of a primitive array is array.length, while the length of an ArrayList (and most other collections) is arrayList.size()
In C, the length of an array is sizeof(array) / sizeof(array[0]), but this is nearly useless since you can't pass arrays as arguments (they degenerate to pointers). The normal way to find the size of an array in C is to pass it as an extra argument to the function, or sometimes to terminate it with a sentinel value (eg. strings are \0 terminated)
There is no way to calculate that. in C (not C++, which has std::array and std::vector) an array is transmitted as its pointer, which you might increase by some offset. So you really don't know the runtime size of an "array", except by some conventions.
In particular for formal arrays, there is no way to know the size of the actual array passed
e.g. as void f(int arr[]) { /*...*/ } unless you give a static dimension.
Likewise, with an external array declared as extern int xarr[]; you cannot get its dimension with sizeof(xarr)/sizeof(xarr[0]).
In C, there is no way of calculating the size of array if you have only a pointer to it. You must store it in separate variable.
In fact you HAVE TO keep the size of an array in separate variable because you have to allocate memory if you want to use dynamic-size array.
And if you want to use fixed-size array you know it's size by the time you're writing your code so why not use #define, variable or const to store it?
Java is totally different language than C and the philosophy of programming is different-you should always keep that in mind.
In Java, you should use array.length, look here for example: http://www.roseindia.net/help/java/a/java-array-length.shtml
Simply, there is no possibility if you recieve only a pointer. That's why main has an argc argument. It defines the number of entries in argv. If you have an array "datatype" (actually the same as a pointer, but the behaviour depends on the context), you can use
int[] arr = new int[10];
sizeof(arr)/sizeof(int) // or whatever type is contained in ``arr``
I'm doing a project in Java in which I'm looking at the frequency characters occur after each other character in a text. When I'm collecting my results, I'm storing them in a 2d array of ints. However, in the same results table, I want to store some results about the whole thing.
Is it possible to have an array where some of the elements are arrays, and others are primitives?
Yes, you can have Object[] and store a mix a types, although it would have to be the wrapper Integer (not the primitive int). (Note: you could "cheat" and save each int as a single-element array int[], thus making it an Object, but "don't try this at home")
But even if you could, an array isn't the right approach. Instead, create a class for this.
Try something like this:
public class FrequencyAnalysis {
private int[] frequencies;
private String info;
private Date lastRun;
// etc
}
No, it is not working. There can only be objects OR primitives in one array. Anything else is just a workaround.
In that case you would have to store Objects (primitives will autobox to Wrapper classes) and then when you read the entries you would need to instanceof the Object and then cast it safely.
You can use boxed types, i.e. java.lang.Integer (So your array type must be Object[]), but better do make a new class for results Storage
What is the reason why Wrapper classes (like Integer, Double, etc.) don't have a setter for their inner primitive value ?
I am asking this because that kind of functionality would have simplified calculus, and have made the Java language a little more flexible .
Let me give you some examples.
1) Let's take the following example:
Integer x = new Integer(5);
x++;
The previous code behind the scenes is performing autoboxing . Something like:
int x_tmp = x.intValue();
x_tmp++;
x = new Integer(x_tmp); // Yes that's a new memory allocation
Because of this problem doing calculus on Wrapper is slower than performing on plain primitive types. With a setter it would've been more easy to increment the inner value, without allocating another object on the heap.
2) Another issue that is bugging me is that is impossible in Java to write a swap function like I can do in C (using pointers) or in C++ (pointers or references).
If i write void swap(Integer x, Integer y) I cannot acces the inner value because, and It is going to be impossible for me to swap the values.
PS:
A friend of mine suggested that i should consider the bigger picture, and think in terms of concurrency and type immutability.
So do you have an explanation for this ?
Thanks!
Wrapper classes are usually not used unless you need to put them into a collection. If they were mutable it would make problems if used inside sets and as keys for hashtables.
Sets and hashtables need the hash value to be always the same.
1) With a setter, the wrapper types would be mutable. Immutability is a good thing in many ways... threading, general understandability of the code etc. Personally I think it's a shame that Calendar and Date are mutable, for example.
In fact, your expansion of x++; isn't quite right - it uses Integer.valueOf which doesn't always create a new value. For example:
Integer x = 5;
x++;
Integer y = 5;
y++;
// This prints true
System.out.println(x == y); // Compare references
Only a limited range of Integer values are cached like this (the spec defines what values must behave this way, but allows for a wider range if the JRE wishes to do so)... but it does mean that it won't always be creating a new object.
2) Yes, Java doesn't have pass by reference. Frankly I very rarely find that to be a problem. How often do you really need to swap the values of variables?
Caching Integers in the range from -128 to 127 requires immutable Integers. Consider the follwoing code:
Integer id = Integer.valueOf(1); // a new Integer, cached in Integer class
// and somewhere else
Integer key = Integer.valueOf(1); // returns the cached value
Now if Integer was mutable and had a setter and someone did
key.setValue(2); // not legal Java code, just for demonstration
this would change the value of id too and, to a lot of peoples surprise:
Integer one = Integer.valueOf(1);
if (one != 1)
System.out.println("Surprise! I know, you expected `1`, but ...");
In Java, Strings and wrapper classes are designed as immutable to avoid accidental changes to the data. You can check the below article for further information.
Why Strings and Wrapper classes are immutable in java?
In which case should you use primitive types(int) or reference types (Integer)?
This question sparked my curiosity.
In which case should you use primitive
types(int) or reference types
(Integer)?
As a rule of thumb, I will use a primitive (such as int) unless I have to use a class that wraps a primitive.
One of the cases were one must use a wrapper class such as Integer is in the case of using generics, as Java does not support the use of primitive types as type parameters:
List<int> intList = new ArrayList<int>(); // Not allowed.
List<Integer> integerList = new ArrayList<Integer>(); // Allowed.
And, in many cases, I will take advantage of autoboxing and unboxing, so I don't have to explicitly perform conversions from primitives to its wrapper class and vice versa:
// Autoboxing will turn "1", "2", "3" into Integers from ints.
List<Integer> numbers = Arrays.asList(1, 2, 3);
int sum = 0;
// Integers from the "numbers" List is unboxed into ints.
for (int number : numbers) {
sum += number;
}
Also, as an additional note, when converting from primitives to its wrapper class objects, and unique instances of objects are not necessary, use the valueOf method provided by the wrapper method, as it performs caching and return the same instance for a certain value, reducing the number of objects which are created:
Integer i1 = Integer.valueOf(1); // Prefer this.
Integer i2 = new Integer(1); // Avoid if not necessary.
For more information on the valueOf methods, the API specification for the Integer.valueOf method can serve as a reference for how those methods will behave in the wrapper classes for primitives.
That really depends on the context. First prefer the primitive, because it's more intuitive and has less overhead. If it is not possible for generics/autoboxing reasons, or if you want it to be nullable, then go for the wrapper type (complex type as you call it).
The general rules I follow when creating an API can be summarized as follows:
If the method must return an value, use a primitive type
If the method may not always apply (eg: getRadioId(...) on an object where such an ID may not exist), then return an Integer and specify in the JavaDocs that the method will return null in some cases.
On #2, look out for NPEs when autoboxing. If you have a method defined as:
public Integer getValue();
And then call it as follows:
int myValue = getValue();
In the case where getValue() returns null you'll get an NPE without an obvious cause.
Since Java does something called auto-boxing and auto-unboxing, you should use the primitive type int in most cases because of less overhead.
The only time you absolutely need to use Integer is in generics.
List<int> list; // won't compile
List<Integer> list; // correct
One case in which Integer might be prefered is when you are working with a database where numerical entries are allowed to be null, since you wouldn't be able to represent a null value with an int.
But of course if you're doing straight math, then int would be better as others have mentioned due to intuitiveness and less overhead.
My rule of thumb is: use boxed primitives only when it's necessary to get the code to compile. The only places in your code where the names of the primitive wrapper classes should appear is in generic type parameters and static method calls:
List<Integer> intList = new ArrayList<Integer>();
int n = Integer.parseInt("123");
That's the advice I would give to new Java programmers. As they learn more, they'll run into situations where they have to be more discerning, like when dealing with Maps or databases, but by then they should also have a better understanding of the difference between primitives and boxed primitives.
Autoboxing tempts us to believe int and Integer (for example) are interchangeable, but it's a trap. If you mix the two kinds of value indiscriminately, you can end up comparing two Integer values with == or trying to unbox a null without realizing it. The resulting bugs can be intermittent and difficult to track down.
It doesn't help that comparing boxed primitives with == sometimes works as if it were doing a value comparison. It's an illusion caused by the fact that values within a certain range are automatically cached in the process of autoboxing. It's the same problem we've always had with String values: comparing them with == sometimes "works" because you're actually comparing two references to the same, cached object.
When dealing with strings we can just tell the n00bs never to compare them with ==, as we've been doing all along. But comparing primitives with == is perfectly valid; the trick (thanks to autoboxing) is being sure the values really are primitives. The compiler will now let us declare a variable as an Integer and use it as if it were an int; that means we have to exercise a greater level of discipline and treat it as an error when someone does so without good reason.
Rather than calling them "complex types", you'd be best served thinking Integer, Double, etc. as "Classes", and int, double, etc. as "primitives".
If you're doing any type of sophisticated math, the Class-based numeric representation like Integer and Double will be cumbersome and slow you down - many math operations can only be done with primitives.
On the other hand, if you're trying to put your numbers into collections like Lists and Maps, those collections can only contain objects - and thus you must use (or convert to) classes like Integer and Double.
Personally, I use primitives whenever I can get away with it, and only convert to the Class representations like Integer when it's time to do input or output, and the transport requires those representations.
However, if you aren't doing any math at all, and instead are just passing the values straight through your code, you might save yourself some trouble by dealing with the Class-based forms (like Integer) exclusively.
I think this is a bit late but I wanted to add my opinion just in case.
in some scenarios, it's required to use the wrappers as the lack of a value is different from the default value.
example,
for one project I worked on, there was a field on screen where the user could enter a double value, the business requirement clearly mentioned that if the user enters a 0 the meaning is different from not entering a value and leaving the field blank and this difference will make an impact later on in a different module.
so in this scenario we had to use the Double object, since I cannot represent a lack of value using the primitive; since the primitive will default to 0 which was a valid input for the field.
I dont think there is any rule as such. I would choose the types over primitives (Integer over int) when I write method signatures, Maps, collections, Data Objects that get passed around. As such I would still like to use Integer instead of int even inside of methods etc. But if you think that is too much trouble (to type extra "eger") then it is okay to use ints for local variables.
If you want to setAttribute to session you have to use Object like Integer,Boolean,String in servlets. If you want to use value you can use primitive types. Objects may be null but primitives not. And if you want to compare types for primitives use == but objects use .equals because in object comparision == looks not values it looks if these are the same objects. And using primitives makes faster the code.
When we deal with Spring getRequest mapping methods, using boolean value does not work.
For instance :
#GetMapping("/goal")
public boolean isValidGoal() {
boolean isValid = true;
return isValid;
}
Always opt for Boolean in those cases.