Example code:
int width = 5;
int area = 8;
int potato = 2;
int stackOverflow = -4;
Now, say I want to have the user input a string:
String input = new Scanner(System.in).nextLine();
Then, say the user inputs potato. How would I retrieve the variable named potato and do stuff with it? Something like this:
System.getVariable(input); //which will be 2
System.getVariable("stackOverflow"); //should be -4
I looked up some things and did not find much; I did find a reference to something called "the Reflection API," but that seems too complicated for this one simple task.
Is there a way to do this, and if so, what is it? If "Reflection" does indeed work and if it is the only way, then how would I use it to do this? The tutorial page for it has all sorts of internal stuff that I can't make any sense of.
EDIT: I need to keep the Strings in the variables for what I am doing. (I can't use a Map)
Using reflection doesn't seem like a good design for what you're doing here. It would be better to use a Map<String, Integer> for example:
static final Map<String, Integer> VALUES_BY_NAME;
static {
final Map<String, Integer> valuesByName = new HashMap<>();
valuesByName.put("width", 5);
valuesByName.put("potato", 2);
VALUES_BY_NAME = Collections.unmodifiableMap(valuesByName);
}
Or with Guava:
static final ImmutableMap<String, Integer> VALUES_BY_NAME = ImmutableMap.of(
"width", 5,
"potato", 2
);
Or with an enum:
enum NameValuePair {
WIDTH("width", 5),
POTATO("potato", 2);
private final String name;
private final int value;
private NameValuePair(final String name, final int value) {
this.name = name;
this.value = value;
}
public String getName() {
return name;
}
public String getValue() {
return value;
}
static NameValuePair getByName(final String name) {
for (final NameValuePair nvp : values()) {
if (nvp.getName().equals(name)) {
return nvp;
}
}
throw new IllegalArgumentException("Invalid name: " + name);
}
}
Variable names are only available at compiler time. Reflection only gives access to class declarations and items declared inside them, but not to local variables. I suspect that a Map of some kind will be a more appropriate solution to your real problem. Specifically, check out HashMap and TreeMap.
Instead of trying to find the value of a variable name, why don't you use a Map with a key/value pair?
Map<String, Integer> vars = new HashMap<String, Integer>();
vars.put("width",5);
vars.put("area",8);
vars.put("potato", 2);
vars.put("stackOverflow",-4);
Then you could access the inputs like so:
vars.get(input); //would be 2
vars.get("stackOverflow"); //would be -4
I have another solution without a map :
class Vars {
Integer potato, stack;
public Vars(a,b) {
potato=a;
stack=b;
}
}
Object object=(Object)new Vars(1,2);
Class<?> c = object.getClass();
Integer result=(Integer)c.getField("potato").get(object);
I have a solution for this problem that does not involve using a map. I ran into this technique because we had several variables that needed to be update based on something within the variable name itself. However, the best way to do this is by using the getters/setters rather than the variables.
After you create your class, you can access the methods by creating Method objects and invoking them individually.
public class FooClass
private String foo1;
private String foo2;
public String getFoo1();
public String getFoo2();
FooClass fooClass = new FooClass();
Method mFoo1 = fooClass.getClass().getMethod("getFoo" + increment + "()");
mFoo1 .invoke(fooClass);
However, this would not be limited to only incremental numbers, as long as you can get the string to match the method exactly.
String value = "Potato";
Method mPotato = myClass.getClass().getMethod("get" + value+ "()");
mPotato.invoke(myClass);
Very redundant, but you can keep your variable names when using a map:
int width = 5;
int area = 8;
int potato = 2;
int stackOverflow = -4;
Map<String, Integer> map = new HashMap<>();
map.put("width", width);
map.put("area", area);
map.put("potato", potato);
map.put("stackOverflow", stackOverflow);
But a statement like this:
width = 42;
would not change the value in the Map:
String input = "width";
map.get(input); // <-- still returns 5.
Only a new call of put fixes that:
width = 42;
map.put("width", width);
// or
map.put("width", 42);
Related
I have two java class with constants, For Ex:
public class FirstClass {
public static final String STRING_A = "STRING_A";
public static final String STRING_B = "STRING_B";
public static final String STRING_C = "STRING_C";
...
}
public class SecondClass {
public static final String STRING_AA = "STRING_AA";
public static final String STRING_BA = "STRING_BA";
public static final String STRING_CA = "STRING_CA";
...
}
Now, I want to load these constants into a,
Map< String,String> classPropertyMap = new HashMap<>(); in Such a way that, Key for this map must be a constant from FirstClass and corresponding value must be a constant from SecondClass.
If it was just one class I could use reflection to load the fields, now since the constants are from two class, how could this be done?
Finally after loading the map, the contents of the map must be something like this:
First element : key and value is < STRING_A, STRING_AA>
Second element : key and value is < STRING_B, STRING_BA>
Third element : key and value is < STRING_C, STRING_CA>
I think if you really need it, the most robust way is to put the properties to a map explicitly.
I mean map.put(FirstClass.STRING_A, SecondClass.STRING_AA); and so on.
If you use reflection you rely on the properties and their declaration order never changes. If some new property is introduced in the library, it can break your code.
Try something like below, seems you can achieve.
FirstClass first = new FirstClass();
Field[] fields = first.getClass().getFields();
SecondClass second = new SecondClass();
Field[] fields1 = second.getClass().getFields();
Try This:
FirstClass first = new FirstClass();
String[] firstStrings = first.getStrings();
//Makes a String array, and fills it with the strings from the first class.
SecondClass second = new SecondClass();
String[] secondStrings = second.getStrings();
//Makes a String array, and fills it with the strings from the second class.
HashMap<String, String> classPropertyMap = new HashMap<String, String>();
int i = 0;
//Now put strings with the same index number from both arrays into the
//HashMap
while(i <= firstStrings.length()){
classPropertyMap.put(firstStrings.get(i), secondStrings.get(i);
i++;
}
Hope that works!
I'm from a PHP background, and I'm trying to create a multidimentional array with a difficulty in understanding the Java way of doing things. I thought this could be achieved using JSON and the GSON library, but I'm failing to understand how this is done having followed several tutorials online.
Here is what I'm after in PHP, how can I achieve the same thing in Java?
function creatCars($id) {
$aCars = array(
0 => array(
'name' => 'vauxhall',
'doors' => 5,
'color' => 'black',
),
1 => array(
'name' => 'peogeot',
'doors' => 3,
'color' => 'red',
),
);
return $aCars[$id];
}
function printFirstCarName($sName) {
$aCar = createCars(0);
echo $aCars['name'];
}
//prints "vauxhall"
printFirstCarName();
Arrays in PHP are not the same as arrays in Java. Here are the differences:
PHP:
PHP arrays are actually dictionaries. They store a value for each key, where a key can be an integer or a string. If you try to use something else as a key, it will be converted to either an integer or a string.
Java:
Arrays in Java
Java arrays are not associative in the same way as they are in PHP. Let's start with one-dimensional arrays in Java:
A one-dimensional array in Java has a fixed length (that cannot be changed) and each key is an integer in the range of 0 to array.length - 1. So keys, actually called indexes, are always integers. Also, in Java, if you have an array with the keys 2 and 4, you also have (at least) the keys 0, 1 and 3, because the length has to be at least 5 then.
Arrays in Java also have exactly one type and each values in the array can only be of the specified type. Neither size nor type of an array can be changed.
When you create an array in Java, you have two possibilities:
explicitly specify the length when creating the array
String[] words = new String[4];
The variable words now holds an array of type String with the length a length of 4. The values of all indexes (0 to 3) are initially set to null.
specify elements when creating the array
String[] words = new String[] {"apple", "banana", "cranberry"};
The variable words now holds an array of type String with a length of 3. The elements contained are as specified with the first element bound to index 0, the second element bound to index 1, and so on.
You can think of multi-dimensional arrays as of an array which holds arrays. A 2-dimensional array could look like this:
String[][] twoD = new String[][] {
{"apple", "banana", "cranberry"},
{"car", "ship", "bicycle"}
}
For this twoD[0][2] would be "cranberry" and twoD[1][1] would be "ship". But the number of dimensions of an array does not influence the fact that the keys are integers.
Maps in Java:
Even though Java has no built-in language construct for associative arrays, it offers the interface Map with various implementations, e.g. HashMap. A Map has a type of which the keys are, and a type of which the values are. You can use maps like this:
HashMap<String, String> map = new HashMap<String, String>();
map.put("car", "drive");
map.put("boat", "swim");
System.out.println("You can " + map.get("car") + " a car.");
System.out.println("And a boat can " + map.get("boat") + ".");
This will output:
You can drive a car.
And a boat can swim.
The answer:
The one-to-one way in Java
The answer to your question is that it is not really possible in a reasonable way becasue some of your values are strings, and some are integers. But this would be the most similar code to your PHP array:
//array of HashMaps which have Strings as key and value types
HashMap<String, String>[] cars = new HashMap<String, String>[2];
HashMap<String, String> first = new HashMap<String, String>();
first.put("name", "vauxhall");
first.put("doors", "5");
first.put("color", "black");
HashMap<String, String> second = new HashMap<String, String>();
second.put("name", "peogeot");
second.put("doors", "3");
second.put("color", "red");
//put those two maps into the array of maps
cars[0] = first;
cars[1] = second;
This solution is not very handy, but it is the way that comes closest to your given datastructure.
The cuter way in Java
It seems however, that each of the entries in your PHP array has exactly three properties: name, doors and color. In this case, you may want to create a class Car with these member variables, and store them in an array. This would look like this:
public class Car {
//member variables
public String name;
public int doors;
public String color;
//constructor
public Car(String name, int doors, String color) {
this.name = name;
this.doors = doors;
this.color = color;
}
}
Now, when you have the class Car, you can create an array of all your cars like this:
Car[] cars = new Car[2];
cars[0] = new Car("vauxhall", 5, "black");
cars[1] = new Car("peogeot", 3, "red");
This is the nicer way to do this in Java.
Instead of creating 2D Array you can create 1 class Car
public class Car{
private String carName;
private String color;
private int noOfDoors;
public car(String carName,int door,String color){
this.carName=carName;
this.door=door;
this.color=color;
}
public String getCarName(){
return getCarName;
}
public void setCarName(String carName){
this.carName=carName;
}
// Same getters(getXXX) and setters(setXXX) for other Variables
}
Now create Objects of above class
Car audi=new Car("audi",2,"Black");
Car bmw=new Car("bmw",4,"White");
Now add these to the List<Cars>
List<Car> listOfCars=new ArrayList<Car>();
listOfCars.add(audi);
listOfCars.add(bmw);
Now to Print First Car Name
Car firstCar=listOfCars.get(0);
System.out.println(firstCar.getCarName()); //here getter Method Helped you
I would suggest to get familiar with HashMaps, Maps and ArrayLists. In Java and many other languages is something analogous to a video game cheat.
private static Map<Integer, HashMap<String, String> > carMap = new HashMap<Integer, HashMap<String, String> >();
But in this case you have to understand how would OO principles help you. You can create a class with Car objects and populate a HashMap etc.
class Car {
private String name, colour;....
public Car(){....}
public void setValues(...){....}
}
To achieve better what you want to I would suggest reading this and getting familiar with some design patterns. It's a bit further down the road, but do it for the lulz and seeing what it's out there. Example : http://howtodoinjava.com/2012/10/23/implementing-factory-design-pattern-in-java/
When moving from scripting to strongly typed languages sometimes you have to change your way of thinking too.
Firstly you should create class Car i.e:
public class Car {
enum ColorType {
BLACK, RED;
}
private String name;
private int doors;
private ColorType color;
Car(String name, int doors, ColorType color) {
this.name = name;
this.doors = doors;
this.color = color;
}
public String getName() {
return name;
}
public int getDoors() {
return doors;
}
public ColorType getColor() {
return color;
}
}
And now you can use arrays but better for you will be use ArrayList:
List<Car> cars = new ArrayList<Car>();
cars.add(new Car("vauxhall", 5, BLACK));
cars.add(new Car("peogeot", 3, RED));
for (Car car : cars ) {
System.out.println("Car name is: " + car.getName());
}
It seems what you are trying to achive is an 'array of cars'. So instead of creating an array of arrays, I recommend to literally implement an 'array of cars'.
To do this, I would define the car first, possibly in a different file:
class Car {
//you can make these private and use 'get' and 'set' methods instead
public String name;
public String color;
public int doors;
public Car() {
name = "";
color = "";
doors = 0;
}
public Car(String name, String color, int doors) {
this.name = name;
this.color = color;
this.doors = doors;
}
}
You can use the car structure in an another module like this:
Car[] cars = new Car[100]; //create one hundred cars
cars[11].doors = 4; //make the 12th car's number of doors to 4
You can use more flexible data structures, like Vectors, List, Maps, etc... Search for Java collections, you will find tones of info.
Java is not a loosely typed language, you have to tell the compiler what each variable is going to be. And to store this kind of structured data in Java, you should first declare a class and instantiate objects of that class. Following is how you would achieve the same thing as your PHP code:
class Car {
private String name, color;
private int doors;
Car(String name, int doors, String color) {
this.name = name;
this.doors = doors;
this.color = color;
}
public String getName() {
return this.name;
}
}
public class CarMainClass {
public static void main(String[] args) {
Car[] aCars = new Car[2];
aCars[0] = new Car("vauxhall", 5, "black");
aCars[1] = new Car("peogeot", 3, "red");
System.out.println("First car name is: " + aCars[0].getName());
}
}
Compile using:
javac CarMainClass.java
Then run:
java CarMainClass
You will have to learn the basics of Java first to understand the above code.
My problem is can't get an object "Item" (value) from my Treemap. I need send that info to my GUI class and display it in JList to get a select list, so can easily select and add songs to playlist, but only what I get as an output is "01, 02, 03, 04, 05" (key). Please help, because I'm beginner and have no idea what to do.
public class LibraryData {
private static class Item {
Item(String n, String a, int r) {
name = n;
artist = a;
rating = r;
}
// instance variables
private String name;
private String artist;
private int rating;
private int playCount;
public String toString() {
return name + " - " + artist;
}
}
private static Map<String, Item> library = new TreeMap<String, Item>();
static {
library.put("01", new Item("How much is that doggy in the window", "Zee-J", 3));
library.put("02", new Item("Exotic", "Maradonna", 5));
library.put("03", new Item("I'm dreaming of a white Christmas", "Ludwig van Beethoven", 2));
library.put("04", new Item("Pastoral Symphony", "Cayley Minnow", 1));
library.put("05", new Item("Anarchy in the UK", "The Kings Singers", 0));
}
public static String[] getLibrary() {
String [] tempa = (String[]) library.keySet().toArray(new String[library.size()]);
return tempa;
}
SOLUTION:
Because I've to pass the values to another class:
JList tracks = new JList(LibraryData.getLibrary());
I made something like that and it's works
public static Object[] getLibrary() {
Collection c = library.values();
return c.toArray(new Item[0]);
Thank You guys, after 10 hours I finally done it!
}
With this code that you have:
String [] tempa = (String[]) library.keySet().toArray(new String[library.size()]);
You are getting all keys from the map. If you want all values, then use:
library.values();
Finally, if you need to get a value by key use V get(Object key):
library.get("01");
Which will return you the first Item from the map.
It's not very clear which one of these you want, but basically these are the options.
** EDIT **
Since you want all values you can do this:
library.values().toArray()
JList expects an array or vector of Object so this should work.
If you want to get value and key by position, you can use:
key: library.keySet().toArray()[0]
value: library.get(key);
OR (if you just want value)
library.values().toArray()[0];
You can use the ArrayList:
1 - The best for flexible-array managing in Java is using ArrayLists
2 - ArrayLists are easy to add, get, remove and more from and to.
3 - Treemaps are a little... arbitrary. What I say is that if you use the get(Object o) method from a Treemap, the Object o must be a key, which is something not very flexible.
If you want them, use this code:
import java.util.ArrayList;
import com.example.Something; // It can be ANYTHING
//...
ArrayList<Something> somethingList = new ArrayList<Something>();
//...
somethingList.add(new Something("string", 1, 2.5, true));
//...
boolean isSomething = somethingList.get(somethingList.size() - 1); // Gets last item added
//...
int listSize = somethingList.size();
//...
somethingList.remove(somethingList.size() - 1); // Removes last item and decrements size
//...
Something[] nativeArray = somethingList.toArray(new Something[somethingList.size()]); // The parameter is needed or everthing will point to null
// Other things...
Or the classic Treemap:
Object keyAtIndex0 = library.keySet.toArray(new Object[library.size()])[0];
Object value = library.get(keyAtIndex0);
Good Luck!
I was returning a list of string values as treemap value. The used approach is
private Map<String, TreeSet<String>> result;
TreeSet<String> names= result.get(key);
for(String contactName: names){
print contactName;
}
I am attempting to reference a variable using a certain string but have no idea how to do it. I know that I can use if statements if I really had to but I am sure that there is a simple way. An example is a Integer named dog. I would try to access the Integer using another string that contained the text dog.
private int dog;
String anything = "dog";
Is there anyway this is possible? Thanks!
Try this:
// use a map for referring to a value given its name
Map<String, Integer> vars = new HashMap<String, Integer>();
// for example, let's use these values
String anything = "dog";
int dog = 10;
// bind a value to a name
vars.put(anything, dog);
// retrieve the value, given its name
vars.get(anything);
=> 10
http://docs.oracle.com/javase/7/docs/api/java/util/HashMap.html
public static void main(String[] args) {
Map<String, MyObject> mapping = new HashMap<>();
}
Or new HashMap<String, MyObject>(); for pre java 7
You should use a Map of String to Integer. For example,
public static void main(String[] args) {
java.util.Map<String, Integer> dogMap = new java.util.HashMap<String, Integer>();
dogMap.put("Snoop", 10);
dogMap.put("doggy", 15);
dogMap.put("dog", 20);
System.out.println(dogMap);
}
Which outputs
{doggy=15, Snoop=10, dog=20}
Two options: create a Map<String, Object> that connects the two, or use reflection. I prefer reflection.
in order to get the field:
public class Test {
private int dog = 10;
private String anything = "dog";
public static void main(String[] args){
Test obj = new Test();
Object field = obj.getClass()
.getDeclaredField(obj.anything)
.get(obj);
System.out.println(field);
}
}
Output:
10
Create an object of the class that you will use. Then use the getDeclaredField() method on the class of that object. This will look into the private fields that are set, getField() holds only the public fields. That's it.
I've removed the try-catch from the post because it just clutters it.
I have the following file named ght.txt in my c: and it contains the following data
Id|ytr|yts
1|W|T
2|W|T
3|W|T
Now the thing is that positions of this columns (Id|ytr|yts) is also not in order means they can be reshuffled also..for ex
Id|ytr|dgfj|fhfjk|fgrt|yts
or they can be as ..
Id|wer|ytr|weg|yts
so I have done the following way and read them in java as shown below
String[] headers = firstLine.split("|");
int id, Ix, Ixt, count = 0;
for(String header : headers) {
if(header.equals("Id")) {
idIx = count;
}elseif (header.equals("Ix")) {
Ixt = count;
} elseif (header.equals("Ixt")) {
Ixt = count;
}
count++;
}
Now I need to store them in a map in such a way that against id I will get the value of column ytr and yts so in map there should be single key but against that key value could be multiple please advise how to store in map in such a way
Using a Map<Integer,List<String>> sounds like a viable first approach.
As it sound like your value is structured, it might be even better to create a value class to hold this, eg. Map<Integer, YourValueClass> where
class YourValueClass
{
String ix;
String ixt;
// constructor, getters and setters
}
Basically, you should think in terms of classes/objects - don't be in object denial :-)
Cheers,
I'm not quite sure what you mean, but if I get it right, you are looking for a multimap.
You can roll one yourself, as #Anders R. Bystrup suggests.
Or you can use an existing implementation like the Google Collections Multimap.
Don't store one key and multiple values. Instead, you can store a Key and Values as a List.
You can use MultiMap from Guava Library:
MultiMap<String,String> map = ArrayListMultimap.create();
map.put("key","value1");
map.put("key","value2");
By using:
System.out.println(map.get("key");
Prints:
["value1","value2"]
Value Class
class TextValues {
final int id;
final String ix;
final String ixt;
private TextValues(final int id, final String ix, final String ixt){
this.id = id;
this.ix = ix;
this.ixt = ixt;
}
public static TextValues createTextValues(int id, String ix, String ixt) {
return new TextValues(id, ix, ixt);
}
}
Usage:
Map<Integer, TextValues> map = new HashMap<Integer, TextValues>();
map.put(1, TextValues.createTextValues(1, "ix value ", "ixt value"));
public static void main(String[] args) {
Map<String, List<String>> map = new HashMap<String, List<String>>();
List<String> valSetOne = new ArrayList<String>();
valSetOne.add("ABC");
valSetOne.add("BCD");
valSetOne.add("DEF");
List<String> valSetTwo = new ArrayList<String>();
valSetTwo.add("CBA");
valSetTwo.add("DCB");
map.put("FirstKey", valSetOne);
map.put("SecondKey", valSetTwo);
for (Map.Entry<String, List<String>> entry : map.entrySet()) {
String key = entry.getKey();
List<String> values = entry.getValue();
System.out.println("Value of " + key + " is " + values);
}
}
You can use Set or List based on your requirement i.e you need elements in ordered or unordered collection.This is a simple method of having single key with multiple values.