Develop Reference

DRY maxima tracking

Suppose I am importing table entries, where a single entry can be stored in a class:
class Foo {
int i1;
int i2;
double d1;
}
After the import is complete, I will need to have access to the imported values themselves, as well as to their normalized versions. So far, I have implemented this functionality as follows:
class FooWithMaxTracking {
private int i1;
private static int i1_max=0;
public void setI1(int value){
this.i1 = value;
if (value > i1_max) { i1_max = value; }}
public int getI1(){
return i1;}
public double normI1(){
return i1/((double)i1_max);}
private int i2;
private static int i2_max=0;
public void setI2(int value){ <code identical to written above> }
public int getI2(){ ... }
public double normI2(){ ... }
// and another set of similar 2 variables & 3 functions for 'double d1'
}
In this implementation I strongly dislike the fact that I had to write the same code many times (only three in this example, but about ten times in the real project). Is there any way to make the code more DRY ("don't repeat yourself")?

If you do not mind a slight loss of performance, you can put all the maxima in a static Map, define a class that holds a getter, a setter, and a norm methods, and replace the individual variables with instances of that class:
private static Map<String,Object> max = new HashMap<String,Object>();
private static class IntMaxTrack {
private final String key;
private int value;
public IntMaxTrack(String k, int v) {
key = k;
value = v;
max.put(key, value);
}
public int get() { return value; }
public void set(int v) {
int m = ((Integer)max.get(key)).intValue();
value = v;
if (value > m) {
max.put(key, value);
}
}
public double norm() {
int m = ((Integer)max.get(key)).intValue();
return val / ((double)m);
}
}
Make a similar class for double, i.e. DblMaxTrack Now you can replace primitives with instances of these classes, and call their get, set, and norm from the corresponding methods of your class.

What about defining one class with the necessary code, like:
public class Bar {
private int i1;
private static int i1_max = 0;
public void setI1(int value) {
// ...
}
public int getI1() {
// ...
}
public double normI1() {
// ...
}
}
And using it sevearl times, like:
class FooWithMaxTracking {
one = new Bar();
two = new Bar();
three = new BarForDouble();
}

Enum with negative/gap value in Java

I have a problem with enum in Java. I have an enum that starts from -1:
public enum AipType {
Unknown(-1),
None(0),
AipMod(1),
AipNoMod(2);
private final int id;
AipType(int id) {this.id = id;}
public int getValue() {return id;}
}
The problem is when I use this code to initialize a var of AipType
AipType at = AipType.getValues()[index];
where index is a number in the interval [-1,0,1,2] the -1 mess up the value.
i.e. 0 returns Unknown, 1 returns AipMod and 2 returns AipNoMod.
I used this implementation because I need to set manually the numeric value for each enum case. In other case I have a gap beetwen the values so I have the same problem: I cannot use values() and then access with [ ].
I tried to initialize in this way
AipType at = AipType(index);
but doesn't work.
Ideas ? Thanks...

We don't know what the getValues() method you're using exactly doing. Is it supposed to be values().
Anyway, you can always add a static method in your enum, which returns the correct enum instance for that value, and invoke it wherever you need it:
public enum AipType {
Unknown(-1),
None(0),
AipMod(1),
AipNoMod(2);
private final int id;
AipType(int id) {this.id = id;}
public int getValue() {return id;}
public static AipType fromValue(int id) {
for (AipType aip: values()) {
if (aip.getValue() == id) {
return aip;
}
}
return null;
}
}
If you're invoking fromValue() too often, you might also want to cache the array returned by values() inside the enum itself, and use it. Or even better, a map would be a better idea.

Enum.getValues() returns an array of the enums based on the definition order in the enum class.
getValues() doesn't know about the id field or the getValue() method you have added to your enum.
What you could do instead of calling getValues()[-1] (by the way, you'll never be able to index an array in Java with -1) is to add a static function like:
static AipType getAipType(int id) {
for (AipType a : getValues()) {
if (a.getId() == id) return a;
}
throw new IllegalArgumentException("id=" + id + " does not exist");
}

Just have a Map<Integer, AipType> instead of using values(), and expose access to it via a method:
public enum AipType {
UNKNOWN(-1),
NONE(0),
MOD(1),
NO_MOD(2);
private static final Map<Integer, AipType> VALUE_TO_ENUM_MAP;
private final int value;
static {
VALUE_TO_ENUM_MAP = new HashMap<>();
for (AipType type : EnumSet.allOf(AipType.class)) {
VALUE_TO_ENUM_MAP.put(type.value, type);
}
}
private AipType(int value) {
this.value = value;
}
public int getValue() {
return id;
}
public static AipType forValue(int value) {
return VALUE_TO_ENUM_MAP.get(value);
}
}
That will be completely flexible about values - or you could still use an array and just offset it appropriately.

Can you declare your enum like below?
public enum AipType {
Unknown(-1),None(0),AipMod(1),AipNoMod(2);
private int value;
private AipType(int value) {
this.value = value;
}
public int getValue() {
return this.value;
}
public static AipType fromValue(int value) {
for (AipType at: values()) {
if (at.getValue() == value) {
return at;
}
}
return null;
}
};
And instantiate like:
AipType at = AipType.fromValue(-1);

If the ids are completely custom, the only chance you'd have would be to create a map and store the id->AipType mapping there.
Example:
public enum AipType {
... //enum definitions here
static Map<Integer, AipType> map = new HashMap<>();
static {
for( AipType a : AipType.values() ) {
map.put(a.id, a);
}
}
public static AipType typeById( int id ) {
return map.get(id);
}
}
Then call it like AipType.typeById(-1);.

can not define int variable in enum - java

i try to define enum with int in it, but i have error in eclipse : "Syntax error on token "int", delete this token"
my code:
package util.enumurations;
public enum BooleanEnum
{
private int value;
static
{
BooleanEnum[] arrayOfBooleanEnum = new BooleanEnum[2];
arrayOfBooleanEnum[0] = False;
arrayOfBooleanEnum[1] = True;
}
private BooleanEnum(int arg3)
{
int j;
this.value = j;
}
public int getValue()
{
return this.value;
}
}

The first thing in an enum have to be the declaration of the possible values.
public enum BooleanEnum
{
False(0), True(1);
private final int value;
static
{
BooleanEnum[] arrayOfBooleanEnum = new BooleanEnum[2];
arrayOfBooleanEnum[0] = False;
arrayOfBooleanEnum[1] = True;
}
private BooleanEnum(int arg3)
{
this.value = arg3;
}
public int getValue()
{
return this.value;
}
}

Use
java.lang.Boolean.TRUE,
java.lang.Boolean.FALSE
instead

Actually The body of an enum type may contain enum constants. An enum constant defines an instance of the enum type.
What you are trying to is You aren't gonna need it.
Just use a simple Boolean almost it self acts as ENUM for true false types.
Use Boolean.valueOf();

How can I declare enums using java

I want to convert this sample C# code into a java code:
public enum myEnum {
ONE = "one",
TWO = "two",
};
Because I want to change this constant class into enum
public final class TestConstants {
public static String ONE = "one";
public static String TWO= "two";
}

public enum MyEnum {
ONE(1),
TWO(2);
private int value;
private MyEnum(int value) {
this.value = value;
}
public int getValue() {
return value;
}
}
In short - you can define any number of parameters for the enum as long as you provide constructor arguments (and set the values to the respective fields)
As Scott noted - the official enum documentation gives you the answer. Always start from the official documentation of language features and constructs.
Update: For strings the only difference is that your constructor argument is String, and you declare enums with TEST("test")

enums are classes in Java. They have an implicit ordinal value, starting at 0. If you want to store an additional field, then you do it like for any other class:
public enum MyEnum {
ONE(1),
TWO(2);
private final int value;
private MyEnum(int value) {
this.value = value;
}
public int getValue() {
return this.value;
}
}

Quite simply as follows:
/**
* #author The Elite Gentleman
*
*/
public enum MyEnum {
ONE("one"), TWO("two")
;
private final String value;
private MyEnum(final String value) {
this.value = value;
}
public String getValue() {
return value;
}
#Override
public String toString() {
// TODO Auto-generated method stub
return getValue();
}
}
For more info, visit Enum Types from Oracle Java Tutorials. Also, bear in mind that enums have private constructor.
Update, since you've updated your post, I've changed my value from an int to a String.
Related: Java String enum.

Well, in java, you can also create a parameterized enum. Say you want to create a className enum, in which you need to store classCode as well as className, you can do that like this:
public enum ClassEnum {
ONE(1, "One"),
TWO(2, "Two"),
THREE(3, "Three"),
FOUR(4, "Four"),
FIVE(5, "Five")
;
private int code;
private String name;
private ClassEnum(int code, String name) {
this.code = code;
this.name = name;
}
public int getCode() {
return code;
}
public String getName() {
return name;
}
}

public enum MyEnum
{
ONE(1),
TWO(2);
private int value;
private MyEnum(int val){
value = val;
}
public int getValue(){
return value;
}
}

public enum NewEnum {
ONE("test"),
TWO("test");
private String s;
private NewEnum(String s) {
this.s = s);
}
public String getS() {
return this.s;
}
}

Can I set enum start value in Java?

I use the enum to make a few constants:
enum ids {OPEN, CLOSE};
the OPEN value is zero, but I want it as 100. Is it possible?

Java enums are not like C or C++ enums, which are really just labels for integers.
Java enums are implemented more like classes - and they can even have multiple attributes.
public enum Ids {
OPEN(100), CLOSE(200);
private final int id;
Ids(int id) { this.id = id; }
public int getValue() { return id; }
}
The big difference is that they are type-safe which means you don't have to worry about assigning a COLOR enum to a SIZE variable.
See http://docs.oracle.com/javase/tutorial/java/javaOO/enum.html for more.

Yes. You can pass the numerical values to the constructor for the enum, like so:
enum Ids {
OPEN(100),
CLOSE(200);
private int value;
private Ids(int value) {
this.value = value;
}
public int getValue() {
return value;
}
}
See the Sun Java Language Guide for more information.

whats about using this way:
public enum HL_COLORS{
YELLOW,
ORANGE;
public int getColorValue() {
switch (this) {
case YELLOW:
return 0xffffff00;
case ORANGE:
return 0xffffa500;
default://YELLOW
return 0xffffff00;
}
}
}
there is only one method ..
you can use static method and pass the Enum as parameter
like:
public enum HL_COLORS{
YELLOW,
ORANGE;
public static int getColorValue(HL_COLORS hl) {
switch (hl) {
case YELLOW:
return 0xffffff00;
case ORANGE:
return 0xffffa500;
default://YELLOW
return 0xffffff00;
}
}
Note that these two ways use less memory and more process units .. I don't say this is the best way but its just another approach.

If you use very big enum types then, following can be useful;
public enum deneme {
UPDATE, UPDATE_FAILED;
private static Map<Integer, deneme> ss = new TreeMap<Integer,deneme>();
private static final int START_VALUE = 100;
private int value;
static {
for(int i=0;i<values().length;i++)
{
values()[i].value = START_VALUE + i;
ss.put(values()[i].value, values()[i]);
}
}
public static deneme fromInt(int i) {
return ss.get(i);
}
public int value() {
return value;
}
}

If you want emulate enum of C/C++ (base num and nexts incrementals):
enum ids {
OPEN, CLOSE;
//
private static final int BASE_ORDINAL = 100;
public int getCode() {
return ordinal() + BASE_ORDINAL;
}
};
public class TestEnum {
public static void main (String... args){
for (ids i : new ids[] { ids.OPEN, ids.CLOSE }) {
System.out.println(i.toString() + " " +
i.ordinal() + " " +
i.getCode());
}
}
}
OPEN 0 100
CLOSE 1 101

The ordinal() function returns the relative position of the identifier in the enum. You can use this to obtain automatic indexing with an offset, as with a C-style enum.
Example:
public class TestEnum {
enum ids {
OPEN,
CLOSE,
OTHER;
public final int value = 100 + ordinal();
};
public static void main(String arg[]) {
System.out.println("OPEN: " + ids.OPEN.value);
System.out.println("CLOSE: " + ids.CLOSE.value);
System.out.println("OTHER: " + ids.OTHER.value);
}
};
Gives the output:
OPEN: 100
CLOSE: 101
OTHER: 102
Edit: just realized this is very similar to ggrandes' answer, but I will leave it here because it is very clean and about as close as you can get to a C style enum.

#scottf
An enum is like a Singleton. The JVM creates the instance.
If you would create it by yourself with classes it could be look like that
public static class MyEnum {
final public static MyEnum ONE;
final public static MyEnum TWO;
static {
ONE = new MyEnum("1");
TWO = new MyEnum("2");
}
final String enumValue;
private MyEnum(String value){
enumValue = value;
}
#Override
public String toString(){
return enumValue;
}
}
And could be used like that:
public class HelloWorld{
public static class MyEnum {
final public static MyEnum ONE;
final public static MyEnum TWO;
static {
ONE = new MyEnum("1");
TWO = new MyEnum("2");
}
final String enumValue;
private MyEnum(String value){
enumValue = value;
}
#Override
public String toString(){
return enumValue;
}
}
public static void main(String []args){
System.out.println(MyEnum.ONE);
System.out.println(MyEnum.TWO);
System.out.println(MyEnum.ONE == MyEnum.ONE);
System.out.println("Hello World");
}
}

public class MyClass {
public static void main(String args[]) {
Ids id1 = Ids.OPEN;
System.out.println(id1.getValue());
}
}
enum Ids {
OPEN(100), CLOSE(200);
private final int id;
Ids(int id) { this.id = id; }
public int getValue() { return id; }
}
#scottf, You probably confused because of the constructor defined in the ENUM.
Let me explain that.
When class loader loads enum class, then enum constructor also called. On what!! Yes, It's called on OPEN and close. With what values 100 for OPEN and 200 for close
Can I have different value?
Yes,
public class MyClass {
public static void main(String args[]) {
Ids id1 = Ids.OPEN;
id1.setValue(2);
System.out.println(id1.getValue());
}
}
enum Ids {
OPEN(100), CLOSE(200);
private int id;
Ids(int id) { this.id = id; }
public int getValue() { return id; }
public void setValue(int value) { id = value; }
}
But, It's bad practice. enum is used for representing constants like days of week, colors in rainbow i.e such small group of predefined constants.

I think you're confused from looking at C++ enumerators. Java enumerators are different.
This would be the code if you are used to C/C++ enums:
public class TestEnum {
enum ids {
OPEN,
CLOSE,
OTHER;
public final int value = 100 + ordinal();
};
public static void main(String arg[]) {
System.out.println("OPEN: " + ids.OPEN.value);
System.out.println("CLOSE: " + ids.CLOSE.value);
System.out.println("OTHER: " + ids.OTHER.value);
}
};