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List of non-static method references in Java

I am trying to use a list of function references as a lookup table (avoiding the need for a long switch statement). The code worked for a list of static methods, but when I tried to use non-static (i.e. instance) methods in the list, Java gives several errors regarding the types not matching.
Here is a minimal example:
import java.util.Arrays;
import java.util.List;
import java.util.function.Function;
public class MethodReferences {
// My original list of static references
private final static List<Function<Integer, Integer>> lookupTable = Arrays.asList(MethodReferences::f1, MethodReferences::f2);
// This doesn't work
// private final List<Function<Integer, Integer>> lookupTable = Arrays.asList(MethodReferences::f3, MethodReferences::f4);
private static int f1(int x) { return x * 2; }
private static int f2(int x) { return x * 3; }
private int f3(int x) { return x * 2; }
private int f4(int x) { return x * 3; }
public void run() {
System.out.println(lookupTable.get(1).apply(3));
}
public static void main(String[] args) {
MethodReferences testClass = new MethodReferences();
testClass.run();
}
}
The errors I received were all for the line containing the non-static definition:
Type mismatch: cannot convert from List<Object> to List<Function<Integer,Integer>>
and:
The target type of this expression must be a functional interface
I tried using this:: instead of MethodReferences:: before the function names. The code then compiled, but when it runs, nothing happens, probably because this has to be used within non-static functions.
I then moved the initialisation of the array (still using this:: to within the class constructor, but it continued to produce no output when run.
I've checked through the documentation and tutorials on method references in Java, but I cannot find an examples of creating references to instance methods within the class it is defined in (and I cannot find any examples of lists of function references either).
I'm aware that in the main method, you can do testClass::f1, but for my specific situation (not the example code) I do not even have a main class (the class is instantiated by another library), so this approach isn't possible. The methods have to be non-static because I need to be able to modify instance variables within them.
Edit:
It turns out that using this:: does work for the example code, although I am still unsure as to why it is valid (surely you can only use this within a non-static function?)

You need to use BiFunction instead of Function. The first argument is the implicit this argument.
public class MethodReferences {
private final static List<BiFunction<MethodReferences, Integer, Integer>> lookupTable
= Arrays.asList(MethodReferences::f3, MethodReferences::f4);
private int f3(int x) { return x * 2; }
private int f4(int x) { return x * 3; }
public void run() {
System.out.println(lookupTable.get(1).apply(this, 3));
}
public static void main(String[] args) {
MethodReferences testClass = new MethodReferences();
testClass.run();
}
}
output:
9

For instance method references which use the ClassName::functionName format, instead of instanceName::functionName, you also need to pass the specific instance of the class to the function when calling .apply().
This means that your method references are actually need to be a BiFunction<MethodReferences, Integer, Integer>, even though there is only one explicit parameter to the function.
When calling the method, you also need to pass this into apply:
import java.util.Arrays;
import java.util.List;
import java.util.function.BiFunction;
public class MethodReferences {
// To refer to non-static methods by class name,
// you must pass in the instance explicitly:
private final List<BiFunction<MethodReferences, Integer, Integer>> lookupTable = Arrays.asList(MethodReferences::f3, MethodReferences::f4);
private int f3(int x) {
return x * 2;
}
private int f4(int x) {
return x * 3;
}
public void run() {
// We need to pass this in, because it isn't implicit
// for ClassName::functionName references:
System.out.println(lookupTable.get(1).apply(3));
}
public static void main(String[] args) {
MethodReferences testClass = new MethodReferences();
testClass.run();
}
}

Is there any way to generate code in Interface on implementing it in a class

I want to add all the abstracts of getter and setter of a class to the interface that I am implementing in that particular interface. I also want to generate a final variable that resembles class variable. This variable can be used as string to access class variable after deserializing.
Eg:
public class Abc implements IAbc{
private String oneVariable;
public String getOneVariable(){
return oneVariable;
}
}
On implementing the above class with interface IAbc. IAbc should contain the following code:
public interface IAbc{
public static final String ONE_VARIABLE = "oneVariable";
public getOneVariable();
}
I have tried googling for the solution but could not get any. Also the methods in class should have the annotation #Override after this code is generated.

TL;DR This is an interesting programming challenge, but I find very little use for this in real life scenarios.
Here the name of the first string variable is known before hand, you can directly store the final value in this instead of the round-about way of storing the name of second variable.
If understand correctly you are trying to access a (String) variable of a class whose name will be in another string variable. This is possible using java reflection.
Additionally you want this code to be placed in an interface such that it can be (re)used in all the classes implementing it.
import java.lang.reflect.Field;
interface Foo {
public default String getStringVar()
throws NoSuchFieldException, IllegalAccessException {
// get varName
Class thisCls = this.getClass();
Field varNameField = thisCls.getField("varName");
String varName = (String) varNameField.get(this);
// get String variable of name `varName`
Field strField = thisCls.getField(varName);
String value = (String) strField.get(this);
return value;
}
}
class FooImpl1 implements Foo {
public final String varName = "str1";
public String str1 = "some value";
}
class FooImpl2 implements Foo {
public final String varName = "str2";
public String str2 = "some other value";
}
class Main {
public static void main(String[] args)
throws NoSuchFieldException, IllegalAccessException {
System.out.println(new FooImpl1().getStringVar());
System.out.println(new FooImpl2().getStringVar());
}
}
Here I have two String members in classes implementing interface Foo. The first varName contains the variable name of the second String, second String variable contains the data.
In the interface using reflection I am first extracting the variable name stored in varName, then using this extracting the value of second String.

How to pass string value from another class file for selenium script

I have one class where I putting all string value.
I have another class where i have to pass that value in driver.get("")
How I can do?

Suppose you have class A where string value kept and method that return value or you can simply make the variable public then you can access the String without mthod
class A
{
private static String a = "value to be passed";
public static String v= "another value to be pass;"
public static String getA(){
return a;
}
}
Another class that required string value
class ScriptTest{
pulic void method()
{
driver.sendKeys(A.getA());//call the method and get the string
driver.sendKeys(A.v);//diretly access to variable
}
}

You can implement like,
Class demo1
{
public static String xyz = "";
}
Another Class, which can directly retrieve it using extends to base class,
class demo2 extends demo1
{
public void testMethod()
{
driver.get(xyz);
}
}
This is the most preferable way for global access.

How can I refer the objcet that calls a method in the method in Java? [duplicate]

This question already has answers here:
When should I use "this" in a class?
(17 answers)
Closed 7 years ago.
I'm trying to get an understanding of what the the java keyword this actually does.
I've been reading Sun's documentation but I'm still fuzzy on what this actually does.

The this keyword is a reference to the current object.
class Foo
{
private int bar;
public Foo(int bar)
{
// the "this" keyword allows you to specify that
// you mean "this type" and reference the members
// of this type - in this instance it is allowing
// you to disambiguate between the private member
// "bar" and the parameter "bar" passed into the
// constructor
this.bar = bar;
}
}
Another way to think about it is that the this keyword is like a personal pronoun that you use to reference yourself. Other languages have different words for the same concept. VB uses Me and the Python convention (as Python does not use a keyword, simply an implicit parameter to each method) is to use self.
If you were to reference objects that are intrinsically yours you would say something like this:
My arm or my leg
Think of this as just a way for a type to say "my". So a psuedocode representation would look like this:
class Foo
{
private int bar;
public Foo(int bar)
{
my.bar = bar;
}
}

The keyword this can mean different things in different contexts, that's probably the source of your confusion.
It can be used as a object reference which refers to the instance the current method was called on: return this;
It can be used as a object reference which refers to the instance the current constructor is creating, e.g. to access hidden fields:
MyClass(String name)
{
this.name = name;
}
It can be used to invoke a different constructor of a a class from within a constructor:
MyClass()
{
this("default name");
}
It can be used to access enclosing instances from within a nested class:
public class MyClass
{
String name;
public class MyClass
{
String name;
public String getOuterName()
{
return MyClass.this.name;
}
}
}

"this" is a reference to the current object.
See details here

The keyword this is a reference to the current object. It's best explained with the following piece of code:
public class MyClass {
public void testingThis()
{
// You can access the stuff below by
// using this (although this is not mandatory)
System.out.println(this.myInt);
System.out.println(this.myStringMethod());
// Will print out:
// 100
// Hello World
}
int myInt = 100;
string myStringMethod()
{
return "Hello World";
}
}
It's not used a lot unless you have code standard at your place telling you to use the this keyword. There is one common use for it, and that's if you follow a code convention where you have parameter names that are the same as your class attributes:
public class ProperExample {
private int numberOfExamples;
public ProperExample(int numberOfExamples)
{
this.numberOfExamples = numberOfExamples;
}
}
One proper use of the this keyword is to chain constructors (making constructing object consistent throughout constructors):
public class Square {
public Square()
{
this(0, 0);
}
public Square(int x_and_y)
{
this(x_and_y, x_and_y);
}
public Square(int x, int y)
{
// finally do something with x and y
}
}
This keyword works the same way in e.g. C#.

An even better use of this
public class Blah implements Foo {
public Foo getFoo() {
return this;
}
}
It allows you to specifically "this" object in the current context. Another example:
public class Blah {
public void process(Foo foo) {
foo.setBar(this);
}
}
How else could you do these operations.

"this" keyword refers to current object due to which the method is under execution. It is also used to avoid ambiguity between local variable passed as a argument in a method and instance variable whenever instance variable and local variable has a same name.
Example ::
public class ThisDemo1
{
public static void main(String[] args)
{
A a1=new A(4,5);
}
}
class A
{
int num1;
int num2;
A(int num1)
{
this.num1=num1; //here "this" refers to instance variable num1.
//"this" avoids ambigutiy between local variable "num1" & instance variable "num1"
System.out.println("num1 :: "+(this.num1));
}
A(int num, int num2)
{
this(num); //here "this" calls 1 argument constructor within the same class.
this.num2=num2;
System.out.println("num2 :: "+(this.num2));
//Above line prints value of the instance variable num2.
}
}

The keyword 'this' refers to the current object's context. In many cases (as Andrew points out), you'll use an explicit this to make it clear that you're referring to the current object.
Also, from 'this and super':
*There are other uses for this. Sometimes, when you are writing an instance method, you need to pass the object that contains the method to a subroutine, as an actual parameter. In that case, you can use this as the actual parameter. For example, if you wanted to print out a string representation of the object, you could say "System.out.println(this);". Or you could assign the value of this to another variable in an assignment statement.
In fact, you can do anything with this that you could do with any other variable, except change its value.*
That site also refers to the related concept of 'super', which may prove to be helpful in understanding how these work with inheritance.

It's a reference of actual instance of a class inside a method of the same class.
coding
public class A{
int attr=10;
public int calc(){
return this.getA()+10;
}
/**
*get and set
**/
}//end class A
In calc() body, the software runs a method inside the object allocated currently.
How it's possible that the behaviour of the object can see itself? With the this keyword, exactly.
Really, the this keyword not requires a obligatory use (as super) because the JVM knows where call a method in the memory area, but in my opinion this make the code more readeable.

It can be also a way to access information on the current context.
For example:
public class OuterClass
{
public static void main(String[] args)
{
OuterClass oc = new OuterClass();
}
OuterClass()
{
InnerClass ic = new InnerClass(this);
}
class InnerClass
{
InnerClass(OuterClass oc)
{
System.out.println("Enclosing class: " + oc + " / " + oc.getClass());
System.out.println("This class: " + this + " / " + this.getClass());
System.out.println("Parent of this class: " + this.getClass().getEnclosingClass());
System.out.println("Other way to parent: " + OuterClass.this);
}
}
}

Think of it in terms of english, "this object" is the object you currently have.
WindowMaker foo = new WindowMaker(this);
For example, you are currently inside a class that extends from the JFrame and you want to pass a reference to the WindowMaker object for the JFrame so it can interact with the JFrame. You can pass a reference to the JFrame, by passing its reference to the object which is called "this".

Every object can access a reference to itself with keyword this (sometimes called the this
reference).
First lets take a look on code
public class Employee {
private int empId;
private String name;
public int getEmpId() {
return this.empId;
}
public String getName() {
return this.name;
}
public void setEmpId(int empId) {
this.empId = empId;
}
public void setName(String name) {
this.name = name;
}
}
In the above method getName() return instance variable name.
Now lets take another look of similar code is
public class Employee {
private int empId;
private String name;
public int getEmpId() {
return this.empId;
}
public String getName() {
String name="Yasir Shabbir";
return name;
}
public void setEmpId(int empId) {
this.empId = empId;
}
public void setName(String name) {
this.name = name;
}
public static void main(String []args){
Employee e=new Employee();
e.setName("Programmer of UOS");
System.out.println(e.getName());
}
}
Output
Yasir Shabbir
this operator always work with instance variable(Belong to Object)
not any class variable(Belong to Class)
this always refer to class non static attribute not any other parameter or local variable.
this always use in non static method
this operator cannot work on static variable(Class variable)
**NOTE:**It’s often a logic error when a method contains a parameter or local variable that has the
same name as a field of the class. In this case, use reference this if you wish to access the
field of the class—otherwise, the method parameter or local variable will be referenced.

What 'this' does is very simply. It holds the reference of current
object.
This keyword holds the reference of instance of current class
This keyword can not be used inside static function or static blocks
This keyword can be used to access shadowed variable of instance
This keyword can be used to pass current object as parameter in function calls
This keyword can be used to create constructor chain
Source: http://javaandme.com/core-java/this-word

Static method in a generic class?

In Java, I'd like to have something as:
class Clazz<T> {
static void doIt(T object) {
// ...
}
}
But I get
Cannot make a static reference to the non-static type T
I don't understand generics beyond the basic uses and thus can't make much sense of that. It doesn't help that I wasn't able to find much info on the internet about the subject.
Could someone clarify if such use is possible, by a similar manner? Also, why was my original attempt unsuccessful?

You can't use a class's generic type parameters in static methods or static fields. The class's type parameters are only in scope for instance methods and instance fields. For static fields and static methods, they are shared among all instances of the class, even instances of different type parameters, so obviously they cannot depend on a particular type parameter.
It doesn't seem like your problem should require using the class's type parameter. If you describe what you are trying to do in more detail, maybe we can help you find a better way to do it.

Java doesn't know what T is until you instantiate a type.
Maybe you can execute static methods by calling Clazz<T>.doit(something) but it sounds like you can't.
The other way to handle things is to put the type parameter in the method itself:
static <U> void doIt(U object)
which doesn't get you the right restriction on U, but it's better than nothing....

I ran into this same problem. I found my answer by downloading the source code for Collections.sort in the java framework. The answer I used was to put the <T> generic in the method, not in the class definition.
So this worked:
public class QuickSortArray {
public static <T extends Comparable> void quickSort(T[] array, int bottom, int top){
//do it
}
}
Of course, after reading the answers above I realized that this would be an acceptable alternative without using a generic class:
public static void quickSort(Comparable[] array, int bottom, int top){
//do it
}

I think this syntax has not been mentionned yet (in the case you want a method without arguments) :
class Clazz {
static <T> T doIt() {
// shake that booty
}
}
And the call :
String str = Clazz.<String>doIt();
Hope this help someone.

It is possible to do what you want by using the syntax for generic methods when declaring your doIt() method (notice the addition of <T> between static and void in the method signature of doIt()):
class Clazz<T> {
static <T> void doIt(T object) {
// shake that booty
}
}
I got Eclipse editor to accept the above code without the Cannot make a static reference to the non-static type T error and then expanded it to the following working program (complete with somewhat age-appropriate cultural reference):
public class Clazz<T> {
static <T> void doIt(T object) {
System.out.println("shake that booty '" + object.getClass().toString()
+ "' !!!");
}
private static class KC {
}
private static class SunshineBand {
}
public static void main(String args[]) {
KC kc = new KC();
SunshineBand sunshineBand = new SunshineBand();
Clazz.doIt(kc);
Clazz.doIt(sunshineBand);
}
}
Which prints these lines to the console when I run it:
shake that booty 'class com.eclipseoptions.datamanager.Clazz$KC' !!!
shake that booty 'class com.eclipseoptions.datamanager.Clazz$SunshineBand' !!!

It is correctly mentioned in the error: you cannot make a static reference to non-static type T. The reason is the type parameter T can be replaced by any of the type argument e.g. Clazz<String> or Clazz<integer> etc. But static fields/methods are shared by all non-static objects of the class.
The following excerpt is taken from the doc:
A class's static field is a class-level variable shared by all
non-static objects of the class. Hence, static fields of type
parameters are not allowed. Consider the following class:
public class MobileDevice<T> {
private static T os;
// ...
}
If static fields of type parameters were allowed, then the following code would be confused:
MobileDevice<Smartphone> phone = new MobileDevice<>();
MobileDevice<Pager> pager = new MobileDevice<>();
MobileDevice<TabletPC> pc = new MobileDevice<>();
Because the static field os is shared by phone, pager, and pc, what is the actual type of os? It cannot be Smartphone, Pager, and
TabletPC at the same time. You cannot, therefore, create static fields
of type parameters.
As rightly pointed out by chris in his answer you need to use type parameter with the method and not with the class in this case. You can write it like:
static <E> void doIt(E object)

Something like the following would get you closer
class Clazz
{
public static <U extends Clazz> void doIt(U thing)
{
}
}
EDIT: Updated example with more detail
public abstract class Thingo
{
public static <U extends Thingo> void doIt(U p_thingo)
{
p_thingo.thing();
}
protected abstract void thing();
}
class SubThingoOne extends Thingo
{
#Override
protected void thing()
{
System.out.println("SubThingoOne");
}
}
class SubThingoTwo extends Thingo
{
#Override
protected void thing()
{
System.out.println("SuThingoTwo");
}
}
public class ThingoTest
{
#Test
public void test()
{
Thingo t1 = new SubThingoOne();
Thingo t2 = new SubThingoTwo();
Thingo.doIt(t1);
Thingo.doIt(t2);
// compile error --> Thingo.doIt(new Object());
}
}

Since static variables are shared by all instances of the class. For example if you are having following code
class Class<T> {
static void doIt(T object) {
// using T here
}
}
T is available only after an instance is created. But static methods can be used even before instances are available. So, Generic type parameters cannot be referenced inside static methods and variables

When you specify a generic type for your class, JVM know about it only having an instance of your class, not definition. Each definition has only parametrized type.
Generics work like templates in C++, so you should first instantiate your class, then use the function with the type being specified.

Also to put it in simple terms, it happens because of the "Erasure" property of the generics.Which means that although we define ArrayList<Integer> and ArrayList<String> , at the compile time it stays as two different concrete types but at the runtime the JVM erases generic types and creates only one ArrayList class instead of two classes. So when we define a static type method or anything for a generic, it is shared by all instances of that generic, in my example it is shared by both ArrayList<Integer> and ArrayList<String> .That's why you get the error.A Generic Type Parameter of a Class Is Not Allowed in a Static Context!

#BD at Rivenhill: Since this old question has gotten renewed attention last year, let us go on a bit, just for the sake of discussion.
The body of your doIt method does not do anything T-specific at all. Here it is:
public class Clazz<T> {
static <T> void doIt(T object) {
System.out.println("shake that booty '" + object.getClass().toString()
+ "' !!!");
}
// ...
}
So you can entirely drop all type variables and just code
public class Clazz {
static void doIt(Object object) {
System.out.println("shake that booty '" + object.getClass().toString()
+ "' !!!");
}
// ...
}
Ok. But let's get back closer to the original problem. The first type variable on the class declaration is redundant. Only the second one on the method is needed. Here we go again, but it is not the final answer, yet:
public class Clazz {
static <T extends Saying> void doIt(T object) {
System.out.println("shake that booty "+ object.say());
}
public static void main(String args[]) {
Clazz.doIt(new KC());
Clazz.doIt(new SunshineBand());
}
}
// Output:
// KC
// Sunshine
interface Saying {
public String say();
}
class KC implements Saying {
public String say() {
return "KC";
}
}
class SunshineBand implements Saying {
public String say() {
return "Sunshine";
}
}
However, it's all too much fuss about nothing, since the following version works just the same way. All it needs is the interface type on the method parameter. No type variables in sight anywhere. Was that really the original problem?
public class Clazz {
static void doIt(Saying object) {
System.out.println("shake that booty "+ object.say());
}
public static void main(String args[]) {
Clazz.doIt(new KC());
Clazz.doIt(new SunshineBand());
}
}
interface Saying {
public String say();
}
class KC implements Saying {
public String say() {
return "KC";
}
}
class SunshineBand implements Saying {
public String say() {
return "Sunshine";
}
}

T is not in the scope of the static methods and so you can't use T in the static method. You would need to define a different type parameter for the static method. I would write it like this:
class Clazz<T> {
static <U> void doIt(U object) {
// ...
}
}
For example:
public class Tuple<T> {
private T[] elements;
public static <E> Tuple<E> of(E ...args){
if (args.length == 0)
return new Tuple<E>();
return new Tuple<E>(args);
}
//other methods
}