While i'm writing some code in intellij, it keeps popping up, says
Anonymous class can be replaced with lambda?
So what does that mean? what is lambda expression and how Anonymous class can be replaced with lambda?
Play.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
JPanel playscreen = new JPanel();
playscreen.setBackground(Color.WHITE);
p.add(playscreen);
}
});
Anonymous new can be replaced with lambda ?
It's not 100% correct, Anonymous class for an interfaces having one abstract method can be replaced with lambda expression (which is called functional interface)
Since ActionListener has only one method actionPerformed(ActionEvent e) you can write it using lambda expression
Play.addActionListener((event) -> {
JPanel playscreen = new JPanel();
playscreen.setBackground(Color.WHITE);
p.add(playscreen);
});
Lambda Expressions
One issue with anonymous classes is that if the implementation of your anonymous class is very simple, such as an interface that contains only one method, then the syntax of anonymous classes may seem unwieldy and unclear. In these cases, you're usually trying to pass functionality as an argument to another method, such as what action should be taken when someone clicks a button. Lambda expressions enable you to do this, to treat functionality as method argument, or code as data.
FunctionalInterface
A functional interface has exactly one abstract method. Since default methods have an implementation, they are not abstract. If an interface declares an abstract method overriding one of the public methods of java.lang.Object, that also does not count toward the interface's abstract method count since any implementation of the interface will have an implementation from java.lang.Object or elsewhere.
Your code can be changed to this:
Play.addActionListener((event) -> {
JPanel playscreen = new JPanel();
playscreen.setBackground(Color.WHITE);
p.add(playscreen);
});
The reason you can do this is because ActionListener is a functional interface, meaning that it has exactly one abstract method that you must implement. A lambda is able to essentially take the place of that single function and act as an implementation of that interface.
The Lambda acts like the implementation of the method actionPerformed and (event) is the argument to that method and the -> { ... } is the body of that method.
Related
This question already has answers here:
Java8 Lambdas vs Anonymous classes
(9 answers)
Closed 3 years ago.
I have a button and I want a text to be displayed when I hover the mouse over the button.
Both of these work but can they be used interchangeably?
// First
EventHandler<MouseEvent> eventHandler = new EventHandler<MouseEvent>() {
#Override
public void handle(MouseEvent e) {
System.out.println("Hello World");
}
};
button.setOnMouseEntered(eventHandler);
// Second. Lambda expression.
button.setOnMouseEntered(e -> {
System.out.println("Hello ");
});
I don't quite understand the first one. I suppose that EventHandler<MouseEvent> is the type and eventHandler is the name of the object. Right?
Yes, they both will execute the same code.
The first one is an so-called anonymous class. This means that you override the methods (handle in this case) of the inherited interface directly without having to do that with additional classes/interfaces.
You then pass the handler instance to the setOnMouseEntered method.
The lambda expression does this also but since the method requires a certain object type, you can write it shorter with the lambda expression.
e is basically the same as eventHandler
The first one is an anonymous inner class, where the handle method is overridden. This was the way to do listeners and handlers before lambda expressions. Now, if the class is an functional interface (only 1 abstract method, like the EventHandler in your case with its only method, handle) then a lambda expression can be used instead of anonim class.
The two methods are indeed the same, but using lambdas are preferred, because the code will be much more compact and easier to understand.
Both of the answers already provided are good, but I wanted to add another consideration.
Whenever possible, lambdas are preferable, not only because they're easier to read, but they also result in very different bytecode when compiled.
In the first example, the compiler creates a separate class (for example: OuterClass$1). This class has to be instantiated on each invocation, which will have to be garbage collected at some point.
In the second example, the lambda is compiled to a method that's local to the containing class. This means that it's a just a simple method invocation.
Depending on how frequently it's invoked, this may be an important consideration.
I believe the first example could be explicitly written as:
private class MouseHandler implements EventHandler<MouseEvent> {
public void handle(MouseEvent e) {
System.out.println("Hello World");
}
}
button.setOnMouseEntered(new MouseHandler());
This can be reduced via the anonymous class and more by the lambda expression. My understanding is that this is only possible because EventHandler is a functional interface.
What is a functional interface in Java SKO
I often see people write inner classes for listeners take swing for example.
button.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
//some action
}
});
At my uni there is a lot of emphasis on designs being decoupled using design patterns such as MCV or MVP.
When would I want to use an inner class over a decoupled way of doing it?
My only guess is when objects don't need to interact? (Or laziness??)
(Student)
This is actually called an anonymous inner class. You would use it if you only need to use the class once.
It would also reduce the size of your code unless it overrides many features of course (personally I prefer anonymous classes to be small in size for readability).
This Kind of Anonymous Inner Classes are called argument-defined inner classes,and automatically instantiated as part of the method invocation.
Here in the method argument, we get an object of a class,which is the subclass of the ActionListener Interface,but the newly created class is without any name(anonymous) so we cannot create its object again after the control passes through this statement.
now in the anonymous inner class which implements ActionPerformed Interface ,we override the actionPerformed() method which is very specific to that button and then new keyword creates an object of this anonymous class and it will automatically passed as argument.
or you may also create a new class(say myListener) which extends ActionListener class and overrides its actionPerformed() method. and then create a new object of myListener and pass it to the addActionListener()method as argument.
if you use metohod 2. you can see if you have 10 buttons and each have to perform different task ,then you have to create 10 ActionPerformed implementor classes and
pass object to the addActionListener() method of these implementor classes.
It is totally foolish to use this approach to create a whole new class just for a single object.
That's why most Listener are passed as an object of anonymous inner classes.
I’m reading an article about inner class. I found an example that demonstrates anonymous inner class (mentioned below).
button1 = new JButton();
button2 = new JButton();
...
button1.addActionListener(
new java.awt.event.ActionListener()
{
public void actionPerformed(java.awt.event.ActionEvent e)
{
// do something
}
}
);
According to the example it creates an inner class for responding to a button using ActionListener interface. As I know an interface does not have a constructor. But I’m wondering, how they call a constructor.
"new java.awt.event.ActionListener(){
}"
An interface does not have a constructor, but an anonymous class does: like all classes, it extends java.lang.Object implicitly, therefore it can call the Object's parameterless constructor.
Moreover, Object's constructor is the only constructor you could call, because anonymous classes cannot define custom constructors.
Of course in addition to extending java.lang.Object your anonymous class implements ActionListener - that's why you can pass it to the addActionListener method.
you are constructing a subclass.
Have a quick look at the Java Specification - specifically the Default Constructor Section. You get a constructor because when you instantiate an instance of an interface it would be an Object.
Quote from the spec:
If a class contains no constructor declarations, then a default
constructor with no formal parameters and no throws clause is
implicitly declared.
Anonymous inner class: An inner class with no name.
Now the only detail here we care about is, this new class should subType the interface and for that we provide the necessary method implementations.
The constructor for this class is a default one and performs the job well because there are no instance variables associated.
new java.awt.event.ActionListener(){ }
This statement creates an anonymous class object that implements ActionListener interface.
That is you are invoking anonymous class default constructor not the interface one.
According to java docs
The anonymous class expression consists of the following:
1.The new operator
2.The name of an interface to implement or a class to extend.
3.Parentheses that contain the arguments to a constructor, just like a normal class instance creation expression. Note: In the case of implementing an interface, there is no constructor, so you use an empty pair of parentheses.
How are Anonymous (inner) classes used in Java?
http://www.programmerinterview.com/index.php/java-questions/java-anonymous-class-example/
You are not instantiating an interface. You are asking the compiler to create an anonymous class implementing that interface and immediately create an instance of this class.
The best way to demonstrate this is to go to the "class" directory. You will find files of the form className$1.class, className$2.class, etc. These files correspond to those anonymous classes. If you were instantiating the interface itself, there would be no need for these new class files (and the anonymous classes they contain, of course).
That is how Anonymous Classes are(syntax wise).
According to the docs
The anonymous class expression consists of the following:
The new operator
The name of an interface to implement or a class to extend.
Parentheses that contain the arguments to a constructor, just like a
normal class instance creation expression. Note: In the case of
implementing an interface, there is no constructor, so you use an
empty pair of parentheses.
A body, which is a class declaration body. More specifically, in the
body, method declarations are allowed but statements are not.
Is it possible to polymorphically pass a function to an object array in Java?
I'm trying to create an event-based system, and interfaces simply isn't as versatile as I would like it to be.
As a result, I would like to be able to pass any function to the event manager, and have it execute the function on next iteration.
I would like to be able to pass any function to the event manager
Wouldn't we all. Unfortunately, Java doesn't have first-class functions or even function pointers. So we're stuck with interfaces and anonymous classes, until Java 8 at least.
Why not have your event manager execute Runnables, and wrap your method call with one? (This is the approach Swing took.)
public class Foo {
public void doSomething(){
System.out.println("Hello");
}
}
EventQueue.doLater(new Runnable(){
public void run(){
new Foo().doSomething();
}
}
I'm not sure why Interfaces aren't versatile enough for you - you can do anything you like with interfaces, certainly as much as you could do with passing pure functions.
Typically you would just define a single event handler interface:
public interface EventHandler {
public ReturnType handle(ParamType param);
}
Then you could implement that interface with any number of different handler classes. The handler objects can even contain their own state if that is useful.
Anything you define that implements the interface can then be used polymorphically, e.g. stored in an array so that they can be called in response to a specific event..
Java doesn't support first-class functions.
But you can do almost that by using anonymous classes or using EventHandler class.
This is from Java Docs.
The simplest use of EventHandler is to install a listener that calls a method on the target object with no arguments. In the following example we create an ActionListener that invokes the toFront method on an instance of javax.swing.JFrame.
myButton.addActionListener(
(ActionListener)EventHandler.create(ActionListener.class, frame, "toFront"));
When myButton is pressed, the statement frame.toFront() will be executed. One could get the same effect, with some additional compile-time type safety, by defining a new implementation of the ActionListener interface and adding an instance of it to the button:
//Equivalent code using an inner class instead of EventHandler.
myButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
frame.toFront();
}
});
In Java, the standard way to create an object is using
MyClass name = new MyClass();
I also often see the construct
new MyClass() { /*stuff goes in here*/ };
I've been looking online for a while and can't find a good explanation of what the second construct style does or how it does it.
Can someone please explain how and why you would use the second construct?
This construct makes actually two things: 1) It declares an anonymous class which extends the class you use in the constructor and 2) creates an instance of this anonymous class.
Edit: When using such a construct you can observe the anonymous class by looking at the generated .class files. There is the normal MyClass.class file and another one for each anonymous subclass: MyClass$1.class for the first and so on.
You would use the second construct in the case that you want to make an anonymous class. if you have a method that takes a callback as an argument, you might want to specify the implementation of the callback inline as opposed to giving it a name and putting it in a separate file or declaring it elsewhere in the same file.
There's also a trick called double brace initialization where you can get around not having syntax for literal maps and lists by using anonymous classes, like this:
Map map = new HashMap() {{put("foo", 1); put("bar", 2);}};
Here the nested braces create an instance initializer. The object bound to map is not a HashMap, its class is an anonymous class extending HashMap. (That means if you have a PMD rule about classes needing to declare serial uids then it will complain about this.)
Double-brace initialization is a fun trick to know but don't use it in real code. It's not safe to pass around the map created like this, because the inner object keeps a reference to the outer instance so if anything in the program holds onto a reference to the map it keeps the outer object from getting garbage-collected. There are also problems with serialization.
As others have already said, it creates an instance of an anonymous class, subclassing Class. Here's an example how it is commonly used:
panel.addMouseListener(
new MouseAdapter () {
#Override
public void mouseEntered(MouseEvent e) {
System.out.println(e.toString());
}
}
);
The above code creates an instance of an anonymous class which extends MouseAdapter. In the anonymous class the method mouseEntered has been overridden to demonstrate that the anonymous class works basically as any other class. This is very convenient and common way to create (usually simple) listeners.
Second construction creates an instance of anonymous class which is a subclass of Class.
If you want to new a object by a protect constructor from another package, you can use:
new Foo() {};
otherwise you will get an access error. It equals anonymous subclass inherited from Foo class.
From jdk8 onwards you may have seen different syntax seems like creating an objects while using lambda expressions.
NOTE: Lambda expressions don't get translated into anonymous inner classes, they use invoke dynamic that was introduced in Java 7 to execute functional methods.
For Example:
public class LambdaSample {
public static void main(String[] args) {
//If implementation is only one statement then {} braces are optional
Runnable oneLineImplRunnable = ()->System.out.println("This is one line lambda expression");
//Multiple statements in the implementation then {} braces are mandatory
Comparator<StudentTest> stdComparator = (StudentTest s1,StudentTest s2)->{
if(s1.getFirstName().equals(s2.getFirstName())) {
return s1.getLastName().compareTo(s2.getLastName());
}else {
return s1.getFirstName().compareTo(s2.getFirstName());
}
};
}
}