Anonymous
Anonymous inner classes make it easier to pass around code as data.
/**
* Anonymous inner classes make it easier to pass around code as data.
*
* Example:
* Using an anonymous inner class to associate behavior with a button click.
*
* There are four lines of boilerplate code in order to call ...
* one line of important logic.
*/
package com.minte9.lambdas.expressions;
import javax.swing.JButton;
import javax.swing.JFrame;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class Anonimous {
public static void main(String[] args) {
JFrame frame = new JFrame();
JButton button = new JButton("Click me");
button.addActionListener(
new ActionListener() { // Look Here
@Override
public void actionPerformed(ActionEvent e) {
System.out.println("Button clicked");
}
}
);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.getContentPane().add(button);
frame.setSize(300, 300);
frame.setVisible(true);
}
}
Behavior
Starting with Java 8 we can use lambdas expressions, to pass behavior not objects.
/**
* Instead of passing an object we are passing a block of code.
*
* We pass a function without a name.
* Javac is inferring the type of event from its context.
*/
package com.minte9.lambdas.expressions;
import javax.swing.JButton;
import javax.swing.JFrame;
public class Behavior {
public static void main(String[] args) {
JFrame frame = new JFrame();
JButton button = new JButton("Click me");
button.addActionListener(
event -> System.out.println("Button clicked") // Look Here
);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.getContentPane().add(button);
frame.setSize(300, 300);
frame.setVisible(true);
}
}
Lambdas
There are different ways of writing lambda expressions.
/**
* There are different ways of writing lambda expressions.
*
* Lambda expression with no arguments.
* Lambda expression in a full block.
* Lambda expression with more arguments.
*
* Method parameters require less boilerplate but still ...
* they are still statically typed!
*
*/
package com.minte9.lambdas.expressions;
import java.util.function.BinaryOperator;
public class Lambdas {
public static void main(String[] args) {
Runnable noArgs = () -> System.out.println("Hello World");
new Thread(noArgs).run();
// Hello World
Runnable multiStatement = () -> {
System.out.println("Hello");
System.out.println("World");
};
new Thread(multiStatement).run();
// Hello
// World
BinaryOperator<Integer> op = (x, y) -> x + y;
System.out.println(op.apply(2, 3));
// 5
}
}
Final Variable
To use a variable from outside an anonymous inner class it must be final.
/**
* To use a variable from outside an anonymous inner class the variable ...
* must be final (or effective final).
*
* The Holder variable is effective final,
* even if the final keyword it is not required (Java 8)
*/
package com.minte9.lambdas.expressions;
import java.util.Arrays;
import javax.xml.ws.Holder;
public class FinalVariable {
public static void main(String[] args) {
Holder<Integer> holder = new Holder<>(0);
Arrays.asList("a", "b", "c").forEach((x) -> {
holder.value ++;
});
System.out.println(holder.value);
// 3
final String str = "A";
Arrays.asList("a", "b", "c").forEach((x) -> {
// str = str + x;
// Error: final local variable cannot be assigned
});
System.out.println(str);
// A
}
}
Type inference
Javac type inference is smart, but it needs enought information.
/**
* Javac type inference is smart, but it needs enought information.
*/
package com.minte9.lambdas.expressions;
import java.util.function.Function;
public class TypeInference {
public static void main(String[] args) {
// Wrong - doesn't compile
/*
Function op = x -> x + 1;
*/
// Correct
Function<Long, Long> op = x -> x + 1;
System.out.println(op.apply(20L)); // 21
}
}
Last update: 451 days ago
