The document discusses closures in programming languages, providing examples of how closures work in JavaScript by allowing inner functions to access variables in an enclosing scope. It explains that while Java 8 introduced lambda expressions, lambda expressions in Java are not true closures since they do not close over variables in enclosing scopes. The document concludes that a closure is a function that closes over the environment in which it was defined, whereas a lambda is just an anonymous function.

1. Gance
淺談CLOSURE

3. Many of modern programming
languages supports Closure.

4. Is Java going to support Closure?

6. A closure (閉包) is a function
that has an environment of its
own

7. Closures are a technique for
implementing lexically scoped
name binding in languages with
first-class functions.

8. Closure Example 1
js> function doSome() {
> var x = 10;
> function f(y) {
> return x + y;
> }
> return f;
> }
js> var foo = doSome();
js> foo(20);
30
js> foo(30);
40

9. Closure Example 2
js> function doOther() {
> var x = 10;
> function f(y) {
> return x + y;
> }
> x = 100;
> return f;
> }
js> var foo = doOther();
js> foo(20);
120
js> foo(30);
130

10. Closure Example 3
js> function doOther() {
> var x = 10;
> function f() {
> x--;
> return x;
> }
> return f;
> }
js> var f1 = doOther();
js> var f2 = doOther();
js> f1();
9
js> f2();
9

11. More About Closure
 Closure是擁有閒置變數（Free variable）的運
算式。
 如果函式的閒置變數與當時語彙環境（Lexical
environment）綁定，該函式才稱為Closure。
 Closure關閉的是變數的reference，而不是變數
的value

12. Java 8 introduces the support
for Lambda. Is it Closure?

13. Lambda Expression
 A lambda expression is an “anonymous
function”
 a method without a declaration (access
modifier, return value declaration, and name)
 Syntax: (argument) -> (body)
(int a, int b) -> { return a + b; }
() -> System.out.println("Hello World");
(String s) -> { System.out.println(s); }
() -> 42
() -> { return 3.1415 };

14. Lambda Example 1
//Old way:
new Thread(new Runnable() {
@Override
public void run() {
System.out.println("Hello from thread");
}
}).start();
//New way:
new Thread(
() -> System.out.println("Hello from thread")
).start();

15. Lambda Example 2
//Old way:
button.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
System.out.println("The button was clicked using old fashion code!");
}
});
//New way:
button.addActionListener( (e) -> {
System.out.println("The button was clicked. From lambda expressions !");
});

16. Lambda Example 3
public static void main(String[] args) {
int number = 5;
ObjectCallback callback = () -> {
return (number = number + 1);
};
Object result = callback.Callback();
System.out.println(result);
}
number should be effectively final

17. Conclusion
 A closure is any function which closes over
the environment in which it was defined.
 A lambda is just an "anonymous function" - a
function defined with no name.
 Lambda in Java 8 is NOT closure!

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