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java 8 lambda 教程

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lambda

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java 8 lambda Concise syntax – More succinct and clear than anonymous inner classes • Deficiencies with anonymous inner classes – Bulky, confusion re “this” and naming in general, no nonfinal vars, hard to optimize • Convenient for new streams library – shapes.forEach(s -> s.setColor(Color.RED)); • Similar constructs used in other languages – Callbacks, closures, map/reduce idiom • Step toward true functional programming

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Lambda Expressions

in Java 8: Part 1 – Basics

Originals of Slides and Source Code for Examples:

http://www.coreservlets.com/java-8-tutorial/

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Ed), & this tutorial. Available at public venues, or

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Topics in This Section

• Intro

– Motivation

– Quick summary of big idea

• New option: lambdas

– Interpretation

– Most basic form

– Type inferencing

– Expression for body

– Omitting parens

– Comparing lambda approaches to alternatives

– Using effectively final variables

– @FunctionalInterface

– Method references

– The java.util.function package

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Motivation and

Overview

Many Languages Let You Pass

Functions Around

• Dynamically (and usually weakly) typed

– JavaScript, Lisp, Scheme, etc.

• Strongly typed

– Ruby, Scala, Clojure, ML, etc.

• Functional approach proven concise,

useful, and parallelizable

– JavaScript sorting

• var testStrings = ["one", "two", "three", "four"];

• testStrings.sort(function(s1, s2) {

return(s1.length - s2.length);});

• testStrings.sort(function(s1, s2) {

return(s1.charCodeAt(s1.length - 1) –

s2.charCodeAt(s2.length - 1));});

Why Lambdas in Java Now?

• Concise syntax

– More succinct and clear than anonymous inner classes

• Deficiencies with anonymous inner classes

– Bulky, confusion re “this” and naming in general, no non-

final vars, hard to optimize

• Convenient for new streams library

– shapes.forEach(s -> s.setColor(Color.RED));

• Similar constructs used in other languages

– Callbacks, closures, map/reduce idiom

• Step toward true functional programming

– Real functions and mutable local vars perhaps in future

Main Advantage of Lambdas:

Concise and Expressive

• Old

– button.addActionListener(

new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

blah();

}

});

• New

– button.addActionListener(e -> blah());

Vigorous writing is concise... This requires not that the writer make all sentences short, or avoid all details

and treat subjects only in outline, but that every word should tell. -- Strunk and White, The Elements of Style

Corollary Advantages:

Support New Way of Thinking

• Encourage functional programming

– When functional programming approach is used, many

classes of problems are easier to solve and result in code

that is clearer to read and simpler to maintain.

• Support streams

– Streams are wrappers around collections that use

lambdas, support map/reduce, use lazy evaluation, and

can be made parallel automatically by compiler.

• Cannot be made concurrent automatically

– for(Shape s: shapes) { s.setColor(Color.RED); }

• Can be automatically turned into concurrent code

– shapes.forEach(s -> s.setColor(Color.RED));

» Streams are covered in a separate tutorial section

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Lambdas:

Most Basic Form

Big Idea

• You write what looks like a function

• Arrays.sort(testStrings, (s1, s2) -> s1.length() - s2.length());

• taskList.execute(() -> downloadSomeFile());

• someButton.addActionListener(event -> handleButtonClick());

• double d = MathUtils.integrate(x -> x*x, 0, 100, 1000);

• You get an instance of a class that implements

the interface that was expected in that place

– The expected type must be an interface that has exactly one

(abstract) method

• Called “Functional Interface” or “Single Abstract Method (SAM)

Interface”

– The designation of a single ABSTRACT method is not redundant,

because in Java 8 interfaces can have concrete methods, called

“default methods”. Java 8 interfaces can also have static methods.

Both default methods and static methods are covered in later section.

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