148 lines
3.2 KiB
Markdown
148 lines
3.2 KiB
Markdown
## 使用Lambda表达式避免重复啰嗦代码
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```java
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// Java8之前比较两个苹果重量
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Collections.sort(inventory, new Comparator<Apple>) {
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public int compare(Apple a1, Apple a2){
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return a1.getWeight().compareTo(a2.getWeight());
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}
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});
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// Java8之后
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invetory.sort(comparing(Apple::getWeight));
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```
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通过stream流，支持许多处理数据的并行操作
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### 流处理
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linux中的流
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```bash
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# cat命令把两个文件连接起来创建一个流
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# tr装换流中的字符串
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# sort对流进行排序
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# tail -3 给出流最后三行
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# 通过|管道符连接在一起
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cat file1 file2 | tr "[A-Z]" "[a-z]" | sort | tail -3
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```
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### 行为参数化传递代码给方法
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把方法作为参数传递给另一个方法
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### 并行与共享的可变数据
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避免使用synchronized打破“不能有共享的可变数据”
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## 函数
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传统意义方法和类为二等公民，通过运行时传递将方法变为一等公民
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### 方法和Lambda作为一等公民
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方法引用
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```java
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// 筛选隐藏文件
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File[] hiddenFiles = new File(".").listFiles(new FileFilter() {
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public boolean accept(File file) {
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return file.isHidden();
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}
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})
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// Java8
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File[] hiddenFiles = new File(".").listFiles(File::isHidden);
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```
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Lambda函数
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(int x) -> x+1
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### 传递代码
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```java
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// 筛选绿色苹果
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public static List<Apple> fileterGreenApples(List<Apple> inventory) {
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List<Apple> resutl = new ArrayList<>();
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for (Apple apple: inventory) {
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if ("green".equals(apple.getColor())) {
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result.add(apple);
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}
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}
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return result;
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}
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// 筛选重的苹果
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public static List<Apple> fileterHeavyApples(List<Apple> inventory) {
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List<Apple> resutl = new ArrayList<>();
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for (Apple apple: inventory) {
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if (apple.getWeight() > 150) {
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result.add(apple);
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}
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}
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return result;
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}
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// Java8
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public static boolean isGreenApple(Apple apple) {
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return "green".equals(apple.getColor());
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}
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public static boolean isHeavyApple(Apple apple) {
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return apple.getWeight() > 150;
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}
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public interface Predicate<T>{
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boolean test(T t);
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}
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static List<Apple> filterApples(List<Apple> inventory,
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predicate<Apple> p) {
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List<Apple> resutl = new ArrayList<>();
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for (Apple apple: inventory) {
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if (p.test(apple)) {
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result.add(apple);
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}
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}
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return result;
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}
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filterApples(inventory, Apple::isGreenApple);
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filterApples(inventory, Apple::isHeavyApple);
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```
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### 从传递方法到Lambda
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匿名函数
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```java
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filterApples(inventory, (Apple a) -> "green".equals(a.getColor()));
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filterApples(inventory, (Apple a) -> a.getColor() > 150);
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filterApples(inventory, (Apple a) -> a.getColor() < 80 ||
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"brown".equals(a.getColor()));
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```
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## 流
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```java
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// 顺序处理：
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import static java.util.stream.Collectors.toList;
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List<Apple> heavyApples=
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inventory.stream().filter((Apple a)-> a.getWeight() > 150)
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.collect(toList());
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//并行处理：
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import static java.util.stream.Collectors.toList;
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List<Apple> heavyApples=
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inventory.parallelStream().filter((Apple a)-> a.getWeight() > 150)
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.collect(toList());
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```
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## 默认方法
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支持库设计师，写出更容易改进的接口。
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给接口添加一个新方法，意味着所有的实体类都必须为其提供一个实现
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## 函数式编程的其他好思想
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通过使用更多描述性数据类型来避免null
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结构（模式）匹配 |