Java_通过行为参数化传递代码

通过行为参数化传递代码

把一个行为（一段代码）封装起来，并通过传递和使用创建的行为（例如对Apple的不同谓词）将方法的行为参数化。

// 定义Apple类
public class Apple{
private Integer weight = null;
private String color = null;
public Apple(Integer weight, String color){
this.weight = weight;
this.color = color;
}
public Integer getWeight(){
return this.weight;
}
public String getColor(){
return this.color;
}
}

① 将筛选条件抽象化，作为参数进行传递

// 定义一个接口来对选择标准建模
public interface ApplePredicate{
boolean test (Apple apple);
}

// 用ApplePredicate的多个实现代表不同的选择标准
public class AppleHeavyWeightPredicate implements ApplePredicate{
public boolean test(Apple apple){
return apple.getWeight() > 150;
}
}

public class AppleGreenColorPredicate implements ApplePredicate{
public boolean test(Apple apple){
return "green".equals(apple.getColor());
}
}

public class AppleRedAndHeavyPredicate implements ApplePredicate{
public boolean test(Apple apple){
return "red".equals(apple.getColor())
&& apple.getWeight() > 150;
}
}

// 根据抽象条件筛选
public static List<Apple> filterApples(List<Apple> inventory, ApplePredicate p){
List<Apple> result = new ArrayList<Apple>();
for(Apple apple: inventory){
if(p.test(apple)){
result.add(apple);
}
}
return result;
}

// 传递代码
List<Apple> inventory = Arrays.asList(new Apple(80,"green"),
new Apple(155, "green"),
new Apple(120, "red"));
List<Apple> heavyApples = filterApples(inventory, new AppleHeavyWeightPredicate());
List<Apple> greenApples = filterApples(inventory, new AppleGreenColorPredicate());

② 将输出信息抽象化，作为参数传递

// 定义一个接口来对输出信息建模
public interface AppleFormatter{
String accept (Apple apple);
}

public class AppleFancyFormatter implements AppleFormatter{
public String accept(Apple apple){
String characteristic = apple.getWeight() > 150 ? "heavy" : "light";
return "A " + characteristic + " " + apple.getColor() +" apple";
}
}

public class AppleSimpleFormatter implements AppleFormatter{
public String accept(Apple apple){
return "An apple of " + apple.getWeight() + "g";
}
}

public static void prettyPrintApple(List<Apple> inventory, AppleFormatter formatter){
for(Apple apple: inventory){
String output = formatter.accept(apple);
System.out.println(output);
}
}

public void main(String[] args){
__1__ my = new __1__();
List<Apple> inventory = new ArrayList<Apple>();
my.prettyPrintApple(inventory, new AppleFancyFormatter());
}

③ 使用匿名类

List<Apple> redApples = filterApples(inventory, new ApplePredicate() {
public boolean test(Apple apple){
return "red".equals(apple.getColor());
}
});

④ 使用Lambda表达式

List<Apple> result = filterApples(inventory, (Apple apple) -> "red".equals(apple.getColor()));

⑤ 将List类型抽象化

public interface Predicate<T>{
boolean test(T t);
}

public static <T> List<T> filter(List<T> list, Predicate<T> p){
List<T> result = new ArrayList<T>();
for(T e: list){
if(p.test(e)){
result.add(e);
}
}
return result;
}

List<Apple> redApples = filter(inventory, (Apple apple) -> "red".equals(apple.getColor()));
List<Integer> evenNumbers = filter(numbers, (Integer i) -> i % 2 == 0);

⑥ 例子：用Comparator 来排序

// java.util.Comparator
public interface Comparator<T> {
public int compare(T o1, T o2);
}

// 给sort()函数传递一个匿名类
inventory.sort(new Comparator<Apple>() {
public int compare(Apple a1, Apple a2){
return a1.getWeight().compareTo(a2.getWeight());
}
});

// 给sort()函数传入一个Lambda表达式
inventory.sort( (Apple a1, Apple a2) -> a1.getWeight().compareTo(a2.getWeight()));

⑦ 例子：用Runnable 执行代码块

// java.lang.Runnable
public interface Runnable{
public void run();
}

// 使用这个接口创建执行不同行为的线程
Thread t = new Thread(new Runnable() {
public void run(){
System.out.println("Hello world");
}
});

// 用Lambda表达式的话，看起来是这样
Thread t = new Thread(() -> System.out.println("Hello world"));

⑧ 例子：GUI 事件处理

Button button = new Button("Send");

button.setOnAction(new EventHandler<ActionEvent>() {
public void handle(ActionEvent event) {
label.setText("Sent
a1c0
!!");
}
});

// Lambda表达式
button.setOnAction((ActionEvent event) -> label.setText("Sent!!"));