Java8中的Lambda表达式

// Java8之前：旧的写法
Runnable runnable = new Runnable() {
  @Override
  public void run() {
    System.out.println("old run");
  }
};
Thread t = new Thread(runnable);
​
// Java8之后：新的写法
Runnable runnable1 = ()->{
  System.out.println("lambda run");
};
Thread t1 = new Thread(runnable1);

@FunctionalInterface
public interface FunctionInterfaceDemo {
    void abstractFun();
    default void fun1(){
        System.out.println("fun1");    
    }
    default void fun2(){
        System.out.println("fun2");
    }   
}

// 编译器会报错，Multiple non-overriding abstract methods found in XXX
@FunctionalInterface
public interface NoFunctionInterfaceDemo extends FunctionInterfaceDemo{
  void abstractFun2();
}

new Thread(() -> System.out.println("run")).start();

public static String processFile() throws IOException {
    // Java7新增的语法，try(){}，可自动关闭资源，减少了代码的臃肿
    try( BufferedReader bufferedReader = 
        new BufferedReader(new  FileReader("D:\\JavaProject\\JavaBasicDemo\\test.txt"))){
        return bufferedReader.readLine();
    }
}

@FunctionalInterface
interface FileReadInterface{
    // 这里接受一个BufferedReader对象，返回一个String对象
    String process(BufferedReader reader) throws IOException;
}

// 读取一行
FileReadInterface fileReadInterface = reader -> reader.readLine();
// 读取两行
FileReadInterface fileReadInterface2 = reader -> reader.readLine() + reader.readLine();
​

// 参数为第二步我们自己手写的函数式接口
public static String processFile(FileReadInterface fileReadInterface) throws IOException {
        try( BufferedReader bufferedReader =
                 new BufferedReader(new FileReader("./test.txt"))){
                    // 这里我们不再自己定义行为，而是交给函数式接口的抽象方法来处理，然后通过lambda表达式的传入来实现多个行为
          return fileReadInterface.process(bufferedReader);
        }
    }

public class FileReaderDemo {
    public static void main(String[] args) throws IOException {
				// 第三步： 
      	// lambda表达式1 传给 函数式接口：只读取一行
      	FileReadInterface fileReadInterface = reader -> reader.readLine();
				// lambda表达式2 传给 函数式接口：只读取两行
      	FileReadInterface fileReadInterface2 = reader -> reader.readLine() + reader.readLine();
      	// 最后一步： 不同的函数式接口的实现，表现出不同的行为
        String str1 = processFile(fileReadInterface);
        String str2 = processFile(fileReadInterface2);
        System.out.println(str1);
        System.out.println(str2);
    }
  	// 第四步： 读取文件方法，接受函数式接口作为参数
    public static String processFile(FileReadInterface fileReadInterface) throws IOException {
        try( BufferedReader bufferedReader =
                 new BufferedReader(new FileReader("./test.txt"))){
					// 调用函数式接口中的抽象方法来处理数据					
          return fileReadInterface.process(bufferedReader);
        }
    }
	// 第一步：
  public static String processFile() throws IOException {
        try( BufferedReader bufferedReader =
                 new BufferedReader(new FileReader("./test.txt"))){
          return bufferReader.readLine();
        }
    }


}

// 第二步： 我们手写的函数式接口
@FunctionalInterface
interface FileReadInterface{
    String process(BufferedReader reader) throws IOException;
}

@FunctionalInterface
public interface Function<T, R> {
	// 都是接受一个参数，返回另一个参数
  R apply(T t);
}

// 简化前
Function<Cat, Integer> function = c->c.getAge();
// 简化后
Function<Cat, Integer> function2 = Cat::getAge;

public class ReferenceDemo {
    public static void main(String[] args) {
        // 第一种:引用对象的实例方法
        Cat cat = new Cat(1);
        Function<Cat, Integer> methodRef1 = cat::getSum; 
        // 第二种:引用类的静态方法
        Supplier<Integer> methodRef2 = Cat::getAverageAge;
        // 第三种：引用类的实例方法
        Function<Cat, Integer> methodRef3 = Cat::getAge;
    }
}
class Cat {
    int age;

    public Cat(int age) {
        this.age = age;
    }

    // 获取猫的平均年龄
    public static int getAverageAge(){
        return 15;
    }
    // 获取两只猫的年龄总和
    public int getSum(Cat cat){
        return cat.getAge() + this.getAge();
    }

    public int getAge() {
        return age;
    }    public void setAge(int age) {
        this.age = age;
    }
}

// 这里调用 new Cat()
Supplier<Cat> constructRef1 = Cat::new;
// 这里调用 new Cat(Integer)
Function<Integer, Cat> constructRef2 = Cat::new;

public static void main(String[] args) {
  String str = "javalover.cc";
  Runnable runnable = ()->{
    str = "1";// 这里会报错，因为修改了str引用的指向
    System.out.println(str);
  }
}

// 重点代码
public class ComposeDemo {
    public static void main(String[] args) {
        List<Dog> list = Arrays.asList(new Dog(1,2), new Dog(1, 1));
        // 1. 先按年龄排序（默认递增）
      	// Dog::getAge, 上面介绍的方法引用
      	// comparingInt, 是Comparator的一个静态方法，返回Comparator<T>
      	Comparator<Dog> comparableAge = Comparator.comparingInt(Dog::getAge);
        // 2. 如果有相同的年龄，则年龄相同的再按体重排序（如果年龄已经比较出大小，则下面的体重就不会再去比较）
        Comparator<Dog> comparableWeight = Comparator.comparingInt(Dog::getWeight);;
        // 3. 调用list对象的sort方法排序，参数是Comparator<? super Dog>
        list.sort(comparableAge.thenComparing(comparableWeight));
        System.out.println(list);
    }
}
// 非重点代码
class Dog{
    private int age;
    private int weight;

    public Dog(int age, int weight) {
        this.age = age;
        this.weight = weight;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public int getWeight() {
        return weight;
    }

    public void setWeight(int weight) {
        this.weight = weight;
    }

    @Override
    public String toString() {
        return "Dog{" +
                "age=" + age +
                ", weight=" + weight +
                '}';
    }
}