J2SE 8的Lambda --- 语法

语法例子

		LambdaGrammarTest lambdaTest = new LambdaGrammarTest();

		// 1. 能够推导出类型的，可以不写类型
		String[] planets = new String[] { "11", "22", "33" };
		Arrays.sort(planets, (first, second) -> first.length() - second.length());

		Arrays.sort(planets, String::compareToIgnoreCase);

		// 2. 删除list中所有的null值, Predicate 接口
		ArrayList<String> arrayList = new ArrayList<String>();
		arrayList.removeIf(x -> x == null);

		// 3. BiFunction<T, U, R>
		// T - 表示第一个参数, U - 表示第二个参数, R - 表示返回结构result
		BiFunction<String, String, Integer> comp = (first, second) -> first.length() - second.length();
		System.out.println(comp.apply("11", "222"));

		java7_biFunction();
		java8_biFunction();

		// 4. ::
		// (1) 对象方法 object::instanceMethod 等同于 x->object.instanceMethod(x);
		// (x,y)->object.instanceMethod(x,y);
		String string = new String("123456");

		MyFunction3 myFunction3 = string::substring;
		System.out.println(myFunction3.test(2));

		MyFunction4 myFunction4 = string::substring;
		System.out.println(myFunction4.test(2, 5));

		// (2) 静态方法 Object::staticMethod 等同于 x->Object.staticMethod(x);
		// (x,y)->Object.staticMethod(x,y);
		MyFunction5 myFunction5 = System.out::println;
		myFunction5.test("*******");

		MyNumeric2 myNumeric2 = Math::max;
		System.out.println(myNumeric2.test(2, 5));

		// (3) 对象方法 Object::instanceMethod, 等同于 x,y->x.instanceMethod(y)
		Arrays.sort(planets, String::compareToIgnoreCase);

		MyFunction6 myNumeric6 = string::equalsIgnoreCase;
		myNumeric6.test("123456");

		MyFunction7 myNumeric7 = String::equalsIgnoreCase;
		myNumeric7.test("123456", "123456");

		//也可用来调用方法
		Person[] personArray = new Person[]{new Person("Tom"),new Person("Jack"),new Person("Alice")};

		Arrays.sort(personArray, Comparator.comparing(Person::getName));
		for (Person person : personArray) {
			System.out.println(person.getName());
		}

		MyFunc<Integer> myClassFunc = MyClass<Integer>::new;  //相当于 x -> new MyClass<Integer>(x);
		MyClass<Integer> func = myClassFunc.func(100);
		System.out.println(func.getVal());

		MyFunc2<MyClass<Integer>, Integer> myClassFunc2 = MyClass<Integer>::new;
		MyClass<Integer> func2 = myClassFactory(myClassFunc2, 1000);
		System.out.println(func2.getVal());

		// (4) 使用this
		lambdaTest.foo();

		// (5) 使用super
		new B().foo("Test Super::method");

		// (6)  构造器引用  ==>可用来做批量的类型转换
		//int[]::new ==> x->new int[x];

		ArrayList<String> personStrings = new ArrayList<String>();
		personStrings.add("Tom");
		personStrings.add("Jack");

		//可用来做批量的类型转换, 结合stream & map 用法， 过滤
		//得到List
		Stream<Person> personStream = personStrings.stream().map(Person::new);  //Person::new 相当于 x->new Person(x);调用对应的构造器
		List<Person> persons = personStream.collect(Collectors.toList());
		for (Person person : persons) {
			System.out.println(person.getName());
		}

		//得到数组
		personStream = personStrings.stream().map(Person::new);
		Person[] personArrays = personStream.toArray(Person[]::new);
		for (Person person : personArrays) {
			System.out.println(person.getName());
		}

		//5. Lambda表达式中的变量名不能和局部的其它变量名相同

		//6. Lambda表达式必须引用值不会改变的变量

		//7. 使用Lambda遍历list集合  forEach
		personStrings.forEach(n->System.out.println(n));

		//8. 其它接口
		MyNumber num = () -> 555;
		System.out.println(num.getValue());

		num = () -> Math.random() * 100;
		System.out.println(num.getValue());
		System.out.println(num.getValue());
		System.out.println(num.getValue());

		MyNumeric numic = (n) -> (n % 2) == 0;
		System.out.println(numic.test(8));

		numic = (n) -> n > 8;
		System.out.println(numic.test(8));

		MyNumeric2 numic2 = (x, y) -> x * y;
		System.out.println(numic2.test(8, 10));

		numic2 = (int x, int y) -> x - y;
		System.out.println(numic2.test(8, 10));

		MyFunction myFunction = (n) -> {
			int result = 1;

			for (int i = 1; i <= n; i++) {
				result = result * i;
			}

			return result;
		};
		System.out.println(myFunction.test(5));

		MyFunction2 myFunction2 = (str) -> {
			return StringUtils.reverse(str);
		};
		System.out.println(myFunction2.test("qwert"));

		SomeFunction<Integer> someFunction1 = (n) -> {
			int result = 1;

			for (int i = 1; i <= n; i++) {
				result = result * i;
			}

			return result;
		};
		System.out.println(someFunction1.test(5));

		SomeFunction<String> someFunction2 = (str) -> {
			return StringUtils.reverse(str);
		};
		System.out.println(someFunction2.test("qwert"));

		String testingStr = "test string";
		String test1 = "test1";

		String tOperation1 = tOperation((str) -> {
			if (StringUtils.isBlank(str)) {
				throw new Exception();
			}

			// Local variable test1 defined in an enclosing scope must be final
			// or effectively final
			// test1="";

			return StringUtils.reverse(str + test1);
		} , testingStr);
		System.out.println(tOperation1);

		String tOperation2 = tOperation((str) -> str.toUpperCase(), testingStr);
		System.out.println(tOperation2);

		Function<Integer, Integer> factorial = (n) -> {
			// ...
			return n;
		};
		System.out.println(factorial.apply(5));

		Timer timer = new Timer(1000, xx -> System.out.println(new Date() + "+++++"));
		timer.start();
		// Thread.slp(1000000);

		Runnable runnable = ()->{
			System.out.println("runnable action 1");
			System.out.println("runnable action 2");
			System.out.println("runnable action 3");
		};

		repeat(2, runnable);
	

辅助方法

	public static void repeat(int n, Runnable action){
		for (int i = 0; i < n; i++) {
			action.run();
		}
	}

	static String tOperation(SomeFunction<String> someFunction, String str) throws Exception {
		return someFunction.test(str);
	}

	private static void java7_biFunction() {
		BiFunction<String, String, String> bi = new BiFunction<String, String, String>() {
			@Override
			public String apply(String t, String u) {
				return t + u;
			}
		};
		Function<String, String> func = new Function<String, String>() {
			@Override
			public String apply(String t) {
				return t + "-then";
			}
		};
		System.out.println(func.apply("test"));// test-then
		System.out.println(bi.apply("java2s.com", "-tutorial"));// java2s.com-tutorial
		System.out.println(bi.andThen(func).andThen(func).apply("java2s.com", "-tutorial"));// java2s.com-tutorial-then-then
	}

	private static void java8_biFunction() {
		// java8新特性，lambda表达式
		BiFunction<String, String, String> bi = (x, y) -> {
			return x + y;
		};
		Function<String, String> func = x -> x + "-then";
		System.out.println(func.apply("test"));// test-then
		System.out.println(bi.apply("java2s.com", "-tutorial"));// java2s.com
																// tutorial
		System.out.println(bi.andThen(func).andThen(func).apply("java2s.com", "-tutorial"));// java2s.com-tutorial-then-then
	}

	private void foo(){
		MyFunction8 myNumeric8 = this::foo2;
		myNumeric8.test("MyFunction8 foo2");
	}

	private void foo2(String x){
		System.out.println("foo2-----"+x);
	}

	//Lambda expression's parameter x cannot redeclare another local variable defined in an enclosing scope.
	private void foo3(String x){
//		MyFunction2 myFunction2 = x->x;
	}

	static <R,T> R myClassFactory(MyFunc2<R, T> cons, T v){
		return cons.func(v);
	}

函数式接口

interface MyNumber {
	double getValue();
}

interface MyNumeric {
	boolean test(int n);
}

interface MyNumeric2 {
	int test(int x, int y);
}

interface MyFunction {
	int test(int n);
}

interface MyFunction2 {
	String test(String str);
}

interface MyFunction3 {
	String test(int x);
}

interface MyFunction4 {
	String test(int x, int y);
}

interface MyFunction5 {
	void test(String x);
}

interface MyFunction6 {
	boolean test(String x);
}

interface MyFunction7 {
	boolean test(String x, String y);
}

interface MyFunction8 {
	void test(String x);
}

interface MyFunction9 extends Runnable{

}

interface SomeFunction<T> {
	T test(T t) throws Exception;
}

class A{
    void foo(String x){
        System.out.println("Parent A foo:"+x);
    }
}

class B extends A{
    void foo(String x){
        MyFunction8 xxx = super::foo;
        xxx.test(x);
    }
}

interface MyFunc<T>{
    MyClass<T> func(T t);
}

interface MyFunc2<R, T>{
    R func(T t);
}
    
class MyClass<T>{
    private T val;
    
    public MyClass() {
        val = null;
    }
    
    public MyClass(T v) {
        val = v;
    }
    
    T getVal(){
        return val;
    }
}

class Person{
    public Person() {
        // TODO Auto-generated constructor stub
    }
    
    public Person(String name) {
        this.name=name;
    }
    
    public Person(String name,String firstName,String lastName) {
        this.name=name;
        this.firstName=firstName;
        this.lastName=lastName;
    }
    
    private String name;
    private String firstName;
    private String lastName;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }
}