Java8新特性学习笔记 Lambda表达式

Posted 2020-10-30 杜先武博客

没有用Lambda表达式的写法:

Comparator<Transaction> byYear = new Comparator<Transaction>() {
            @Override            public int compare(Transaction o1, Transaction o2) {                return o1.getValue().compareTo(o2.getValue());
            }
        };

 

Lambda表达式的写法:

Comparator<Transaction> byYear = (o1, o2) -> o1.getValue().compareTo(o2.getValue());

 

Lambda表达式的三部分:

1. 参数列表  这里采用了Comparator中compare的方法的参数,两个Transaction

2. 箭头     箭头->把参数列表和Lambda主体分隔开.

3. Lambda主体   比较两个Transaction的年份,表达式就是Lambda的返回值

下面提供了一些Lambda的例子和使用案例

(List<String> list) -> list.isEmpty();
() -> new Transaction();
(Transaction t) -> {
            System.out.println("Year = " + t.getYear());
}
(String s) -> s.length();
(int a, int b) -> a * b;

 

在哪里使用Lambda接口

　　可以在函数式接口上使用Lambda表达式,

1. 函数式接口 就是只定义了一个抽象方法的接口,继承的也不行 作用: 是函数式接口一个具体的实现实例

例子:

Runnable r2 = new Runnable() {//使用匿名类            @Override            public void run() {
                System.out.println("hello world 2");
            }
        };
        process(r1);//打印hello world 1
        process(r2);//打印hello world 2
        process(()-> System.out.println("hello world 3"));//利用直接传递Lambda表达式作为实现实例 打印hello world 3

 

　　如果我们去看看新的java Api,会发现函数式接口带有@FunctionalInterface标注,这个标注表示该接口被设计成一个函数式接口,如果你用了该注解,而它不是却不是函数式接口,编译器会返回一个提示错误.

/**
 * The <code>Runnable</code> interface should be implemented by any
 * class whose instances are intended to be executed by a thread. The
 * class must define a method of no arguments called <code>run</code>.
 * <p>
 * This interface is designed to provide a common protocol for objects that
 * wish to execute code while they are active. For example,
 * <code>Runnable</code> is implemented by class <code>Thread</code>.
 * Being active simply means that a thread has been started and has not
 * yet been stopped.
 * <p>
 * In addition, <code>Runnable</code> provides the means for a class to be
 * active while not subclassing <code>Thread</code>. A class that implements
 * <code>Runnable</code> can run without subclassing <code>Thread</code>
 * by instantiating a <code>Thread</code> instance and passing itself in
 * as the target.  In most cases, the <code>Runnable</code> interface should
 * be used if you are only planning to override the <code>run()</code>
 * method and no other <code>Thread</code> methods.
 * This is important because classes should not be subclassed
 * unless the programmer intends on modifying or enhancing the fundamental
 * behavior of the class.
 *
 * @author  Arthur van Hoff
 * @see     java.lang.Thread
 * @see     java.util.concurrent.Callable
 * @since   JDK1.0 */@FunctionalInterfacepublic interface Runnable {    /**
     * When an object implementing interface <code>Runnable</code> is used
     * to create a thread, starting the thread causes the object‘s
     * <code>run</code> method to be called in that separately executing
     * thread.
     * <p>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     * @see     java.lang.Thread#run()     */
    public abstract void run();
}

 

方法引用

方法引用让你可以重复使用现有的方法定义,并像Lambda一样传递他们.例如下面排序的例子:

List<Apple> inventory = new ArrayList<>();
inventory.sort(Comparator.comparing(Apple::getWeight)//按重量排序
 .reversed()//递减 倒序
 .thenComparing(Apple::getCountry));//如果重量一样 按国家排序

 

当你需要使用方法引用时,目标引用放在分隔符::前,方法名称放在后面,例如Apple::getWeight 就是引用了Apple类中定义的getWeight,请记住,不需要括号,因为你没有实际调用这个方法.

方法引用主要有三类:

1. 指向静态方法的方法引用(例如Integer的parseInt方法 写作Integer::parseInt) .

2. 指向任意类型实例方法的方法引用(例如String的length方法,写作String::length)

3. 指向现有对象的实例方法的方法引用(假设你有一个局部变量expensive用于存放Transaction类型的对象,他支持实例方法getValue,那么你就可以写expensive::getValue).

构造函数引用

　　对于一个现有构造函数,你可以利用它的名称和关键字new 来创建他的一个引用:ClassName::new,它的功能与指向静态方法的引用类型类似.