Java Lambda expression example analysis

1. Introduction

First of all, Lambda expression is a new feature of Java8. It provides support for functional programming in Java programming and helps The simplicity of the code can replace most of the anonymous functions, especially for collection traversal and collection operations, which greatly simplifies the code.

The body of the Lambda expression:

Functional interface:

Note: Lambda expressions must be used with functional interfaces The so-called functional interface means that an interface with only one abstract method in the interface is a functional interface. We can customize it. JDK also has a large number of built-in functional interfaces.

1. The @FunctionalInterface annotation modifies the interface. Then this interface is a functional interface. It can only have one method. The following is a functional interface:

@FunctionalInterface
public interface MyInteface {

    void eat();

}

2. If @FunctionalInterface is not added **Note, if you write only one abstract method in the interface, it can also be considered as a functional interface:

public interface MyInteface {
    void eat();
}

This is also possible.

3. There is only one case where functional interfaces do not only have abstract methods, that is, they can inherit the methods of the Object class:

@FunctionalInterface
public interface MyInteface3 {

    void eat();

    @Override
    String toString();
    
    @Override
    int hashCode();
}

2. The use of Lambda expressions:

1. Use in ordinary methods

Student class:

@FunctionalInterface
public interface Student {

    void eat();

}

Test class:

public class Test {
    public static void main(String[] args) {

        Student stu = new Student() {

            //普通方法，重写并使用
            @Override
            public void eat() {
                System.out.println("我是学生");
            }
        };
        stu.eat();


        //lambda表达式写法：
        //参数1:重写了Student接口中唯一的那个无参数的eat抽象方法做了具体的实现，所以重写不 需要署名
        //参数2:-> 表达式 固定的
        //参数3：{具体的实现} 对Student接口中唯一的eat方法做了具体的实现
            Student stu2 = () -> {
                System.out.println("学生吃饭");
            };
        stu2.eat();
        
    }
}

Output:

I am a student
Students eat

2. Use of methods with parameters

Student class:

@FunctionalInterface
public interface Student {

    void eat(String food);
}

Test class:

public class Test {
    public static void main(String[] args) {

        //lambda重写Student接口唯一的有参方法：
        Student stu2 = (foodName)->{
            System.out.println("学生在吃"+foodName);
        };

        stu2.eat("肉");
    }
}

// Output: Students are eating meat

3. Lambda expressions to implement multi-threading

The method of creating multi-threads was introduced in the article on multi-threading (1). Here we use lambda to create threads:

public class Test {
    public static void main(String[] args) {

        Thread t = new Thread(() -> {
            System.out.println("这个线程是由lambda来创建的");
        });

        t.start();

    }
}

4. Lambda expression operation operations

We use lambda to operate operations and can save a lot of code:

Functional interface:

@FunctionalInterface
public interface Calculator<T> {

    T operation(T v1,T v2);

}

Test class:

public class Test {

    //计算方法
    public static Integer operator(Integer v1,Integer v2,Calculator<Integer> calculator){
        return calculator.operation(v1, v2);
    }

    public static void main(String[] args) {


        //使用lambda表达式：
        //这里的意思就是传入两个参数，返回运行后的值
        int add = Test.operator(5,10,(x,y)->{
            return x+y;
        });


        //简写：可以少写很多代码，比上面更简介了
        int num1 = Test.operator(5,10,(x,y)->x+y);
        int num2 = Test.operator(10,5,(x,y)->x-y);

        System.out.println(add);
        System.out.println(num1);
        System.out.println(num2);

    }
}

Output:

15, 15, 5

5, Lambda expression method reference

Sometimes we don’t have to rewrite the interface methods to achieve specific implementation. If we have existing methods that can be implemented, we can also refer to existing methods through method references to make the methods in the interface specific. The advantage of this implementation is code reuse, such as the following:

Functional interface:

public interface ResultOneParam {

    int method(int a);
}

Test class:

public class Test {

    public int addTo(int a){
        return  a+10;
    }

    public  static int addTo2(int a){
        return  a+10;
    }

    public static void main(String[] args) {
        //lambda重写了method方法
        ResultOneParam lambda1=(a)->a+10;

        //方法引用：就是在Test里面的addTo2方法用来替代method被重写的方法
        ResultOneParam lambda2= Test::addTo2;
        int result1= lambda2.method(9);
        System.out.println(result1);

        //方法引用 ::引用现成的方法来替代方法重写，这样可以方法重用
        Test test=new Test();
        ResultOneParam lambda3=test::addTo;
        int result2= lambda3.method(9);
        System.out.println(result1);

    }
}

6. Lambda expressions for collections Using

Of course Lambda is also very convenient to operate collections, and you can save a lot of code:

public class Test {
    public static void main(String[] args) {

        List<Integer> list = Arrays.asList(5,1,3,4,5,0,9,7,0,1,5);

        //lambda表达式遍历集合，重写了Consumer接口的方法
        list.forEach((element)->{
            System.out.println(element);
        });

        //简写：
        list.forEach(element-> System.out.println(element));

        //lambda表达式方法引用，用于遍历输出list集合:
        list.forEach(System.out::print);

        //输出list的偶数:
        list.forEach(element->{
            if(element%2==0){
                System.out.println(element);
            }

        });
    }
}

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