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Java8: lambda expression and functional interface

1、

Comparator Interface :

@FunctionalInterface
public interface Comparator<T> {
    int compare(T o1, T o2);

(1) Anonymous inner class

   @Test
    public void test1() {
        Comparator<Integer> com = new Comparator<Integer>() {
            @Override
            public int compare(Integer o1, Integer o2) {
                return Integer.compare(o1, o2);
            }
        };
    }

(2)Lambda expression

  @Test
    public void test2(){
        Comparator<Integer> com=(o1,o2)->Integer.compare(o1,o2);
    }

(3) Case study ( Get older than 35 Employee information )

The conventional way :

Define an employee class , Traverse all employee information , Older than 35 The employee's information is stored in a new collection . however , If we change the demand to get more wages than 5000 Employee information , What needs to be changed is the judgment condition of the program , This creates code redundancy

Design patterns : Strategy design pattern

Create a new employee class

@Data
@AllArgsConstructor
public class Employee {
    private int id;
    private String name;
    private int age;
    private double salary;
}

Create an interface , This interface defines a method for filtering

@FunctionalInterface
public interface MyPredicate<T> {

    public boolean test(T t);
    
}

Create implementation classes for two interfaces : Methods that implement the class rewrite interface filter age and salary respectively

Wages

public class FilterEmployeeForSalary implements MyPredicate<Employee> {

    @Override
    public boolean test(Employee t) {
        return t.getSalary() >= 5000;
    }

}

Age

public class FilterEmployeeForAge implements MyPredicate<Employee>{

    @Override
    public boolean test(Employee t) {
        return t.getAge() <= 35;
    }

}

Test class :

Add employee information

Define filtering methods , The first parameter of this method is a Employee Type of List aggregate , The second parameter is an interface type , That is to say, all implementation classes of the interface can be passed , That is to say, the method is not written to death , The method is more flexible

The third method is to test the class , The implementation class of transferring employee information and interface , What is returned is the filtered result

public class TestLambda {
    List<Employee> emps = Arrays.asList(
            new Employee(101, " Zhang San ", 18, 9999.99),
            new Employee(102, " Li Si ", 59, 6666.66),
            new Employee(103, " Wang Wu ", 28, 3333.33),
            new Employee(104, " Zhao Liu ", 8, 7777.77),
            new Employee(105, " Panax notoginseng ", 38, 5555.55)
    );

    public List<Employee> filterEmployee(List<Employee> list, MyPredicate<Employee> myPredicate) {
        List<Employee> employees = new ArrayList<>();
        for (Employee employee : list) {
            if (myPredicate.test(employee)) {
                employees.add(employee);
            }
        }
        return employees;
    }

    @Test
    public void test() {
        List<Employee> employeeList = filterEmployee(emps, new FilterEmployeeForAge());
        for (Employee employee : employeeList) {
            System.out.println(employee);
        }
        System.out.println("-------------------");
        List<Employee> list = filterEmployee(emps, new FilterEmployeeForSalary());
        for (Employee employee : list) {
            System.out.println(employee);
        }
    }

test result :

Employee(id=101, name= Zhang San , age=18, salary=9999.99)
Employee(id=103, name= Wang Wu , age=28, salary=3333.33)
Employee(id=104, name= Zhao Liu , age=8, salary=7777.77)
-------------------
Employee(id=101, name= Zhang San , age=18, salary=9999.99)
Employee(id=102, name= Li Si , age=59, salary=6666.66)
Employee(id=104, name= Zhao Liu , age=8, salary=7777.77)
Employee(id=105, name= Panax notoginseng , age=38, salary=5555.55)

Anonymous inner class mode

public List<Employee> filterEmployee(List<Employee> list, MyPredicate<Employee> myPredicate) {
        List<Employee> employees = new ArrayList<>();
        for (Employee employee : list) {
            if (myPredicate.test(employee)) {
                employees.add(employee);
            }
        }
        return employees;
    }

    @Test
    public void test() {
        List<Employee> list = filterEmployee(emps, new MyPredicate<Employee>() {
            @Override
            public boolean test(Employee employee) {
                return employee.getSalary() < 7000;
            }
        });
        for (Employee employee : list) {
            System.out.println(employee);
        }
    }

The anonymous inner class approach is compared to the policy pattern , Less writing of implementation classes , But the way anonymous inner classes are not simple enough

Lambda expression

    public List<Employee> filterEmployee(List<Employee> list, MyPredicate<Employee> myPredicate) {
        List<Employee> employees = new ArrayList<>();
        for (Employee employee : list) {
            if (myPredicate.test(employee)) {
                employees.add(employee);
            }
        }
        return employees;
    }

    @Test
    public void test() {
        List<Employee> list = filterEmployee(emps,(e)->e.getSalary()<7000);
        for (Employee employee : list) {
            System.out.println(employee);
        }
    }

Compared with the way the function is interfaced ,Lambda Expressions make the code more concise

The simplest way :Stream API

  @Test
    public void test() {
        emps.stream()
                .filter((e)->e.getSalary()>=5000)
                .forEach(System.out::println);
    }

In this way, only one entity class is needed

 

2、 Functional interface

(1) Define a functional interface

@FunctionalInterface
public interface MyFun {

    public Integer getValue(Integer num);
    
}

(2) Define methods and test

public class TestLambda {
    public Integer operation(Integer num, MyFun mf) {
        return mf.getValue(num);
    }

    @Test
    public void test() {
        Integer num = operation(100, (x) -> x * x);
        System.out.println(num);

        System.out.println(operation(200, (y) -> y + 200));
    }
}

The parameter of method one is an interface type , This method calls the method in the interface

Method two is the test method ,Lambda The expression needs to pass an argument

 

3、Lambda Application of expressions

(1) Sort

Functional interface

int compare(T o1, T o2);

Methods that call functional interfaces

   public static <T> void sort(List<T> list, Comparator<? super T> c) {
        list.sort(c);
    }

Test class

    List<Employee> emps = Arrays.asList(
            new Employee(101, " Zhang San ", 18, 9999.99),
            new Employee(102, " Li Si ", 59, 6666.66),
            new Employee(103, " Wang Wu ", 28, 3333.33),
            new Employee(104, " Zhao Liu ", 8, 7777.77),
            new Employee(105, " Panax notoginseng ", 38, 5555.55)
    );

    @Test
    public void test1() {
        Collections.sort(emps, (e1, e2) -> {
            if (e1.getAge() == e2.getAge()) {
                return e1.getName().compareTo(e2.getName());
            } else {
                return Integer.compare(e1.getAge(), e2.getAge());
            }
        });

        for (Employee emp : emps) {
            System.out.println(emp);
        }
    }

(2) Processing strings

Define a functional interface

@FunctionalInterface
public interface MyFunction {
    
    public String getValue(String str);

}

Define string processing and testing methods

public class TestLambda {
    public String strHandler(String str, MyFunction mf){
        return mf.getValue(str);
    }
    @Test
    public void test2(){
        String trimStr = strHandler("    Hello    ", (str) -> str.trim());
        System.out.println(trimStr);

        String upper = strHandler("qqqqeeeabc", (str) -> str.toUpperCase());
        System.out.println(upper);

        String newStr = strHandler("1234556", (str) -> str.substring(2, 5));
        System.out.println(newStr);
    }
}

(3) Data processing

Function value interface

public interface MyFunction2<T, R> {

    public R getValue(T t1, T t2);

}

Test class

public class TestLambda {
    public void op(Long l1, Long l2, MyFunction2<Long, Long> mf){
        System.out.println(mf.getValue(l1, l2));
    }
    @Test
    public void test3(){
        op(100L, 200L, (x, y) -> x + y);

        op(100L, 200L, (x, y) -> x * y);
    }
}

 

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