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关于Java中optional，stream，lambda

news 2024/10/27 13:25:46

关于Java中optional，stream，lambda

Lambda表达式高效使用

// 1. 结合Comparator进行排序
List<Person> persons = Arrays.asList(new Person("John", 25),new Person("Alice", 22),new Person("Bob", 30)
);// 按年龄排序
persons.sort((p1, p2) -> p1.getAge() - p2.getAge());// 2. 事件处理
button.addActionListener(e -> System.out.println("Button clicked"));// 3. 自定义函数式接口使用
@FunctionalInterface
interface MathOperation {int operation(int a, int b);
}// 加法lambda
MathOperation add = (a, b) -> a + b;
// 乘法lambda
MathOperation multiply = (a, b) -> a * b;System.out.println(add.operation(5, 3));      // 输出: 8
System.out.println(multiply.operation(5, 3));  // 输出: 15

Stream API高效使用

List<Person> persons = Arrays.asList(new Person("John", 25, "US", 3000),new Person("Alice", 22, "UK", 4000),new Person("Bob", 30, "US", 5000)
);// 1. 复杂数据处理链
Map<String, Double> avgSalaryByCountry = persons.stream().filter(p -> p.getSalary() > 2000)              // 过滤.collect(Collectors.groupingBy(                  // 分组Person::getCountry,                         Collectors.averagingDouble(Person::getSalary)));// 2. 并行处理大数据集
long count = persons.parallelStream().filter(p -> p.getAge() > 25).count();// 3. 数值统计
DoubleSummaryStatistics salaryStats = persons.stream().mapToDouble(Person::getSalary).summaryStatistics();
System.out.println("Average salary: " + salaryStats.getAverage());
System.out.println("Max salary: " + salaryStats.getMax());// 4. 数据转换和收集
List<String> names = persons.stream().map(Person::getName).collect(Collectors.toList());// 5. 查找和匹配
boolean anyMatch = persons.stream().anyMatch(p -> p.getAge() > 30);

Optional高效使用

public class OptionalDemo {// 1. 优雅处理可能为空的对象public String getCarInsuranceName(Optional<Person> person) {return person.flatMap(Person::getCar).flatMap(Car::getInsurance).map(Insurance::getName).orElse("Unknown");}// 2. 链式调用避免空指针public Optional<String> getUppercaseName(Person person) {return Optional.ofNullable(person).map(Person::getName).map(String::toUpperCase);}// 3. 条件处理public void printNameIfPresent(Optional<String> name) {name.ifPresent(System.out::println);name.ifPresentOrElse(System.out::println,    // 值存在时执行() -> System.out.println("Name not found")  // 值不存在时执行);}// 4. 默认值处理public String getNameWithDefault(Optional<String> name) {return name.orElseGet(() -> "Default Name");  // 延迟加载默认值}// 5. 组合多个Optionalpublic Optional<String> getCombinedName(Optional<String> firstName, Optional<String> lastName) {return firstName.flatMap(f -> lastName.map(l -> f + " " + l));}// 6. 过滤值public Optional<Person> getAdultPerson(Optional<Person> person) {return person.filter(p -> p.getAge() > 18);}
}// 实体类示例
class Person {private String name;private int age;private Optional<Car> car;  // 使用Optional作为字段类型public Optional<Car> getCar() {return car;}
}class Car {private Optional<Insurance> insurance;public Optional<Insurance> getInsurance() {return insurance;}
}class Insurance {private String name;public String getName() {return name;}
}