Optional容器类

爱被打了一巴掌 2022-05-17 03:09 239阅读 0赞

# Optional类 #

Optional<T>类(java. util. Optional)是一个容器类,代表一个值存在或不存在,原来用null表示一个值不存在,现在Optional可以更好的表达这个概念。并且可以避免空指针异常。

常用方法:

Optional.of (T t) :创建一个Optional实例

Optional. empty() :创建一个空的Optional实例

Optional. ofNullable(T t):若t不为nul1,创建Optional实例,否则创建空实例

isPresent () :判断是否包含值

orElse(T t) : 如果调用对象包含值,返回该值,否则返回t

orElseGet (Supplier s) :如果调用对象包含值,返回该值,否则返回s获取的值

map (Function f):如果有值对其处理,并返回处理后的0ptional,否则返回Optional. empty()

flatMap(Function mapper) :与map类似,要求返回值必须是Optional

Optional.of (T t) :创建一个Optional实例

@Test
    public void test1() {
        Optional<Employee> optional = Optional.of(new Employee());
        Employee employee = optional.get();
        System.out.println(employee);//Employee{name='null', age=0, salary=0, status='null'}
    }

Optional. empty() :创建一个空的Optional实例

@Test
    public void test2() {
        Optional<Employee> optional = Optional.empty();//创建一个空的Optional实例
        Employee employee = optional.get();//java.util.NoSuchElementException: No value present
        System.out.println(employee);
    }

Optional. ofNullable(T t):若t不为nul1,创建Optional实例,否则创建空实例

@Test
    public void test3() {
        Optional<Employee> optional = Optional.ofNullable(new Employee());
        Employee employee = optional.get();
        System.out.println(employee);//Employee{name='null', age=0, salary=0, status='null'}
    
        Optional<Employee> optional2 = Optional.ofNullable(null);
        Employee employee2 = optional2.get();//java.util.NoSuchElementException: No value present
        System.out.println(employee2);
    }

public static <T> Optional<T> ofNullable(T value) {
        return value == null ? empty() : of(value);
    }

isPresent () :判断是否包含值orElse(T t) : 如果调用对象包含值,返回该值,否则返回t

@Test
    public void test3() {
        Optional<Employee> optional = Optional.ofNullable(new Employee());
        if (optional.isPresent()) {
            Employee employee = optional.get();
            System.out.println(employee);//Employee{name='null', age=0, salary=0, status='null'}
        }
        Optional<Employee> optional2 = Optional.ofNullable(null);
        if (optional2.isPresent()) {//没有值跳过
            Employee employee2 = optional2.get();
            System.out.println(employee2);
        }
    }

orElse(T t) : 如果调用对象包含值,返回该值,否则返回t

@Test
    public void test3() {
        Optional<Employee> optional = Optional.ofNullable(new Employee());
        Employee employee = optional.orElse(new Employee("张三", 18, 3000, Status.BUSY));
        System.out.println(employee);//Employee{name='null', age=0, salary=0, status='null'}
    
        Optional<Employee> optional2 = Optional.ofNullable(null);
        Employee employee2 = optional2.orElse(new Employee("张三", 18, 3000, Status.BUSY));
        System.out.println(employee2);//Employee{name='张三', age=18, salary=3000, status='BUSY'}
    }

orElseGet (Supplier s) :如果调用对象包含值,返回该值,否则返回s获取的值

@Test
    public void test3() {
        Optional<Employee> optional = Optional.ofNullable(new Employee());
        Employee employee = optional.orElseGet(()->new Employee("张三", 18, 3000, Status.BUSY));
        System.out.println(employee);//Employee{name='null', age=0, salary=0, status='null'}
    
        Optional<Employee> optional2 = Optional.ofNullable(null);
        Employee employee2 = optional2.orElseGet(()->new Employee("张三", 18, 3000, Status.BUSY));
        System.out.println(employee2);//Employee{name='张三', age=18, salary=3000, status='BUSY'}
    }

map (Function f):如果有值对其处理,并返回处理后的0ptional,否则返回Optional. empty()

@Test
    public void test4() {
        Optional<Employee> optional = Optional.ofNullable(new Employee("张三", 18, 3000, Status.BUSY));
        Optional<String> optionalS = optional.map((x) -> x.getName());
       System.out.println(optionalS .get());//张三
    }

flatMap(Function mapper) :与map类似,要求返回值必须是Optional

@Test
    public void test4() {
        Optional<Employee> optional = Optional.ofNullable(new Employee("张三", 18, 3000, Status.BUSY));
        Optional<String> optional1 = optional.flatMap((x) -> optional.of(x.getName()));
        System.out.println(optional1.get());//张三
    }