关于Optional orElseGet的一些注意点

小咪咪 2022-05-12 10:10 264阅读 0赞

前提 : 现在有A B C三个类, 代码如下 :

A.java

package com.ahut.essearch.Optional;
    
    /** * desc : * create_user : cheng * create_date : 2018/9/14 14:30 */
    public class A { 
    
        private B b;
    
        public B getB() {
            return this.b;
        }
    
        public void setB(B b) {
            this.b = b;
        }
    }

B.java

package com.ahut.essearch.Optional;
    
    /** * desc : * create_user : cheng * create_date : 2018/9/14 14:30 */
    public class B { 
    
        private C c;
    
        public C getC() {
            return this.c;
        }
    
        public void setC(C c) {
            this.c = c;
        }
    }

C.java

package com.ahut.essearch.Optional;
    
    /** * desc : * create_user : cheng * create_date : 2018/9/14 14:30 */
    public class C { 
    
        private String desc;
    
        public String getDesc() {
            return desc;
        }
    
        public void setDesc(String desc) {
            this.desc = desc;
        }
    }

测试 :

/** * desc : * create_user : cheng * create_date : 2018/9/14 14:32 */
        @Test
        public void testOptional() {
            A a = new A();
    
            String desc = Optional.ofNullable(a)
                    .map(m -> m.getB())
                    .map(m -> m.getC())
                    .map(m -> m.getDesc())
                    .orElseGet(() -> null);
    
            System.out.println(desc);// null
        }

/** * desc : * create_user : cheng * create_date : 2018/9/14 14:32 */
        @Test
        public void testOptional() {
            A a = new A();
    
            String desc = Optional.ofNullable(a)
                    .map(m -> m.getB())
                    .map(m -> m.getC())
                    .map(m -> m.getDesc())
                    .orElseGet(null);
    
            System.out.println(desc);// 抛出NullPointerException
        }

查看Optional中orElseGet方法源码 :

/**
         * Return the value if present, otherwise invoke {@code other} and return
         * the result of that invocation.
         *
         * @param other a {@code Supplier} whose result is returned if no value
         * is present
         * @return the value if present otherwise the result of {@code other.get()}
         * @throws NullPointerException if value is not present and {@code other} is
         * null
         */
        public T orElseGet(Supplier<? extends T> other) {
     return value != null ? value : other.get();
        }

可知 : 因为需要调用other的get方法, 所以other不能为null