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 :
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
