用Java 8 Optional替换重复的get语句

问题描述

我有一个用例，其中有嵌套类和顶级类的对象.我想获得第N级的值.为了避免NPE，我反复使用吸气剂来实现此目的.示例代码(假设有吸气剂存在)

I have a use case, where I have nested classes and an object of the top class. I want to get a value which is at the Nth level. I'm using getters repetitively to achieve this to avoid NPE. Sample code (assuming getters are there)

class A {
    String a1;
    String getA1() {
        return a1;
    }
}

class B {
    A a;
    A getA() {
        return a;
    }
}

class C {
    B b;
    B getB() {
        return b;
    }
}

class D {
    C c;
    C getC() {
        return c;
    }
}

如果我有类D的对象d，并且想要获取A的String a1，我正在做的事情是:

If I have an object d of class D, and want to get the String a1 of A, what I'm doing is following:

String getAValue(D d) {
    String aValue = null;
    if(d != null && d.getC() != null && d.getC().getB() != null && d.getC().getB().getA() != null) {
        aValue = d.getC().getB().getA().getA1();
    }
    return aValue;
}

这个重复的a看起来非常丑陋.如何通过使用java8 Optional来避免这种情况?

This repetitive a is looking really ugly. How do I avoid it by using java8 Optional?

我无法修改以上类.假设此d对象作为服务调用返回给我.我只接触这些吸气剂.

I can't modify the above classes. Assume this d object is returned to me as a service call. I'm exposed to these getters only.

推荐答案

在一系列map()调用中将Optional与一个好的内衬一起使用:

Use Optional with a series of map() calls for a nice one-liner:

String getAValue(D d) {
   return Optional.ofNullable(d)
       .map(D::getC).map(C::getB).map(B::getA).map(A::getA1).orElse(null);
}

如果链中的任何东西null，包括d本身，将执行orElse().

If anything is null along the chain, including d itself, the orElse() will execute.

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