java - 两个对象值为null；为什么？

Question

这个问题已经有答案了:

Is Java "pass-by-reference" or "pass-by-value"? (91 个回答)

已关闭 3 年前。

方法参数sd & ed是LocalDate，我需要返回日期对象，但是sd & ed返回null。我定义了一个私有(private)方法:

private void setQuarter(LocalDate sd,LocalDate ed,LocalDate today){
            switch (getQuarter(today.getMonthValue())){
                case 1:
                    sd=LocalDate.of(today.getYear(),1,1);
                    ed=LocalDate.of(today.getYear(),3,31);
                    break;
                case 2:
                    sd=LocalDate.of(today.getYear(),4,1);
                    ed=LocalDate.of(today.getYear(),6,30);
                    break;
                case 3:
                    sd=LocalDate.of(today.getYear(),7,1);
                    ed=LocalDate.of(today.getYear(),9,30);
                    break;
                case 4:
                    sd=LocalDate.of(today.getYear(),10,1);
                    ed=LocalDate.of(today.getYear(),12,31);
                    break;
                default:
                    sd=LocalDate.of(today.getYear()+1,1,1);
                    ed=LocalDate.of(today.getYear()+1,1,1);
            }
        }

调用此方法:

LocalDate sd=null;
LocalDate ed=null;

setQuarter(sd,ed,today);

log.info("==> sd:{},ed:{}",sd,ed);

输出消息:==> sd:null,ed:null

Answer 1

因为在Java中方法的参数是按值传递的。引用(即指针)通过引用的值(对象的地址)传递。

代码应如下所示:

Pair<LocalDate, LocalDate> dates = getSdAndEd(today);
LocalDate sd = dates.getLeft();
LocalDate ed = dates.getRight();

private Pair<LocalDate, LocalDate> getSdAndEd(LocalDate today){
    switch (getQuarter(today.getMonthValue())){
        case 1:
            return Pair.of(
                    LocalDate.of(today.getYear(),1,1),
                    LocalDate.of(today.getYear(),3,31)
            )
        case 2:
          ...
    }
} 

在示例中 Pair-来自 Apache Commons Lang 的类 » group: 'org.apache.commons', name: 'commons-lang3', version: “3.9”。