为什么 IntelliJ 告诉我我的类从不返回 null？

Question

private void addCompoundsFrom(Verse verse) {
    Optional<List<Compound>> compounds = Optional.of(verse.getCompounds());
    if (compounds.isPresent()) {
        for (Compound compound : compounds.get()) {
            addCompoundsFrom(compound);
        }
    }
}

The IntelliJ inspector tells me that the if-statement is always true. How can it know that? This is the Compounds class:

public class Compounds extends PositionalIds {

    @XmlElement(name = "verse")
    private List<Verse> verses;

    public List<Verse> getVerses() {
        return verses;
    }
}


@XmlTransient
public abstract class PositionalIds {

    @XmlAttribute(name = "start")
    private String startId;

    @XmlAttribute(name = "end")
    private String endId;

    public String getStartId() {
        return startId;
    }

    public String getEndId() {
        return endId;
    }
}

And the Verse class:

public class Verse extends PositionalIds {

    @XmlElement(name = "compound")
    private List<Compound> compounds;

    @XmlAttribute(name = "notation")
    private String notation;

    public List<Compound> getCompounds() {
        return compounds;
    }

    public String getNotation() {
        return notation;
    }
}

If I stop using Optional to wrap the verse.getCompounds() result and instead just do a null check, the inspection message goes away.

I'm using Java 8.

Answer 1

The Optional class has two methods:

• Optional.of -> throws exception if the parameter is null
• Optional.ofNullable -> creates an empty optional if the parameter is null

Hence if your method returns null, the of() method will throw an exception and empty optional will never reach your if statement

Answer 2

Optional.of(verse.getCompounds()); returns an Optional that contains a valid value.

The isPresent check that follows will always be true because the Optional compounds will never not have a value, since you just set it to a valid value on the line above.