oop - 下面是可变类还是不可变类？

Question

In object-oriented and functional programming, an immutable object is an object whose state cannot be modified after it is created. This is in contrast to a mutable object, which can be modified after it is created.

下面是可变类还是不可变类？我认为它是可变的，因为静态字段 singleton 在 new MavenPush(project) 之后发生了变化，但我需要 100% 正确的答案。这是 Groovy 代码，但它就像 Java 代码

final class MavenPush {

    private static volatile MavenPush singleton = null

    private final def project

    private MavenPush(project) {
        this.project = project
    }

    static MavenPush with(project) {
        if (singleton == null) {
            synchronized (MavenPush.class) {
                if (singleton == null) {
                    singleton = new MavenPush(project)
                }
            }  
        }
        return singleton
    }
}

Answer 1

类的实例是不可变的，当类的字段发生变化时，类是可变的。