java - 代码调用 new GenericObject() 但 GenericObject.java 没有构造函数

Question

我正在查看我正在使用的一些代码，其中有一点特别让我印象深刻:

文件中有一个 block :

public void prepare(){
    if (this.GenericObjectID != null) 
         doStuff();
    else{
         this.GenericObjet = new GenericObject();
    }

但是，当我查看 GenericObject.java 时，根本没有构造函数。代码运行了，但我没有写它，所以我不确定如何(还!)。所以我的问题是:这怎么可能？当 Java 编译器看到此调用但随后在描述该对象的文件中没有构造函数时，它在做什么？

Answer 1

如果没有显式构造函数，则编译器会创建一个隐式默认构造函数，不带任何参数，除了隐式调用父类(super class)构造函数之外什么都不做。

Section 8.8.9 of the JLS谈论默认构造函数:

If a class contains no constructor declarations, then a default constructor is implicitly declared. The form of the default constructor for a top level class, member class, or local class is as follows:

• The default constructor has the same accessibility as the class (§6.6).

• The default constructor has no formal parameters, except in a non-private inner member class, where the default constructor implicitly declares one formal parameter representing the immediately enclosing instance of the class (§8.8.1, §15.9.2, §15.9.3).

• The default constructor has no throws clauses.

• If the class being declared is the primordial class Object, then the default constructor has an empty body. Otherwise, the default constructor simply invokes the superclass constructor with no arguments.