java - 从什么时候开始在 Java 中必须包含一个空的构造函数？

Question

当我遇到这个问题时，我正在练习 dome Java OO 原则。我创建 POJO 并尝试从中创建对象时，如果未定义空构造函数，它将无法编译。

我觉得这很奇怪，因为我曾经这样做过，而且 JVM 为我提供了一个默认的。它是 Java 7 中的新功能吗？我错过了什么吗？

这是我做的示例代码

public class Dog {
String name;
String race;
int age;
/*
public Dog() {
    If this isn't included, it doesn't compile if I try to
    create no-args objects. 
}*/

public Dog (String _name) {
    this.name = _name;
}

public Dog (String _name, String _race) {
    this.name = _name;
    this.race = _race;
}

public Dog (String _name, String _race, int _age) {
    this.name = _name;
    this.race = _race;
    this.age = _age;
}

Answer 1

使用 Dog newDog = new Dog(); 在您当前的代码中将不起作用，因为您尚未定义它。

默认构造函数将仅在没有其他构造函数存在的情况下自动生成。

You don't have to provide any constructors for your class, but you must be careful when doing this. The compiler automatically provides a no-argument, default constructor for any class without constructors. This default constructor will call the no-argument constructor of the superclass. In this situation, the compiler will complain if the superclass doesn't have a no-argument constructor so you must verify that it does. If your class has no explicit superclass, then it has an implicit superclass of Object, which does have a no-argument constructor.

来源:http://docs.oracle.com/javase/tutorial/java/javaOO/constructors.html