Java要求,如果在构造函数中调用this()或super(),它必须是第一条语句。为什么?
例如:
public class MyClass {
public MyClass(int x) {}
}
public class MySubClass extends MyClass {
public MySubClass(int a, int b) {
int c = a + b;
super(c); // COMPILE ERROR
}
}
Sun编译器说,调用super必须是构造函数中的第一条语句。Eclipse编译器说,构造函数调用必须是构造函数中的第一个语句。
然而,你可以通过稍微重新安排代码来解决这个问题:
public class MySubClass extends MyClass {
public MySubClass(int a, int b) {
super(a + b); // OK
}
}
下面是另一个例子:
public class MyClass {
public MyClass(List list) {}
}
public class MySubClassA extends MyClass {
public MySubClassA(Object item) {
// Create a list that contains the item, and pass the list to super
List list = new ArrayList();
list.add(item);
super(list); // COMPILE ERROR
}
}
public class MySubClassB extends MyClass {
public MySubClassB(Object item) {
// Create a list that contains the item, and pass the list to super
super(Arrays.asList(new Object[] { item })); // OK
}
}
因此,它不会阻止您在调用super()之前执行逻辑。它只是阻止您执行无法放入单个表达式中的逻辑。
调用this()也有类似的规则。编译器说,调用this必须是构造函数中的第一条语句。
为什么编译器有这些限制?你能给出一个代码例子,如果编译器没有这个限制,就会发生不好的事情吗?
父类的构造函数需要在子类的构造函数之前调用。这将确保如果在构造函数中调用父类上的任何方法,父类已经正确设置。
你要做的是,将参数传递给超级构造函数是完全合法的,你只需要像你所做的那样内联构造这些参数,或者将它们传递给你的构造函数,然后将它们传递给super:
public MySubClassB extends MyClass {
public MySubClassB(Object[] myArray) {
super(myArray);
}
}
如果编译器没有强制执行,你可以这样做:
public MySubClassB extends MyClass {
public MySubClassB(Object[] myArray) {
someMethodOnSuper(); //ERROR super not yet constructed
super(myArray);
}
}
在父类有默认构造函数的情况下,编译器会自动插入对super的调用。由于Java中的每个类都继承自Object,所以必须以某种方式调用Object的构造函数,并且必须首先执行它。编译器可以自动插入super()。强制super首先出现,强制构造函数主体以正确的顺序执行,即:Object -> Parent -> Child -> ChildOfChild -> SoOnSoForth
你问为什么,其他的答案,在我看来,并没有真正说为什么它是可以调用你的超级构造函数,但只有当它是第一行。原因是您并没有真正调用构造函数。在c++中,等价的语法是
MySubClass: MyClass {
public:
MySubClass(int a, int b): MyClass(a+b)
{
}
};
When you see the initializer clause on its own like that, before the open brace, you know it's special. It runs before any of the rest of the constructor runs and in fact before any of the member variables are initialized. It's not that different for Java. There's a way to get some code (other constructors) to run before the constructor really starts, before any members of the subclass are initialized. And that way is to put the "call" (eg super) on the very first line. (In a way, that super or this is kind of before the first open brace, even though you type it after, because it will be executed before you get to the point that everything is fully constructed.) Any other code after the open brace (like int c = a + b;) makes the compiler say "oh, ok, no other constructors, we can initialize everything then." So it runs off and initializes your super class and your members and whatnot and then starts executing the code after the open brace.
如果几行之后,它遇到一些代码说“哦,是的,当你构造这个对象时,这是我想让你传递给基类构造函数的参数”,那就太晚了,这没有任何意义。所以你会得到一个编译器错误。