为什么this（）和super（）必须是构造函数中的第一条语句？

问题：

Java requires that if you call this() or super() in a constructor, it must be the first statement. Java要求，如果您在构造函数中调用this（）或super（），则它必须是第一条语句。 Why? 为什么？

For example: 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
    }
}

The Sun compiler says "call to super must be first statement in constructor". Sun编译器说“对super的调用必须是构造函数中的第一条语句”。 The Eclipse compiler says "Constructor call must be the first statement in a constructor". Eclipse编译器说“构造函数调用必须是构造函数中的第一条语句”。

However, you can get around this by re-arranging the code a little bit: 但是，您可以通过重新安排一些代码来解决此问题：

public class MySubClass extends MyClass {
    public MySubClass(int a, int b) {
        super(a + b);  // OK
    }
}

Here is another example: 这是另一个示例：

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
    }
}

So, it is not stopping you from executing logic before the call to super. 因此，这不会阻止您在调用super之前执行逻辑 。 It is just stopping you from executing logic that you can't fit into a single expression. 这只是在阻止您执行无法包含在单个表达式中的逻辑。

There are similar rules for calling this() . 调用this()有类似的规则。 The compiler says "call to this must be first statement in constructor". 编译器说“对此的调用必须是构造函数中的第一条语句”。

Why does the compiler have these restrictions? 为什么编译器有这些限制？ Can you give a code example where, if the compiler did not have this restriction, something bad would happen? 您能否举一个代码示例，如果编译器没有此限制，那么会发生不好的事情吗？

解决方案：

参考一： https://stackoom.com/question/4twH/为什么this-和super-必须是构造函数中的第一条语句
参考二： https://oldbug.net/q/4twH/Why-do-this-and-super-have-to-be-the-first-statement-in-a-constructor

来源：oschina

链接：https://my.oschina.net/u/3797416/blog/4411251

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