Java 构造函数调用必须是构造函数中的第一条语句
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原文地址: http://stackoverflow.com/questions/20198279/
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Constructor call must be the first statement in a constructor
提问by beginner
I do not understand why the below code displays the error Constructor call must be the first statement in a constructorif I shift this(1);to the last line in the constructor.
Constructor call must be the first statement in a constructor如果我this(1);移到构造函数的最后一行，我不明白为什么下面的代码会显示错误。
package learn.basic.corejava;
public class A {
int x,y;
A()
{
// this(1);// ->> works fine if written here
System.out.println("1");
this(1); //Error: Constructor call must be the first statement in a constructor
}
A(int a)
{
System.out.println("2");
}
public static void main(String[] args) {
A obj1=new A(2);
}
}
I've checked many answers on this topic on StackOverflow but I still could not understand the reason for this. Please help me to make clear about this error with some easy example and explanation.
我已经在 StackOverflow 上检查了很多关于这个主题的答案，但我仍然无法理解这样做的原因。请通过一些简单的示例和解释帮助我澄清此错误。
采纳答案by óscar López
As you know, this works:
如您所知，这有效：
A() {
this(1);
System.out.println("1");
}
Why? because it's a rule of the language, present in the Java Language Specification: a call to another constructor in the same class (the this(...)part) or to a constructor in the super class (using super(...)) must go in the first line. It's a way to ensure that the parent's state is initialized beforeinitializing the current object.
为什么？因为它是语言的规则，出现在 Java 语言规范中：对同一类（this(...)部分）中的另一个构造函数或超类（using super(...)）中的构造函数的调用 必须放在第一行。这是一种确保在初始化当前对象之前初始化父级状态的方法。
For more information, take a look at this post, it explains in detail the situation.
有关更多信息，请查看此帖子，它详细说明了情况。
回答by T I
The error tells you the problem
错误告诉你问题
A()
{
System.out.println("1");
this(1); //Error: Constructor call must be the first statement in a constructor
}
i.e. you must call the constructor first
即您必须先调用构造函数
A()
{
this(1);
System.out.println("1");
}
this also applies to calls to super
这也适用于调用 super
class B extends A
{
B()
{
super();
System.out.println("1");
}
}
the reason being is answered here
原因在这里得到了回答
