java初始化的过程

Posted 2020-06-27 慢慢来

面试和笔试中总能遇到这样的问题，像下面一样的代码具体的执行顺序（ABCD都是类）

public class A {
    public A(String text){
        System.out.println("A was built :"+text);
    }

    D b1=new D("d in A");

    static{
        System.out.println("A Static Code out of constructor");
    }
    static D b2=new D("static b in A");
    {
        System.out.println("A Code out of constructor");
    }
}

之前自己已经知道了，今天复习《Thinking in Java》又看到了这一段，在这里用代码来看看结果。最后给大家一个结论 代码如下（虽然有点长，但是仔细看还是挺简单的）

public class A {
    public A(String text){
        System.out.println("A was built :"+text);
    }
    D b1=new D("d in A");

    static{
        System.out.println("A Static Code out of constructor");
    }
    static D b2=new D("static b in A");



    {
        System.out.println("A Code out of constructor");
    }


}
class B extends A{
    static{
        System.out.println("B Static Code out of constructor");
    }

    {
        System.out.println("B Code out of constructor");
    }
    public B(String text){
        super(text);
        System.out.println("B was built :"+text);
    }
}
class D{
    public D(String text){
        System.out.println("D was built :"+text);
    }
    public static void fsdaf(){

    }
}

class C{

    static{
        System.out.println("C static code out of constructor");
    }
    public C(String text){
        System.out.println("C was built :"+text);
    }
}

class TestABC{
    public static void main(String[] args) {
     // A testA=null;
        A testA=new A("testA");
        System.out.println("********************************");
        A testAf=new A("testA");
        System.out.println("--------------------------------");
        B testB=new B("testB");
        System.out.println("********************************");
        C testC=new C("testC");

    }
}

结果如下

A Static Code out of constructor
D was built :static b in A
D was built :d in A
A Code out of constructor
A was built :testA
********************************
D was built :d in A
A Code out of constructor
A was built :testA
--------------------------------
B Static Code out of constructor
D was built :d in A
A Code out of constructor
A was built :testB
B Code out of constructor
B was built :testB
********************************
C static code out of constructor
C was built :testC

从上面可以看出，如果没有继承父类 那么执行的顺序是

1.static类型的对象和代码段(那个在前面先执行哪个 且该类没有被实例化过)

2.类中非static的代码段

3.构造函数 如果该类已被实例化过，那么static的内容就不会再次被执行。

如果继承了父类 那么执行顺序是

1.static类型的对象和代码段(那个在前面先执行哪个 且该类没有被实例化过)

2.父类中的代码段

3.父类的构造函数

4.子类中的非静态代码段

5.子类构造函数

 

 

另外static的内容并不是代码写好就已经被执行，而是必须在使用到或者实例化的时候才会运行，这一点需要注意