在堆上创建对象（Java）之后是否调用对象的构造函数？

Question

When you instantiate a new object by calling constructor, ie Foo bar = new Foo(var);

When does the code in the constructor actually gets invoked in relations to object creation heap? When the constructors modifies member variables of bar, is storage for the variables already allocated and contain default values?

Answer 1

Once new is called it knows how much memory needs to be allocated into the heap for a variable of type, in your case Foo. Once that memory is allocated only then are the values set. Think about it how else are you going to assign member variables if you don't have memory for the member variable? If there is no memory new will throw an exception which you need to handle.

Process:

1. JVM sees new
2. Allocates memory on the heap to store the object (Ref type)
3. Assigns default values
4. If of object type assigns null
5. Call constructor

Answer 2

当JVM遇到new关键字时，它将为该类类型分配所需的内存，如果没有初始化，则它将所有成员初始化为其默认值，如果该成员是一个对象，则将其初始化为null。

Answer 3

Here Foo bar = new Foo(var); we are creating bar object.When we use new keyword memory is allocated on the heap.The amount of memory allocated depends on the instance variables of the class.The JVM will calculate the amount of memory to be allocated, then using new it will allocate the memory.Here bar is a reference variable pointing on the heap where the object is allocated.

Answer 4

The constructor cannot be called until the memory exists.

For member variables, it is a recursive application of the same rule.