java – 在实例化领域和实例化构造函数有什么区别？

这样做有什么区别？

public class SomeClass {
    SomeObject obj = new SomeObject();
    //rest of the code
}

和这个

public class SomeClass {
    SomeObject obj;
    public SomeClass(){
       obj = new SomeObject();
    }
    //rest of the code
}

解决方法

根据Java语言规范第 12.5 Creation of New Class Instances章：

Just before a reference to the newly
created object is returned as the
result,the indicated constructor is
processed to initialize the new object
using the following procedure:

1. Assign the arguments for the constructor to newly created parameter
   variables for this constructor
   invocation.
2. If this constructor begins with an explicit constructor invocation of
   another constructor in the same class
   (using this),then evaluate the
   arguments and process that constructor
   invocation recursively using these
   same five steps. If that constructor
   invocation completes abruptly,then
   this procedure completes abruptly for
   the same reason; otherwise,continue
   with step 5.
3. This constructor does not begin with an explicit constructor
   invocation of another constructor in
   the same class (using this). If this
   constructor is for a class other than
   Object,then this constructor will
   begin with an explicit or implicit
   invocation of a superclass constructor
   (using super). Evaluate the arguments
   and process that superclass
   constructor invocation recursively
   using these same five steps. If that
   constructor invocation completes
   abruptly,then this procedure
   completes abruptly for the same
   reason. Otherwise,continue with step
4. Execute the instance initializers and instance variable initializers for
   this class,assigning the values of
   instance variable initializers to the
   corresponding instance variables,in
   the left-to-right order in which they
   appear textually in the source code
   for the class. If execution of any of
   these initializers results in an
   exception,then no further
   initializers are processed and this
   procedure completes abruptly with that
   same exception. Otherwise,continue
   with step 5. (In some early
   implementations,the compiler
   incorrectly omitted the code to
   initialize a field if the field
   initializer expression was a constant
   expression whose value was equal to
   the default initialization value for
   its type.)
5. Execute the rest of the body of this constructor. If that execution
   completes abruptly,then this
   procedure completes abruptly for the
   same reason. Otherwise,this procedure
   completes normally.

所以区别只是一步(步骤4.或步骤5.),但结果是一样的.