What's the difference between instantiating in the field and instantiating in the constructor?

Question

What's the difference between doing this:

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

and this

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

Answer 1

Time of construction. The first is done before main is entered. The other object's construction is delayed until SomeClass's ctor is called.

Answer 2

In the first instance, obj will be initialized before the constructor is run. This is an important nuance when you have subclasses. The ordering of constructors and initialization blocks will be:

1. superclass initializers
2. superclass constructor
3. subclass initializers
4. subclass constructor

Answer 3

The only difference is in at which step the reference is initialized. The final effect is the same.

Answer 4

According to the chapter 12.5 Creation of New Class Instances of the Java Language Specification:

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.

So the difference is just the step (step 4. or step 5.) but the result is the same.

Answer 5

This is only a matter of style, it compiles to the same code.

Personally I tend to put all initialization of instances in constructors, because that works uniformly for all cases.