以这两种方式创建对象有什么区别？

Question

i wonder what the difference is between these:

1-)

JFrame frame = new JFrame();
JLabel label = new JLabel("example");
frame.add(label);

2-)

JFrame frame = new JFrame();
frame.add(new Label("example"));

also, we can use the syntax like this:

1-)

new Timer(10, new ActionListener() {...}).start();

but why cannot we use it like this:

2-)

Timer timer = new Timer(10, new ActionListener() {...}).start(); // we cannot use it this way it has to be like:

//Timer timer = new Timer(10, new ActionListener() {...});
//timer.start();

Answer 1

the difference between these two are like this,

if you don't require the reference variable to object while sending that as an argument to a method you can directly generate a object while calling the method.

like this frame.add(new Label("example"));

but if you do require the object you would be passing as argument, it would be better to have it's reference variable so you can do something with it latter in code,

let's say you want to change some instance variable or get he status of Object after the method completed it's task. In such cases you would require the reference variable to the Object

and yes answer to your other question is given by Ramanlfc

which says new Timer(10, new ActionListener() {...}).start();

this can be done as the method return type is ignored here, this task will be done as the statement issued,

but

Timer timer = new Timer(10, new ActionListener() {...}).start(); 

this is not possible as start() does not return Timer which is being assigned to the Timer reference variable

Answer 2

Timer timer = new Timer(10, new ActionListener() {...}).start();

assignments are right to left ,so what your expression evaluates to should be assignable to the operand on the left of = . start() will not return Timer in this case ,so it fails.

Answer 3

In the 2 first examples you're creating a JFrame and assigning it to a JFrame variable. Same with the label.

Timer timer = new Timer(10, new ActionListener() {...}).start();

Will not work because the return type of

start();

is void.

new Timer(10, new ActionListener() {...}).start();

Works because you never try to assign the void return value to a Timer.

Answer 4

The other Answers are correct.

Another way to think about it is that the call to the new JLabel creates the JLabel object in memory immediately. Only after construction does the object get assigned to your JLabel label variable. The label variable is not your JLabel object. It is a reference, a name that can take us to the JLabel object. The JLabel object itself is floating around in memory someplace.

Astronaut Tether

The JLabel object is like an astronaut on a spacewalk. Assigning that object to a label variable is like putting the end of the astronaut's tether in your hand. Using that label variable is like tugging on that tether to get to the astronaut.

Your alternate code passes a reference to the JLabel object off to the JFrame object. Again, the JLabel object already exists wholly and intact before the hand-off to the JFrame. After the hand-off, the local code has "forgotten" about the JLabel. No local reference variable means no way to get back to the JLabel object. The JLabel object still exists in memory some place, but is out of reach to our local code. In our spacewalk analogy, handing the reference off to the JFrame is like handing over the astronaut's tether to somebody else.

If we kept a local JLabel reference variable and handed off to the JFrame, that would be like hooking up two tethers to our space-walking astronaut. You hold one tether in your hand (the local var) while passing the second tether to someone else (the JFrame object).

If all the references are set to null or go out of scope, that is like releasing all the tethers to our astronaut. The object still lives in memory for a while but becomes a candidate for garbage collection. Actual removal from memory happens later, when the garbage collector runs and decides it is time to recycle that memory space.