Java循环传递值

Question

I think this question is related to this question: Is Java "pass-by-reference" or "pass-by-value"? , but not really the same.

Suppose I have this loop code:

ArrayList<foo> list = new ArrayList<foo>();
Calendar cal = Calendar.getInstance();
for(int i = 0; i < 10; ++i) {
    cal.set(Calendar.HOUR, i);
    list.add(new foo(cal));
}
for(int i = 0; i < 10; ++i) {
    System.out.print(list.get(i).calToString());
}

foo.class:

public class foo {
    private Calendar mCal;
    public foo(Calendar cal) {
        mCal = cal;
    }
    public String calToString() {
        return String.valueOf(mCal.get(Calendar.HOUR));
    }
}

The resulting list has all its items Calendar.HOUR set to 9 . It prints 9999999999 . How can I make it so that each item will have 0-9 respectively? Will instantiating variable cal inside the loop be a performance issue (if in case foo is a more complex class)?

Answer 1

Actually you are overwriting your object, you declared it outside the loop and you keep changing it's value.

You should create new instance inside the loop. No, you won't notice any performance issues.

Answer 2

You would need to initialize a new Calendar for each iteration of your loop.

Something like:

for(int i = 0; i < 10; ++i) {
    Calendar cal = Calendar.getInstance();

Otherwise, the same instance is referenced everytime.

Therefore, the set invocation modifies the same Calendar , which is why you are getting all 9 s.

More elegantly perhaps, you could initialize your Foo class with a constructor taking the actual hour int .

Then you would change Foo to have its own Calendar instance.

Answer 3

class Main

{ public static void main(String[] args) {

    ArrayList<foo> list = new ArrayList<foo>();
    Calendar cal = null;
    for (int i = 0; i < 10; ++i) {
        cal=Calendar.getInstance();
        cal.set(Calendar.HOUR, i);
        list.add(new foo(cal));
    }
    for (int i = 0; i < 10; ++i) {
        System.out.print(list.get(i).calToString());
    }
}

}

class foo { private Calendar mCal;

public foo(Calendar cal) {
    mCal = cal;
}

public String calToString() {
    return String.valueOf(mCal.get(Calendar.HOUR));
}

}

you are overwriting your calender object