Confused with object references

Question

I am preparing myself to the Java certification and confused with object references in this case. In this piece of code I can't understand why ArrayList's and arrays's elements aren't affected when we assign a new object to them?

    ArrayList<StringBuilder> myArrList = new ArrayList<StringBuilder>();
    StringBuilder sb1 = new StringBuilder("Jan");
    StringBuilder sb2 = new StringBuilder("Feb");
    myArrList.add(sb1);
    myArrList.add(sb2);

    StringBuilder[] array = myArrList.toArray(new StringBuilder[2]);
    for(StringBuilder val : array) {
        System.out.println(val);
    }

    StringBuilder sb3 = new StringBuilder("NNN");
    sb2 = sb3;

    for(StringBuilder val : array) {
        System.out.println(val);
    }

    for(StringBuilder val : myArrList) {
        System.out.println(val);
    }

Output:

Jan
Feb
Jan
Feb
Jan
Feb

I will be grateful if you could provide simple explanation. Thank you.

Read this http://stackoverflow.com/questions/40480/is-java-pass-by-reference — Christian Tapia, Feb 11 '14 at 06:46

JB Nizet · Accepted Answer · 2014-02-11T07:02:36.423

References are pointers. Assigning a new value to a variable consists in making this pointer point to another object. So, at the beginning, you have an array with two elements:

array[0] ---> Jan <--- sb1
array[1] ---> Feb <--- sb2

Then you create another StringBuilder, referenced by sb3:

array[0] ---> Jan <--- sb1
array[1] ---> Feb <--- sb2
sb3 --------> NNN

Then you say that the sb2 variable should reference the same object as the sb3 variable:

array[0] ---> Jan <--- sb1
array[1] ---> Feb 
sb3 --------> NNN <--- sb2

As you're seeing, array[0] and array[1] still reference the same objects Jan and Feb.

Confused with object references

1 Answers1