How to change value of ArrayList element in java

Question

Please help me with below code , I get the same output even after changing the value

import java.util.*;

class Test {
    public static void main(String[] a         


        

Answer 1

I think the problem is that you think the statement ...

x = Integer.valueOf(9);

... causes that the value of '9' get 'stored' into(!) the Object on which x is referencing.

But thats wrong.

Instead the statement causes something similar as if you would call

x = new Integer(9); 

If you have a look to the java source code, you will see what happens in Detail.

Here is the code of the "valueOf(int i)" method in the "Integer" class:

public static Integer valueOf(int i) {
    assert IntegerCache.high >= 127;
    if (i >= IntegerCache.low && i <= IntegerCache.high)
        return IntegerCache.cache[i + (-IntegerCache.low)];
    return new Integer(i);
}

and further, whenever the IntegerCache class is used for the first time the following script gets invoked:

static {
        // high value may be configured by property
        int h = 127;
        String integerCacheHighPropValue =
            sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
        if (integerCacheHighPropValue != null) {
            int i = parseInt(integerCacheHighPropValue);
            i = Math.max(i, 127);
            // Maximum array size is Integer.MAX_VALUE
            h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
        }
        high = h;

        cache = new Integer[(high - low) + 1];
        int j = low;
        for(int k = 0; k < cache.length; k++)
            cache[k] = new Integer(j++);
    }

You see that either a new Integer Object is created with "new Integer(i)" in the valueOf method ... ... or a reference to a Integer Object which is stored in the IntegerCache is returned.

In both cases x will reference to a new Object.

And this is why the reference to the Object in your list get lost when you call ...

x = Integer.valueOf(9);

Instead of doing so, in combination with a ListIterator use ...

i.set(Integer.valueOf(9));

... after you got the element you want to change with ...

i.next();