Find max element in Java ArrayList

Question

I have such simple code:

import java.util.ArrayList;
import java.util.List;

public class Main {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        Integer[] tab = {2324,1,2,2324,3,45,1,5,0,9,13,2324,1,3,9,8,4,2,1};
        Integer max = 2324;
        List<Integer> indexes = new ArrayList<Integer>();
            for (int e = 0; e < tab.length; e++) {
                if (tab[e] == max) {
                    indexes.add(new Integer(e));
                    System.out.println("Found max");
                }
            }
    }
}

The main problem here is I want to find every index in my tab where the max value is. For now on, it doesnt work - it doesnt display Found max message even once, although it should do it 3 times. Wheres the problem?

Ok, this finally worked, thanks all of you people:

public static void main(String[] args) {
        Integer[] tab = {2324,1,2,2324,3,45,1,5,0,9,13,2324,1,3,9,8,4,2,1};
        Integer max = 2324;
        List<Integer> indexes = new ArrayList<Integer>();
            for (int e = 0; e < tab.length; e++) {
                if (tab[e].intValue() == max.intValue()) {
                    indexes.add(Integer.valueOf(e));
                    System.out.println("Found max");
                }
            }
    }

One hint. Use Integer.valueOf(...) instead of new Integer() since it is more memory efficient (uses cached values for small numbers). — Michal Borek, May 13 '13 at 12:13
http://stackoverflow.com/questions/8304767/how-to-get-maximum-value-from-the-list-arraylist — Raghunandan, May 13 '13 at 12:16
Integer.valueOf(e) instead of new Integer(e). Semantically the same, but JVM will return pre-cached objects of numbers between -128 and 127 — Michal Borek, May 13 '13 at 12:17
@Raghunandan: OPs title is misleading. Your linked question has nothing in common to this one. — jlordo, May 13 '13 at 12:17

jlordo · Answer 1 · 2013-05-13T12:17:40.407

9

Change

Integer[] tab = {2324,1,2,2324,3,45,1,5,0,9,13,2324,1,3,9,8,4,2,1};

to

int[] tab = {2324,1,2,2324,3,45,1,5,0,9,13,2324,1,3,9,8,4,2,1};

Integer objects are only precached for the values from -128 to 127.

If you want to leave it Integer you can change

if (tab[e] == max) {

to

if (tab[e].equals(max)) {

because it will then check for object equality, and not reference equality.

edited May 13 '13 at 12:17

answered May 13 '13 at 12:11

jlordo

37,490
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score 3 · Answer 2 · answered May 13 '13 at 12:12

3

That's because you are comparing with == and not equals.

answered May 13 '13 at 12:12

Michal Borek

4,584
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Manuel Reis · Answer 3 · 2015-08-02T17:16:41.947

3

You are using the == operator in something that it is not the primitive int, but an instance of class Integer. Basically, you are comparing the references of both objects, which are different. Try using :

if(tab[e].equals(max))

edited Aug 02 '15 at 17:16

answered May 13 '13 at 12:13

Manuel Reis

574
8
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score 2 · Answer 4 · answered May 13 '13 at 12:14

The basic problem you have is that you are using Integer, not int One difference being that as Integer is an object == compares the references to two different objects. (Not he contents of those objects)

I suggest you use primitives like int instead of objects where you can.

score 2 · Answer 5 · edited May 23 '17 at 12:21

2

You can only compare primitive values with ==. Since Integer is an object, change tab[e] == max to tab[e].equals(max).

Look for equals vs ==

Also read: Java: int vs integer

edited May 23 '17 at 12:21

Community

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1

answered May 13 '13 at 12:14

rawphl

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darijan · Answer 6 · 2015-08-06T13:57:42.370

2

The JVM is caching Integer values.

== only works for numbers between -128 and 127

See explanation here: http://www.owasp.org/index.php/Java_gotchas#Immutable_Objects_.2F_Wrapper_Class_Caching

edited Aug 06 '15 at 13:57

answered May 13 '13 at 12:19

darijan

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score 1 · Answer 7 · answered May 13 '13 at 12:13

1

Use one of the three : 1. tab[e].intValue() == max or 2. int max = 2324; or 3. Use equals() method of Integer class.

answered May 13 '13 at 12:13

AllTooSir

48,828
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score 1 · Accepted Answer · answered May 13 '13 at 12:13

1

try this:

if (tab[e].intValue() == max.intValue()) {

or

if (tab[e].intValue() == max) {

If you are using Integer object rather than primitive int, then with comparison operator like == , atleast one operand should be primitive one (other will be converted implicitly).

Or you should use equals method for equality

answered May 13 '13 at 12:13

vishal_aim

7,636
1
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Hmm, strange. Changed it as you suggested but nothing changed. – Brian Brown May 13 '13 at 12:15
@BrianBrown it changed on my system, can you provide the code again what you tried? – vishal_aim May 13 '13 at 12:19

Find max element in Java ArrayList

8 Answers8