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
– Michal Borek, Commented May 13, 2013 at 12:13
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
– Michal Borek, Commented May 13, 2013 at 12:17
@Raghunandan: OPs title is misleading. Your linked question has nothing in common to this one. — jlordo
– jlordo, Commented May 13, 2013 at 12:17

jlordo · Accepted Answer · 2013-05-13 12:17:40Z

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, 2013 at 12:17

answered May 13, 2013 at 12:11

jlordo

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Comments

Michal Borek · Accepted Answer · 2013-05-13 12:12:12Z

3

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

answered May 13, 2013 at 12:12

Michal Borek

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Comments

Manuel Reis · Accepted Answer · 2015-08-02 17:16:41Z

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 2, 2015 at 17:16

answered May 13, 2013 at 12:13

Manuel Reis

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Comments

Peter Lawrey · Accepted Answer · 2013-05-13 12:14:01Z

2

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.

answered May 13, 2013 at 12:14

Peter Lawrey

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Comments

Community · Accepted Answer · 2017-05-23 12:21:57Z

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 ==

Comments

darijan · Accepted Answer · 2015-08-06 13:57:42Z

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 6, 2015 at 13:57

answered May 13, 2013 at 12:19

darijan

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Comments

vishal_aim · Accepted Answer · 2013-05-13 12:13:50Z

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, 2013 at 12:13

vishal_aim

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2 Comments

Brian Brown Over a year ago

Hmm, strange. Changed it as you suggested but nothing changed.

vishal_aim Over a year ago

@BrianBrown it changed on my system, can you provide the code again what you tried?

AllTooSir · Accepted Answer · 2013-05-13 12:13:05Z

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, 2013 at 12:13

AllTooSir

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