Get the position of a given item in an ArrayList

Question

I have an ArrayList of type Vertex. Vertex is my own class which contains only one data member of String type. It has a member function getName() which returns the name of the Vertex. I want to get the position in the ArrayList, if a particular string is given. I've written the below code to do it. But it always returns -1, which is the initial value. What is the problem with my code?

    public int map(String vname)
    {
        int pos=-1;
        for(int i=0;i<nodes.size();i++)
        {
            if(nodes.get(i).getName()==vname)
            {
                pos=i;
                break;  
            }
        }
        return pos;
    }

In the above code, nodes is an ArrayList of type Vertex.

NOOOOO.. not agaaaaaain :_( Don't use `==` to compare strings in Java. Use `equals`. — Maroun, May 24 '13 at 06:17
Everytime a string is compared with `==` for equality in Java, a developer cries (@MarounMaroun case and point) — Craig, May 24 '13 at 06:20
For reference types, please understand that there is a conceptual difference between identity and equality. Using the '==' operator for reference types asks "is this reference type the very same one as the other." To ask, "are the values of these objects equal (even if they are different objects)" you must use the .equals() method. — scottb, May 24 '13 at 06:21
@AparnaShajiPeter See the String API, it should help you to better understand it: http://docs.oracle.com/javase/7/docs/api/java/lang/String.html#equals(java.lang.String) — Maroun, May 24 '13 at 06:24

score 4 · Answer 1 · edited May 24 '13 at 06:23

4

In Java == is for object equality, string1.equals(string2) is for string equality

edited May 24 '13 at 06:23

Maroun

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answered May 24 '13 at 06:18

Craig

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Good Guy MarounMaroun, doesn't post an answer, makes everyones answers better – Craig May 24 '13 at 06:25
3

I got enough reputation points from `==` vs `equals`, it's your time to shine now :D – Maroun May 24 '13 at 06:26

score 4 · Accepted Answer · edited May 24 '13 at 06:20

4

Use String#equals: nodes.get(i).getName().equals(vname) instead. == in Java compares address of the string.

edited May 24 '13 at 06:20

Maroun

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answered May 24 '13 at 06:19

NcJie

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score 4 · Answer 3 · edited May 24 '13 at 06:22

4

== compares references not the content.

Use String#equals() if case is important otherwise use String#equalsIgnoreCase().

edited May 24 '13 at 06:22

Maroun

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answered May 24 '13 at 06:20

PSR

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@Maroun Maroun Thanks for edit – PSR May 24 '13 at 06:24
@Maroun Maroun you are doing good job man – PSR May 24 '13 at 06:28

score 2 · Answer 4 · answered May 24 '13 at 06:18

2

You String comparison is wrong...

if(nodes.get(i).getName()==vname)

Should be

if(nodes.get(i).getName().equals(vname))

answered May 24 '13 at 06:18

MadProgrammer

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score 2 · Answer 5 · edited May 24 '13 at 06:47

2

Replace it with

if(nodes.get(i).getName()==vname)

to

if(nodes.get(i).getName().equals(vname))

for comparing String with the case And for Comparing vname without case use

if(nodes.get(i).getName().equalsIgnoreCase(vname))

edited May 24 '13 at 06:47

Marco Forberg

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answered May 24 '13 at 06:33

Hitesh Jain

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Get the position of a given item in an ArrayList

5 Answers5