foreach not applicable to java.lang.object and compilation error when trying to iterate through array list of array lists in java

Question

I have an array list of array lists. And I am trying to iterate through them. However I keep getting compilation error. Where am I going wrong. Is there a better way to iterate.

Code:

import java.util.ArrayList;

public class ListofLists {
    public static ArrayList family() {
        // ArrayList of ArrayLists
        ArrayList<ArrayList<String>> couples = new ArrayList<ArrayList<String>>();
        ArrayList<String> husbands = new ArrayList<String>();
        husbands.add("brad");
        husbands.add("jessie");
        couples.add(husbands);
        ArrayList<String> wives = new ArrayList<String>();
        wives.add("jolie");
        wives.add("jena");
        couples.add(wives);
        return couples;
    }


    public static void main(String[] args) {
        ArrayList couples = family();

        for (Object couple : couples) {
            for (String person: couple) {
                System.out.println(person);
            }
        }
    }
}

Compilation Error:

required: array or java.lang.Iterable
  found:    Object

Expected Output:

brad
jessie
jolie
jena

Although the error is obvious from inspection, it is generally required that you tell us the actual compiler error you encountered. It might not be so obvious in all cases. — Jim Garrison
– Jim Garrison, Commented Jan 26, 2014 at 2:06

anon · Accepted Answer · 2014-01-26 02:10:09Z

4

WHat you need is the following:

public static void main(String[] args) {
    ArrayList<ArrayList<String>> couples = family();

    for (ArrayList<String> couple : couples) {
        for (String person : couple) {
            System.out.println(person);
        }
    }
}

Basically, you were storing the results from your call to familiy() in an ArrayList of unknown type. It was automatically getting boxed to Object, and for-each doesn't work for Objects.

answered Jan 26, 2014 at 2:10

anon

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Comments

mojiayi · Accepted Answer · 2014-01-26 02:26:45Z

2

iterator is a feature of collection,class Object itself does not provide such feature. Look at my code.It's work well.

package com.test;

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

public class ListOfLists {
public static List<List<String>> family() {
    // ArrayList of ArrayLists
    List<List<String>> couples = new ArrayList<List<String>>();
    List<String> husbands = new ArrayList<String>();
    husbands.add("brad");
    husbands.add("jessie");
    couples.add(husbands);
    List<String> wives = new ArrayList<String>();
    wives.add("jolie");
    wives.add("jena");
    couples.add(wives);
    return couples;
}

public static void main(String[] args) {
    List<List<String>> couples = family();

    for (List<String> couple : couples) {
        for (String person : couple) {
            System.out.println(person);
        }
    }
}

}

edited Jan 26, 2014 at 2:26

answered Jan 26, 2014 at 2:12

mojiayi

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4 Comments

Sotirios Delimanolis Over a year ago

This is not a useful answer. Anyone can just post code. The point is to explain it.

mojiayi Over a year ago

Ok,I just check java doc,iterator is a feature of collection,class Object itself does not provide such feature.

Shankar Over a year ago

what is the advantage of using List<List<String>> over ArrayList<ArrayList<String>> ??

mojiayi Over a year ago

In current code,there is no difference between two ways.Generally speaking,I prefer to List<String>.You can get more details from this link type-list-vs-type-arraylist-in-java.

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