Getting different output from main

Question

package com.test.scjp;

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

public class TestGeneric {
    public static void main(String args[]){
        try{
            List<String> l = new ArrayList<String>();
            TestGeneric t = new TestGeneric();
        //t.test(l);
            System.out.println("Test");
            l=t.test(l);
            System.out.println("Test2");

            System.out.println(l.get(0));
        }
        catch(Exception e){
            e.printStackTrace();
        }
    }
    public List<String> test(List l){
        System.out.println("Test0");
        l.add(new Integer(1));
        System.out.println("Test1");
        return l;
    }
}

When I run this code 20 times I am getting different outputs:

Output 1:

Test
Test0
Test1
Test2
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
    at com.test.scjp.TestGeneric.main(TestGeneric.java:16)

Output 2:

Test
Test0
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
    at com.test.scjp.TestGeneric.main(TestGeneric.java:16)
Test1
Test2

Test Test0 Test1 Test2 java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String at com.test.scjp.TestGeneric.main(TestGeneric.java:16) — Rajesh Sahoo, May 29 '13 at 21:52
`l.add(new Integer(1))` <-- you are trying to add an integer to a list containing String elements. — fge, May 29 '13 at 21:54
Output in console comes from two streams, `System.out`, and `System.err`, and as you can see, sometimes output from `err` can be printed while printing output from `out`. To make it more readable IDEs like Eclipse use different color on `err`. — Pshemo, May 29 '13 at 21:57
@JBNizet Nah, answer should fully explain this problem. I proffered to post general idea as comment and let others explain it better, since I am too tired and going to sleep now. Goodnight all :D — Pshemo, May 29 '13 at 22:12

score 2 · Answer 1 · edited May 23 '17 at 11:57

2

System.out and System.err work on different threads. Both threads access a shared resource (the console) at different times.

See also Random printing order for System.out & System.err calls and Java: System.out.println and System.err.println out of order

edited May 23 '17 at 11:57

Community

1
1

answered May 29 '13 at 22:00

Paul Vargas

41,222
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score 0 · Answer 2 · answered May 29 '13 at 22:00

0

Inside test, your list isnt Generic - so you're allowed to add any Object to it.

The problem is that outside of it you're using a List that is restricted to Strings.

Its this conflict thats causing your problems.

answered May 29 '13 at 22:00

pauljwilliams

19,079
3
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79

Getting different output from main

2 Answers2