ArrayList没有打印出来

Question

I have created two arrays. I added them up. Then I wanted to print out the new array by defining a method. But the method can't print out the array. Where is the error in my code?

package stringpractice;

import java.util.LinkedList;
import java.util.List;

public class StringPractice {

    public static void main(String[] args) {

        String[] boy = {"John", "Russel", "Ryan"};
        List<String> l1 = new LinkedList<String>();

        for (String x : l1) {
            l1.add(x);

        }

        String[] girl = {"Sara", "Leena", "Emilia"};
        List<String> l2 = new LinkedList<String>();
        for (String y : l2) {

            l2.add(y);
            l1.addAll(l2);
            l2 = null;
            printMe(l1);

            /*removeStuff(l1, 1,2);
             reverseMe(l1);
             */
        }

    }

    public static void printMe(List<String> l1) {
        for (String p : l1) {
            System.out.printf("%s ", p);
        }
        System.out.println();

    }

}

Answer 1

There is nothing to print in your list. You need to change

for (String x : l1) // l1 don't have a single element
                    // at this moment 

and

for (String y : l2) // same as l1 

To

for (String x : boy)

and

for (String x : girl) 

Again

 l2.add(y);
 l1.addAll(l2);
 l2 = null; // inside for lopp l2 become null

So you will get NPE from here.

Corrected code

  public static void main(String[] args) {
    String[] boy = {"John", "Russel", "Ryan"};
    List<String> l1 = new LinkedList<>();
    for (String x : boy) {
        l1.add(x);
    }
    String[] girl = {"Sara", "Leena", "Emilia"};
    List<String> l2 = new LinkedList<>();
    for (String y : girl) {
        l2.add(y);
        l1.addAll(l2);
        printMe(l1);
    }
    l2 = null;
}

public static void printMe(List<String> l1) {
    for (String p : l1) {
        System.out.printf("%s ", p);
    }
    System.out.println();
}

Answer 2

The for-loops walk through the still empty lists.

   for (String x : boys) {
        l1.add(x);
   }

Or

   Collections.addAll(l1, boys);

Setting l2 to null will not loop well. And the addAll can be done outside the loop.

Answer 3

your both the List L1 and L2 are empty. so your both the for loop body is not executed.

Answer 4

public class StringPractice { public static void main(String[] args) {

    String[] boy = { "John", "Russel", "Ryan" };
    List<String> l1 = new ArrayList<String>(Arrays.asList(boy));

    String[] girl = { "Sara", "Leena", "Emilia" };
    List<String> l2 = new ArrayList<String>(Arrays.asList(girl));

    l1.addAll(l2);
    l2 = null;
    printMe(l1);

    /*removeStuff(l1, 1,2);
     reverseMe(l1);
     */

}

public static void printMe(List<String> l1) {
    for (String p : l1) {
        System.out.printf("%s ", p);
    }
    System.out.println();

}

Answer 5

You arent traversing the boy[] and girl[] array anywhere. You need to iterate like for (String x : boy) for some result.

Answer 6

您需要遍历boy []和girl []数组以在其中打印内容