When a HashMap contains null keys or values,
converting any entries (as returned by the entrySet
method) to strings throws a NullPointerException

import java.util.Map;
import java.util.Set;
import java.util.HashMap;

public class HashMapTest {
    public static void main(String[] arg) {
        Map map = new HashMap();
        map.put("hello", "world");
        System.out.println(map);
        System.out.println(map.entrySet());
        
        try {
            map = new HashMap();
            map.put(null, "world");
            System.out.println(map);
            System.out.println(map.entrySet());
        }
        catch (NullPointerException e) {
            e.printStackTrace();
        }

        try {
            map = new HashMap();
            map.put("hello", null);
            System.out.println(map);
            System.out.println(map.entrySet());
        }
        catch (NullPointerException e) {
            e.printStackTrace();
        }
    }
}

Output generated is:

{hello=world}
[hello=world]
{null=world}
java.lang.NullPointerException:
        at java.util.HashMap$Entry.toString(HashMap.java:630)
        at java.lang.String.valueOf(String.java:1540)
        at java.util.AbstractCollection.toString(Compiled Code)
        at java.lang.String.valueOf(String.java:1540)
        at java.io.PrintStream.print(PrintStream.java:423)
        at java.io.PrintStream.println(PrintStream.java:542)
        at HashMapTest.main(HashMapTest.java:35)
{hello=null}
java.lang.NullPointerException:
        at java.util.HashMap$Entry.toString(HashMap.java:630)
        at java.lang.String.valueOf(String.java:1540)
        at java.util.AbstractCollection.toString(Compiled Code)
        at java.lang.String.valueOf(String.java:1540)
        at java.io.PrintStream.print(PrintStream.java:423)
        at java.io.PrintStream.println(PrintStream.java:542)
        at HashMapTest.main(HashMapTest.java:45)