Q1. Difference between HashMap and Hashtable?
Ans. Hashtable is synchronized whereas HashMap is not.
HashMap allows null values whereas Hashtable doesn’t allow null values.
Q2. Can we create a null as a key for a map collection ?
Ans. Yes , for Hashtable. Hashtable implements Map interface.
Q3. How would you go about debugging a NullPointerException?
Ans. Open ended Question.
Q4. What will be the output of following code ?
public static void main(String[] args){
String name = null;
File file = new File("/folder", name);
System.out.print(file.exists());
}
Ans. NullPointerException at line:
"File file = new File("/folder", name);"
Q5. What will be the output of following code ?
public static void main(String[] args){
String child = null;
File file = new File("/folder", child);
try {
file.createNewFile();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
Ans. NullPointerException at line:
File file = new File("/folder", child);
Q6. Can we have null keys in TreeMap ?
Ans. No, results in exception.
Q7. Can value be null in TreeMap ?
Ans. Yes.
Ans. Hashtable is synchronized whereas HashMap is not.
HashMap allows null values whereas Hashtable doesn’t allow null values.
Q2. Can we create a null as a key for a map collection ?
Ans. Yes , for Hashtable. Hashtable implements Map interface.
Q3. How would you go about debugging a NullPointerException?
Ans. Open ended Question.
Q4. What will be the output of following code ?
public static void main(String[] args){
String name = null;
File file = new File("/folder", name);
System.out.print(file.exists());
}
Ans. NullPointerException at line:
"File file = new File("/folder", name);"
Q5. What will be the output of following code ?
public static void main(String[] args){
String child = null;
File file = new File("/folder", child);
try {
file.createNewFile();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
Ans. NullPointerException at line:
File file = new File("/folder", child);
Q6. Can we have null keys in TreeMap ?
Ans. No, results in exception.
Q7. Can value be null in TreeMap ?
Ans. Yes.