How do you sort collection of StringBuffer object in java?

Problem statement: Given the collection of StringBuffer objects.
StringBuffer sb1 = new StringBuffer("Z");
StringBuffer sb2 = new StringBuffer("A");
StringBuffer sb3 = new StringBuffer("P");

You have to sort it either way, meaning natural order or custom order.

Answer:
// ASCENDING ORDER:

1. import java.util.Comparator;
2. import java.util.TreeSet;
3. public class Test {
4. public static void main(String[] args) {
5. SortStringBufferObject s = new SortStringBufferObject();
6. TreeSet<StringBuffer> t = new TreeSet<>(s);
7. t.add(new StringBuffer("Z"));
8. t.add(new StringBuffer("A"));
9. t.add(new StringBuffer("P"));
10. System.out.println(t);
11. }
12. }
13. class SortStringBufferObject implements Comparator<StringBuffer> {
14. public int compare(StringBuffer sb1, StringBuffer sb2) {
15. return sb1.toString().compareTo(sb2.toString());
16. }
17. }

Output:

[A, P, Z] 

// DESCENDING ORDER:

1. import java.util.Comparator;
2. import java.util.TreeSet;
3. public class Test {
4. public static void main(String[] args) {
5. SortStringBufferObject s = new SortStringBufferObject();
6. TreeSet<StringBuffer> t = new TreeSet<>(s);
7. t.add(new StringBuffer("Z"));
8. t.add(new StringBuffer("A"));
9. t.add(new StringBuffer("P"));
10. System.out.println(t);
11. }
12. }
13. class SortStringBufferObject implements Comparator<StringBuffer> {
14. public int compare(StringBuffer sb1, StringBuffer sb2) {
15. return sb2.toString().compareTo(sb1.toString());
16. }
17. }

Output:

[Z, P, A]
Reference

What will be the output when you add String object in HashSet?

Problem statement: Given code is 

1. public class Test {
2. public static void main(String[] args) {
3. HashSet<String> s = new HashSet<>();
4. s.add("A");
5. s.add("B");
6. s.add("C");
7. s.add("A");
8.   System.out.println("Elements: \n" + s);
9.   System.out.println("Size: " + s.size());
10.   System.out.println("Objects: " + s);
11. }
12. }

Answer:
Elements: 
[A, B, C]
Size: 3
Objects: [A, B, C]

What will be the output when we add elements in collection of ArrayList in java?

Problem statement: In given java code, can you please let me know, what will be the output.

1. import java.util.ArrayList;
2. class A {}
3. class B {}
4. class C {}
5. public class Test {
6. public static void main(String[] args) {
7. ArrayList list = new ArrayList();
8. list.add(new A());
9. list.add(new B());
10. list.add(new C());
11.   list.add(new A());
12. System.out.println(list);
13.   System.out.println("Size: " + list.size());
14. }
15. }

Answer:
[com.practice.core.A@15db9742, com.practice.core.B@6d06d69c, com.practice.core.C@7852e922, com.practice.core.A@4e25154f]
Size: 4

Note: fully_qualified_class_name@hashcode_value

How do you sort the keys in HashMap?

Problem statement:
Write the java code to sort the keys in HashMap.

1. import java.util.HashMap;
2. import java.util.Set;
3. import java.util.TreeSet;
4. public class Test {
5. public static void main(String[] args) {
6. HashMap<String, String> m = new HashMap<String, String>();
7. m.put("key45", "aa");
8. m.put("key12", "zz");
9. m.put("key39", "cc");
10. m.put("key27", "bb");
11. Set<String> s = m.keySet();
12. s = new TreeSet<String>(s);
13. System.out.println(s);
14. }
15. }

Output:
[key12, key27, key39, key45]

How do you sort the values in HashMap?

Problem statement:
Write the java code to sort the values in HashMap.

1. import java.util.Collection;
2. import java.util.HashMap;
3. import java.util.TreeSet;
4. public class Test {
5. public static void main(String[] args) {
6. HashMap<String, String> m = new HashMap<String, String>();
7. m.put("key45", "aa");
8. m.put("key12", "zz");
9. m.put("key39", "cc");
10. m.put("key27", "bb");
11. Collection<String> c = m.values();
12. c = new TreeSet<String>(c);
13. System.out.println(c);
14. }
15. }

Output:

[aa, bb, cc, zz]

Which of the following option will sort the keys in HashMap ?

Problem statement:

Given java code as follows, can you find out the correct option to sort the keys in HashMap?

1. import java.util.HashMap;
2. import java.util.Set;
3. import java.util.TreeSet;
4. public class Test {
5. public static void main(String[] args) {
6. HashMap<String, String> m = new HashMap<String, String>();
7. m.put("key45", "aa");
8. m.put("key12", "bb");
9. m.put("key39", "cc");
10. Set<String> s = m.keySet();
11.   // code goes here
12. s = new TreeSet(s);
13.  }
14. }

• Arrays.sort(s);
• s = new TreeSet(s);
• Collections.sort(s);
• s = new SortedSet(s);

Correct option: 

s = new TreeSet(s);
Explanation:
Arrays.sort(s) // sorts for static array 
Collection.sort(s) // sort the List type of object not Set type
s =new SortedSet(s); // SortedSet is an interface

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