1. If you have not completed the exercise on The Time Class, do so now.
2. Open TheTimeClass.java and add the toString method and add the following constructor.

public String toString(){
  return this.hour + ":" + this.minute + ":" + this.second;
}
public TheTimeClass(int hour, int minute, int second){
  this.hour = hour;
  this.minute = minute;
  this.second = second;
}

3. Attempt to compile the class.

$ javac TheTimeClass.java
TheTimeClass.java:17: error: constructor TheTimeClass in class TheTimeClass cannot be applied to given types;
    TheTimeClass myTime = new TheTimeClass();
                          ^
  required: int,int,int
  found: no arguments
  reason: actual and formal argument lists differ in length
1 error

The class doesn’t compile due to the new constructor added. If a class has an overloaded constructor then Java does not create a default no-argument constructor.

4. Add the following constructor.

public TheTimeClass(){
  this.hour = 0;
  this.minute = 0;
  this.second = 0;
}

5. Compile and run the program.

$ javac TheTimeClass.java
$ java TheTimeClass
0:0:0

6. Modify main to call the constructor that takes parameters.

public static void main(String[] args){
  TheTimeClass myTime = new TheTimeClass(3,33,55);
  System.out.println(myTime.toString());
}

7. Compile and run the program.

$ javac TheTimeClass.java
$ java TheTimeClass
3:33:55

The complete class should appear as follows.

public class TheTimeClass {
  private int hour;
  private int minute;
  private int second;

  public String toString(){
    return this.hour + ":" + this.minute + ":" + this.second;
  }

  public TheTimeClass(){
    this.hour = 0;
    this.minute = 0;
    this.second = 0;
  }

  public TheTimeClass(int hour, int minute, int second){
    this.hour = hour;
    this.minute = minute;
    this.second = second;
  }

  public static void main(String[] args){
    TheTimeClass myTime = new TheTimeClass(3,33,55);
    System.out.println(myTime.toString());
  }
}

For more practice with constructors refer to the following video tutorial by LearningLad.
 

 
And for another tutorial on constructors and explaining the this keyword, refer to this tutorial by LearningLad.