Module 5: Conditional Statements

An example with integers

We will write an applet to customize a greeting based on age:

import java.awt.*;
import java.applet.Applet;
import java.awt.event.*; 

public class AgeSpecificGreetingApplet extends Applet implements ActionListener {

  // The string variable for the greeting.
  String greeting;

  // The text fields in which the user enter strings.
  TextField nameField;
  TextField ageField;
  

  public void init ()
  {
    // -------------------------------------------------------------
    // GUI stuff
    setLayout (new BorderLayout());
    Panel bottomPanel = new Panel ();

    Label nameLabel = new Label ("Name: ");
    bottomPanel.add (nameLabel);

    nameField = new TextField (15);
    bottomPanel.add (nameField);

    Label ageLabel = new Label (" Age: ");
    bottomPanel.add (ageLabel);

    ageField = new TextField (2);
    bottomPanel.add (ageField);

    Button doneButton = new Button ("Press when done");
    bottomPanel.add (doneButton);

    doneButton.addActionListener (this);

    add (bottomPanel, BorderLayout.SOUTH);
    // -------------------------------------------------------------

    // Initial value of greeting:
    greeting = "";
  }
   
   
  // The method actionPerformed is called when the button is clicked.

  public void actionPerformed (ActionEvent click)
  {
    // Retrieve the strings from the two text fields:
    String name = nameField.getText ();
    int age = new Integer (ageField.getText().trim()).intValue();

    if (age < 2) {
      greeting = "Coo coo, little baby " + name;
    }
    else if (age < 10) {
      greeting = "Yo, kid, what's up?";
    }
    else if (age < 17) {
      greeting = "Hi " + name + ", how's school these days?";
    }
    else {
      greeting = "Good day " + name + "!";
    }

    // Redraw the applet:
    repaint ();
  }
  
      
  public void paint (Graphics g) 
  {
    // The Face, as before:
    g.drawOval (40,40, 120,150);         // Head
    g.drawOval (57,75, 30,20);           // Left eye
    g.drawOval (110,75, 30,20);          // Right eye
    g.fillOval (68,81, 10,10);           // pupil (left)
    g.fillOval (121,81, 10,10);          // pupil (right)
    g.drawOval (85,100, 30,30);          // Nose
    g.fillArc (60,125,80,40, 180,180);   // Mouth
    g.drawOval (25,92, 15,30);           // Left ear
    g.drawOval (160,92, 15,30);          // Right ear

    // Draw the greeting:
    g.drawString (greeting, 190,150); 
  }
  
}

Note:

Notice the structure of an if statement:

       if (some condition) {
         some statements
       }
       else if (some other condition) {
         some other statements
       }
       else {
         yet other statements
       }

Both if and else are reserved words.

You can have a single if all by itself, as in:

  public void actionPerformed (ActionEvent click)
  {
    // Retrieve the strings from the two text fields:
    String name = nameField.getText ();
    int age = new Integer (ageField.getText().trim()).intValue();

    if (age < 2) {
      greeting = "Coo coo, little baby " + name;
    }

    // Redraw the applet:
    repaint ();
  }

You can pair and else with an if part as in:

  public void actionPerformed (ActionEvent click)
  {
    // Retrieve the strings from the two text fields:
    String name = nameField.getText ();
    int age = new Integer (ageField.getText().trim()).intValue();

    if (age < 2) {
      greeting = "Coo coo, little baby " + name;
    }
    else {
      greeting = "Good day " + name + "!";
    }

    // Redraw the applet:
    repaint ();
  }

Or, like in the example, you can have as many else if parts in between as you like.
The logic works like this:
- First the condition in the if clause is evaluated. If true, the statement within executes and then execution skips out to past the else clause.
- If the condition fails, the nearest else clause is executed, which above is an else if clause (else if (age < 10)).
- And so on.
Thus, in the example, only one greeting assignment occurs.
Link to applet

Let's consider some alternate ways of writing the same logic:

Consider this way:

    if (age < 2) {
      greeting = "Coo coo, little baby " + name;
    }
    else if ( (age >= 2) && (age < 10) ) {
      greeting = "Yo, kid, what's up?";
    }
    else if ( (age >= 10) && (age < 17) ) {
      greeting = "Hi " + name + ", how's school these days?";
    }
    else if (age >= 17) {
      greeting = "Good day " + name + "!";
    }

Here, each condition is more explicitly written.
The result is the same because, for example, the first else if clause will execute only if age is at least 2.
Notice the use of the && (AND) operator to require both conditions to hold.

Another way:

    if (age < 2) {
      greeting = "Coo coo, little baby " + name;
    }
    else {
      if (age < 10) {
        greeting = "Yo, kid, what's up?";
      }
      else {
        if (age < 17) {
          greeting = "Hi " + name + ", how's school these days?";
        }
        else {
          greeting = "Good day " + name + "!";
        }
      }
    }

Yet another way:

   if (age >= 17) {
      greeting = "Good day " + name + "!";
   }
   else {
     // Now we know age < 17
     if (age >= 10) {
      greeting = "Hi " + name + ", how's school these days?";
     }
     else {
       // Now we know age < 10
       if (age >= 2) {
         greeting = "Yo, kid, what's up?";
       }
       else {
         greeting = "Coo coo, little baby " + name;
       }
     }
   }

In-Class Exercise: Add a few more age ranges and responses to the applet above.

String comparisons

Consider this example in which two names are compared for alphabetical order:

import java.awt.*;
import java.applet.Applet;
import java.awt.event.*; 

public class AlphabeticalNamesApplet extends Applet implements ActionListener {

  // Once they are alphabetically sorted, we'll place
  // the names in these variables for use in paint()
  String firstOne, secondOne;

  // The text fields in which the user enter strings.
  TextField nameField1, nameField2;
  

  public void init ()
  {
    // -------------------------------------------------------------
    // GUI stuff
    Label nameLabel1 = new Label ("Name #1: ");
    add (nameLabel1);

    nameField1 = new TextField (10);
    add (nameField1);

    Label nameLabel2 = new Label ("Name #2: ");
    add (nameLabel2);

    nameField2 = new TextField (10);
    add (nameField2);

    Button doneButton = new Button ("Press when done");
    add (doneButton);

    doneButton.addActionListener (this);
    // -------------------------------------------------------------

    // Initial values of names: note the use of the reserved word "null"
    firstOne = null;
    secondOne = null;
  }
   
   
  // The method actionPerformed is called when the button is clicked.

  public void actionPerformed (ActionEvent click)
  {
    // Retrieve the strings from the two text fields:
    String name1 = nameField1.getText ();
    String name2 = nameField2.getText ();

    if (name1.compareTo (name2) < 0) {
      // name1 comes alphabetically ahead.
      firstOne = name1;
      secondOne = name2;
    }
    else {
      // name2 comes alphabetically ahead.
      firstOne = name2;
      secondOne = name1;
    }
    
    // Redraw the applet:
    repaint ();
  }
  
      
  public void paint (Graphics g) 
  {
    // If neither string has been set, don't do anything.
    if ( (firstOne == null) || (secondOne == null) ) {
      // Executing a "return" simply quits the method.
      return;
    }
    
    // Now we know both strings are non-null, so draw the strings.
    g.drawString ("In alphabetical order: ", 30, 120);
    g.drawString ("  " + firstOne, 30, 140);
    g.drawString ("  " + secondOne, 30, 160);
  }
  
}

Note:

Note the logic in the program:
- Variables name1 and name2 are used to capture the strings from the text field.
- The method paint() simply paints the two strings firstOne and secondOne as long as they are non-empty.
- Thus, once the button is clicked, it's up to actionPerformed to set these variables correctly. This is where we use a conditional statement.
Object variables such as firstOne are initialized to null (meaning "nothing").
Notice how strings are compared:
```
    if (name1.compareTo (name2) < 0) {
      // ...
    }
    
```
- The compareTo method of one string is given the other string.
- It returns -1 if the first is alphabetically less.
- It returns 0 if they are the same.
- It returns 1 if the second is alphabetically less.
Notice how in paint, we first check to see whether variables firstOne and secondOne have values assigned to them:
- The equality operator == is used to see whether firstOne is set to null.
- Observe the use of the || (OR) operator: the condition holds true if any of the two are null.
Executing return (a Java reserved word) causes a method to terminate immediately. Thus, once the return is executed, nothing else in the method is executed.
Link to applet

In-Class Exercise: Add a third name to the above applet and display in correct alphabetical order.

For illustration, let's consider another way of writing the same program:

import java.awt.*;
import java.applet.Applet;
import java.awt.event.*; 

public class AlphabeticalNamesApplet2 extends Applet implements ActionListener {

  String name1, name2;

  // The text fields in which the user enter strings.
  TextField nameField1, nameField2;
  

  public void init ()
  {
    // -------------------------------------------------------------
    // GUI stuff
    Label nameLabel1 = new Label ("Name #1: ");
    add (nameLabel1);

    nameField1 = new TextField (10);
    add (nameField1);

    Label nameLabel2 = new Label ("Name #2: ");
    add (nameLabel2);

    nameField2 = new TextField (10);
    add (nameField2);

    Button doneButton = new Button ("Press when done");
    add (doneButton);

    doneButton.addActionListener (this);
    // -------------------------------------------------------------

    // Initial values of names: note the use of the reserved word "null"
    name1 = null;
    name2 = null;
  }
   
   
  // The method actionPerformed is called when the button is clicked.

  public void actionPerformed (ActionEvent click)
  {
    // Retrieve the strings from the two text fields:
    name1 = nameField1.getText ();
    name2 = nameField2.getText ();

    // Redraw the applet:
    repaint ();
  }
  
      
  public void paint (Graphics g) 
  {
    // Print only if both strings are set.
    if ( (name1 != null) && (name2 != null) ) {
    
      g.drawString ("In alphabetical order: ", 30, 120);

      if (name1.compareTo (name2) < 0) {
        // name1 comes alphabetically ahead.
        g.drawString ("  " + name1, 30, 140);
        g.drawString ("  " + name2, 30, 160);
      }
      else {
        // name2 comes alphabetically ahead.
        g.drawString ("  " + name2, 30, 140);
        g.drawString ("  " + name1, 30, 160);
      }
    }
    // Nothing is needed in else clause
  }
  
}

Note:

Now we use only the name1 and name2 variables. and perform the comparison directly in paint.
The paint method has one large if block that's executed only after the names have been entered.
Notice the use of the != (NOT EQUALS) operator.
Link to applet

In-Class Exercise: Suppose we change the code in the above program so that the method actionPerformed looks like this:

  public void actionPerformed (ActionEvent click)
  {
    // Retrieve the strings from the two text fields:
    String name1 = nameField1.getText ();
    String name2 = nameField2.getText ();

    // Redraw the applet:
    repaint ();
  }

What happens? Does it compile? Does it work? Explain.

In-Class Exercise: Add a third name to the above applet and display in correct alphabetical order. Add colors to improve the display.