7.7: Handling Text in a Graphics Context (Optional)

In order to create attractive GUIs, it is often necessary to be able to select and control the font that is used. Even a simple drawing task, such as being able to center a message in a panel, requires that we know the font’s dimensions and be able to manipulate them. In this section, we learn how to work with Java’s fonts and font control methods.

Each graphics context has an associated Font and FontMetrics object, and the Graphics class (Fig. [fig-fontmethods]) provides several methods to access them. A FontMetrics is an object that encapsulates important data about a font, such as its height and width. Java assigns a default font to each Graphics object. For example, this is the font used by the drawString() method, which we used in our very first Java programs back in Chapter [ch:intro2]. The particular font used is system dependent, but to override the default one can simply invoke the setFont() method:

g.setFont(new Font("TimesRoman", Font.ITALIC, 12));

In this case, the Font() constructor is used to specify a 12-point, italicized, TimesRoman font. Once the font is set, it will be used in all subsequent drawings.

The Font and FontMetrics Classes

The Font class (Fig. 7.19) provides a platform-independent representation of an individual font. A font is distinguished by its name, size, and style, and the Font class includes protected instance variables for these properties, as well as a constructor method that allows these three characteristics to be specified.

In order to understand how fonts work, it is necessary to distinguish between a character, which is a symbol that represents a certain letter or digit, and a glyph, which is a shape used to display the character. When you display a string, such as “Hello”, Java maps each individual character into a corresponding shape, as defined by the particular font that is selected.

Java distinguishes between physical and logical fonts. A physical font is an actual font library that contains the data and tables needed to associate the correct glyph with a given character. Generally speaking, a given platform (host computer plus operating system) will have a collection of such fonts available on it.

A logical font is one of five font families that are supported by the Java runtime environment. These include Serif, SansSerif, Monospaced, Dialog, and DialogInput. Java also supports the following font styles: PLAIN, BOLD, ITALIC, and BOLD+ITALIC. Whereas the physical fonts are platform dependent, the logical fonts and styles are platform independent. When used in a program, they are mapped to real fonts available on the host system. If the host system does not have an exact match for the specified font, it will supply a substitute. For example, if you specify a 48-point, italic, Monospaced font,

Font myFont = new Font("Monospaced", Font.ITALIC, 48);

the system may map this to a 24-point, italic Courier font, if that is the largest fixed-spaced font available.

The Font() constructor is designed to work with any set of arguments. Thus, if you supply the name of a font that is not available, the system will supply a default font as a substitute. For example, on my system, specifying a nonexistent font named Random,

g.setFont(new Font("Random", Font.ITALIC, 12) );
g.drawString("Hello, World!! (random, italic, 12)", 30, 45);

produces the same font used as the mapping for a font named Dialog.

The Component.setFont() method can be used to assign a specific font to a button or window or other graphics component. All AWT and JFC components have an associated font, which can be accessed using the Component.setFont() and Component.getFont() methods. For example, the following code could be used to override a Button’s font:

Button b = new Button("Label");
b.setFont(new Font("Times", Font.ITALIC, 14));

If 14-point, italic, Times font is not available on the host system, a substitute will be supplied.

Font Metrics

To illustrate how to use the FontMetrics class, let’s write a “Hello, World!” application that centers its message both horizontally and vertically in its window. The message should be centered regardless of the size of the application window. Thus, we will have to position the text relative to the window size, which is something we learned in positioning geometric shapes. The message should also be centered no matter what font is used. This will require us to know certain characteristics of the font itself, such as the height and width of its characters, whether the characters have a fixed or variable width, and so on. In order to get access to these properties, we will use the FontMetrics class.

Figure 7.20 illustrates the various properties that are associated with a font. The baseline of a font refers to the line on which the bottom of most characters occurs. When drawing a string, the x- and y-coordinates determine the baseline of the string’s first character. Thus, in

g.drawString("Hello, World!", 10, 40);

the bottom left of the H in “Hello, World!” would be located at (10, 40).

All characters ascend some distance above the baseline. This is known as the character’s ascent. Some characters, such as y, may extend below the baseline, into what’s known as the descent. Each font has a maximum descent. Similarly, some characters, such as accent characters, may extend above the maximum ascent into a space known as the leading.

The height of a font is defined as the sum (in pixels) of the ascent, descent, and leading values. The height is a property of the font itself rather than of any individual character. Except for fixed-width fonts, in which the width of all characters is the same, the characters that make up a font have varying widths. The width of an individual character is known as its advance.

The FontMetrics class (Fig. [fig-fontmetrics2]) provides methods for accessing a font’s properties. These can be useful to control the layout of text on a GUI. For example, when drawing multiple lines of text, the getHeight() method is useful for determining how much space should be left between lines. When drawing character by character, the charWidth() method can be used to determine how much space must be left between characters. Alternatively, the stringWidth() method can be used to determine the number of pixels required to draw the entire string.

Example: Centering a Line of Text

Given this background, let’s take on the task of centering a message in an application window. In order for this application to work for any font, we must take care not to base its design on characteristics of the particular font that we happen to be using. To underscore this point, let’s design it to work for a font named Random, which, as we noted earlier, will be mapped to some font by the system on which the application is run. In other words, we will let the system pick a font for this application’s message. An interesting experiment would be to run the application on different platforms to see what fonts are chosen.

The only method we need for this application is the paint() method. Let’s begin by setting the font used by the graphics context to a random font. To get the characteristics of this font, we create a FontMetrics object and get the font metrics for the font we just created:

g.setFont(new Font("Random", Font.BOLD, 24));
FontMetrics metrics = g.getFontMetrics();

The next step is to determine the JFrame’s dimensions using the getSize() method. This method returns an object of type Dimension. The java.awt.Dimension class (Fig. 7.22) represents the size (width and height) of a GUI component. A Dimension makes it possible to manipulate an object’s width and height as a single entity. Note that the height and width variables are defined as public, which is an exception from the usual convention of defining instances variables as private or protected. The justification for this exception is probably to simplify the syntax of referring to an object’s width and height. For example, the following syntax can be used to refer to a component’s dimensions:

Dimension d = new Dimension(100, 50);
System.out.println("width = " + d.width + 
                   " height = " + d.height);

Note the redundancy built into the Dimension class. For example, in addition to being able to set a Dimension’s instance variables directly, public access methods are provided. Also, by defining more than one version of some access methods, the class achieves a higher level of flexibility. The same can be said for providing several different constructors, including a copy constructor. Finally, note how it overrides the equals() and toString() methods. These are all examples of good object-oriented design.

The Dimension object is used to calculate the x- and y-coordinates for the string. In order to center the string horizontally, we need to know its width, which is supplied by the metrics object. If the JFrame is d.width pixels wide, then the following expression subtracts the width of the string from the width of the JFrame and then divides the leftover space in half:

                            // Calculate coordinates
int x = (d.width - metrics.stringWidth(str)) / 2; 

Similarly, the following expression adds the height of the string to the height of the JFrame and divides the leftover space in half:

int y = (d.height + metrics.getHeight()) / 2;

Taken together, these calculations give the coordinates for the lower left pixel of the first character in “Hello, World!!” The only remaining task is to draw the string (Fig. [fig-centertext]). Because the paint() method is called automatically whenever the JFrame is resized, this application, whose output is shown in Figure [fig-centertextscreen], will re-center its message whenever it is resized by the user.

import java.awt.*;
import javax.swing.*;

public class CenterText extends JFrame {
                // Print hello world! in center of frame
public void paint(Graphics g) {
  String str = "Hello, World!";
  g.setFont(new Font("Random", Font.PLAIN, 24)); // Random font
  FontMetrics metrics = g.getFontMetrics(); //  And its metrics

  Dimension d = getSize();       // Get the frame's size
             // Clear the frame
  g.setColor(getBackground());
  g.fillRect(0,0,d.width,d.height);
  g.setColor(Color.black);
                                // Calculate coordinates
  int x = (d.width - metrics.stringWidth(str)) / 2; 
  int y = (d.height + metrics.getHeight()) / 2;

  g.drawString( str, x, y );          // Draw the string
} // paint()

public static void main(String args[]) {
  CenterText ct = new CenterText();
  ct.setSize(400,400);
  ct.setVisible(true);
}
} // CenterText

ascent

baseline

concatenation

copy constructor

data structure

delimited string

delimiter

empty string

garbage collection

glyph

lexicographic order

logical font

off-by-one error

orphan object

physical font

read only

string

string index

string literal

token

unit indexed

zero indexed

A String literal is a sequence of 0 or more characters enclosed within double quotation marks. A String object is a sequence of 0 or more characters, plus a variety of class and instance methods and variables.

A String object is created automatically by Java the first time it encounters a literal string, such as “Socrates,” in a program. Subsequent occurrences of the literal do not cause additional objects to be instantiated. Instead, every occurrence of the literal “Socrates” refers to the initial object.

A String object is created whenever the new operator is used in conjunction with a String() constructor—for example, new String("hello").

The String concatenation operator is the overloaded \(+\) symbol; it is used to combine two Strings into a single String: “hello” + “world” ==> “helloworld”.

Strings are indexed starting at 0. The indexOf() and lastIndexOf() methods are used for finding the first or last occurrence of a character or substring within a String. The valueOf() methods convert a nonstring into a String. The length() method determines the number of characters in a String. The charAt() method returns the single character at a particular index position. The various substring() methods return the substring at particular index positions in a String.

The overloaded equals() method returns true if two Strings contain the same exact sequence of characters. The == operator, when used on Strings, returns true if two references designate the same String object.

String objects are immutable. They cannot be modified.

A StringBuffer is a string object that can be modified using methods such as insert() and append().

A StringTokenizer is an object that can be used to break a String into a collection of tokens separated by delimiters. The whitespace characters—tabs, blanks, and newlines—are the default delimiters.

The FontMetrics class is used to obtain the specific dimensions of the the various Fonts. It is useful when you wish to center text. Fonts are inherently platform dependent. For maximum portability, it is best to use default fonts.

= 0pt

4

1. silly
2. silly
3. silly stuff

1. String str1 = "";
2. String str2 = new String("stop");
3. String str3 = str1 + str2;

= 0pt

4

1. 15
2. "551"
3. "5175"

See Figure [fig-strobjects3]. [ans-strobjects2]

4

1. "45"
2. "121"
3. "X"

1. String.valueOf(100)
2. String.valueOf('V');
3. String s = new String(String.valueOf(X * Y));

4

1. 0
2. 1
3. \(-1\)

4

1. 16
2. "16"
3. 1
4. 15
5. 1
6. 13
7. 7
8. 3
9. 7
10. 7
11. 3

Evaluate the following expression:

String tricky = "abcdefg01234567";
tricky.indexOf(String.valueOf(tricky.indexOf("c")));
tricky.indexOf(String.valueOf(2));
tricky.indexOf("2");
Answer: 9

4

1. "uvwxyz"
2. "bcde"
3. "xyz"
4. "xy"
5. "xyz"

4

1. "uvwxyz"
2. "bcde"
3. "xyz"
4. "xyz"
5. "xyz"

A class to test the string methods.

public class StringProcessorTest {
  public static void main(String[] args) {
    KeyboardReader kb = new KeyboardReader();
    kb.prompt("Input a String or - stop - to quit: ");
    String str = kb.getKeyboardInput();
    while (!str.equals("stop")){
      kb.display("Testing printLetters()\n");
      StringProcessor.printLetters(str);
      kb.display("testing countChars()\n");
      kb.display("Total occurences of e = ");
      kb.display(StringProcessor.countChar(str,'e') + "\n");
      kb.display("Testing reverse()\n");
      kb.display(StringProcessor.reverse(str)+ "\n");
      kb.display("Testing capitalize()\n");
      kb.display(StringProcessor.capitalize(str) + "\n\n");
      kb.prompt("Input a String or - stop - to quit: ");
      str = kb.getKeyboardInput();
    } // while
  } //main()
} // StringProcessorTest class

Method to remove all blanks from a string:

 // Pre: s is a non null string
 // Post: s is returned with all its blanks removed
 public String removeBlanks(String s) {
   StringBuffer result = new StringBuffer();
   for (int k = 0; k < s.length();  k++)
     if (s.charAt(k) != ' ')      // If this is not a blank
       result.append(s.charAt(k));  //  append it to result
   return result.toString();
 }

A Alpha Z Zero Zeroes a alpha bath bin z zero

To modify precedes so that it also returns true when its two string arguments are equal, just change the operator in the final return statement to <=:

    if (s1.charAt(k) <= s2.charAt(k) )
      return true;

4

1. true
2. true
3. false
4. false
5. false
6. true
7. false
8. false
9. false

The variables in TestStringEquals are declared static because they are used in static methods. Whenever you call a method directly from main(), it must be static because main() is static. Remember that static elements are associated with the class, not with its instances. So main() can only use static elements because they don’t depend on the existence of instances.

1. String s3 = s1.substring(s1.indexOf('n'))
+ s1.substring(0,s1.indexOf('n'));

2. String s4 = s2.substring(6) + " " + s2.substring(0,5);
3. String s5 = s2.substring(0,6) + s1.substring(0,3);

Explain the difference between the following pairs of terms:

Unit indexing and zero indexing.

Data structure and data type.

StringBuffer and String.

String and StringTokenizer.

Declaring a variable and instantiating a String.

A Font and a FontMetrics object.

Fill in the blanks.

=14pt

When the first character in a string has index 0, this is known as

---

 .

A sequence of characters enclosed within quotes is known as a

---

 .

=11pt

Given the String str with the value “to be or not to be that is the question,” write Java expressions to extract each of the substrings shown below. For each substring, provide two sets of answers. One that uses the actual index numbers of the substrings—for example, the first “to” goes from 0 to 2—and a second more general solution that will also retrieve the substring from the following string “it is easy to become what you want to become.” (Hint: In the second case, use length() and indexOf() along with substring() in your expressions. If necessary, you may use local variables to store intermediate results. The answer to (a) is provided as an example.)

the first “to” in the string

str.substring(0, 2)                            // Answer 1
str.substring(
     str.indexOf("to"), str.indexOf("to") + 2) // Answer 2

2

the last “to” in the string

the first “be” in the string

the last “be” in the string

the first four characters in the string

the last four characters in the string

Identify the syntax errors in each of the following, assuming that s is the literal string “exercise”:

2

s.charAt("hello")

s.indexOf(10)

s.substring("er")

s.lastIndexOf(er)

s.length

Evaluate each of the following expressions, assuming that s is the literal string “exercise”:

2

s.charAt(5)

s.indexOf("er")

s.substring(5)

s.lastIndexOf('e')

s.length()

Write your own equalsIgnoreCase() method using only other String methods.

Write your own String equality method without using String. . (Hint: Modify the precedes() method.)

Even though Java’s String class has a built-in toLowerCase() method, write your own implementation of this method. It should take a String parameter and return a String with all its letters written in lowercase.

Write a method that converts its String parameter so that letters are written in blocks five characters long. For example, consider the following two versions of the same sentence:

Plain :   This is how we would ordinarily write a sentence.
Blocked : Thisi showw ewoul dordi naril ywrit easen tence.

Design and implement a Java Swing program that lets the user type a document into a TextArea and then provides the following analysis of the document: the number of words in the document, the number of characters in the document, and the percentage of words that have more than six letters.

Design and write a Java Swing program that searches for single-digit numbers in a text and changes them to their corresponding words. For example, the string “4 score and 7 years ago” would be converted into “four score and seven years ago”.

A palindrome is a string that is spelled the same way backward and forward. For example, mom, dad, radar, 727 and able was i ere i saw elba are all examples of palindromes. Write a Java Swing program that lets the user type in a word or phrase and then determines whether the string is a palindrome.

Write a maze program that uses a string to store a representation of the maze. Write a method that accepts a String parameter and prints a two-dimensional representation of a maze. For example, the maze shown here, where O marks the entrance and exit can be generated from the following string:

String: XX_XXXXXXXX__XXX_XXXX_XX____XXX_XX_XX_XXX____X____XXXXXXXX_X
  O
XX XXXXXXX
X  XXX XXX
X XX    XX
X XX XX XX
X    X    O
XXXXXXXX X

Write a method that takes a delimited string to store a name and address, from which you can print a mailing label. For example, if the string contains “Sam Penn:14 Bridge St.:Hoboken, NJ 01881,” the method should print the label shown in the margin.

Design and implement a Java Swing program that plays Time Bomb with the user. Here’s how the game works. The computer picks a secret word and then prints one asterisk for each letter in the word: * * * * *. The user guesses at the letters in the word. For every correct guess, an asterisk is replaced by a letter:e * * *. For every incorrect guess, the time bomb’s fuse grows shorter. When the fuse disappears, after say, six incorrect guesses, the bomb explodes. Store the secret words in a delimited string and invent your own representation for the time bomb.

Challenge: The global replace function is a string-processing algorithm found in every word processor. Write a method that takes three String arguments: a document, a target string, and a replacement string. The method should replace every occurrence of the target string in the document with the replacement string. For example, if the document is “To be or not to be, that is the question,” and the target string is “be,”, and the replacement string is “see,” the result should be, “To see or not to see, that is the question.”

Challenge: Design and implement a Java Swing Program that plays the following game with the user. Let the user pick a letter between A and Z. Then let the computer guess, the secret letter. For every guess the player has to tell the computer whether it’s too high or too low. The computer should be able to guess the letter within five guesses. Do you see why?

Challenge: A list is a sequential data structure. Design a List class that uses a comma-delimited String—such as, “a,b,c,d,12,dog”—to implement a list. Implement the following methods for this class:

void addItem( Object o );      // Use Object.toString()
String getItem(int position);
String toString();
void deleteItem(int position);
void deleteItem(String item);
int getPosition(String item);
String getHead();              // First element
List getTail();                // All but the first element
int length();                  // Number of items

Challenge: Use a delimited string to create a PhoneList class with an instance method to insert names and phone numbers, and a method to look up a phone number when a user provides a person’s name. Since your class will take care of looking things up, you don’t have to worry about keeping the list in alphabetical order. For example, the following string could be used as such a directory:

-4pc

mom:860-192-9876::bill g:654-0987-1234::mary lancelot:123-842-1100

Design and implement an application that displays a multi-line message in various fonts and sizes input by the user. Let the user choose from among a fixed selection of fonts, sizes, and styles.