Download ch03-3

Using Objects (continued)
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suggested reading: 3.3
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String objects
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One of the most common types of objects in Java is
type String.
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String: A sequence of text characters.
String variables can be declared and assigned, just like
primitive values:
String <name> = "<text>";
String <name> = <expression that produces a String>;
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Unlike most other objects, Strings are not created with new.
Examples:
String name = "Marla Singer";
int x = 3, y = 5;
String point = "(" + x + ", " + y + ")";
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Indexes
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The characters in a String are each internally numbered
with an index, starting with 0 for the first character:
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Example:
String name = "M. Stepp";
index
0
character 'M'
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1
2
3
4
5
6
7
'.'
' '
'S'
't'
'e'
'p'
'p'
Individual text characters are represented inside the
String by a primitive type called char. Literal char
values are surrounded with apostrophe (single-quote)
marks, such as 'a' or '4'.
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An escape sequence can be represented as a char, such as
'\n' (new-line character) or '\'' (apostrophe).
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String methods
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Here are several of the most useful String methods:
Method name
Description
charAt(index)
character at a specific index
indexOf(String)
index where the start of the given String
appears in this String (-1 if it is not there)
length()
number of characters in this String
substring(index1, index2)
the characters from index1 to just before
index2
toLowerCase()
a new String with all lowercase letters
toUpperCase()
a new String with all uppercase letters
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String method examples
//
index 012345678901
String s1 = "Stuart Reges";
String s2 = "Marty Stepp";
System.out.println(s1.length());
System.out.println(s1.indexOf("e"));
System.out.println(s1.substring(1, 4));
// 12
// 8
// tua
String s3 = s2.toUpperCase();
System.out.println(s3.substring(6, 10)); // STEP
String s4 = s1.substring(0, 6);
System.out.println(s4.toLowerCase());
// stuart
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String methods
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Given the following String:
String book = "Building Java Programs";
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How would you extract the word "Java" ?
How would you change book to store:
"BUILDING JAVA PROGRAMS" ?
How would you extract the first word from any general String?
Method name
Description
charAt(index)
character at a specific index
indexOf(String)
index where the start of the given String
appears in this String (-1 if it is not there)
length()
number of characters in this String
substring(index1, index2)
the characters from index1 to just before
index2
toLowerCase()
a new String with all lowercase letters
toUpperCase()
a new String with all uppercase letters
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Point objects
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Java has a type of objects named Point.
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To use Point, you must write: import java.awt.*;
Constructing a Point object, general syntax:
Point <name> = new Point(<x>, <y>);
Point <name> = new Point(); // the origin, (0, 0)
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Example:
Point p1 = new Point(5, -2);
Point p2 = new Point(); // 0, 0
Point objects are useful for several reasons:
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In programs that do a lot of 2D graphics, it can be nice to be
able to store an (x, y) pair in a single variable.
Points have several useful geometric methods we can call in our
programs.
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Point object methods
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Data stored in each Point object:
Field name
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x
Point's x-coordinate
y
Point's y-coordinate
Useful methods of each Point object:
Method name
Description
distance(Point)
how far apart these two Points are
setLocation(x, y)
changes this Point's x and y to be the given
values
adjusts this Point's x and y by the given
difference amounts
translate(dx, dy)
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Description
Point objects can also be output to the console using
println statements.
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Using Point objects
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An example program that uses Point objects:
import java.awt.*;
public class PointMain {
public static void main(String[] args) {
// create two Point objects
Point p1 = new Point(7, 2);
Point p2 = new Point(4, 3);
// print each point and their distance apart
System.out.println("p1 is " + p1);
System.out.println("p2: (" + p2.x + ", " + p2.y + ")");
System.out.println("distance = " + p1.distance(p2));
// translate the point to a new location
p2.translate(1, 7);
System.out.println("p2 is " + p2);
System.out.println("distance = " + p1.distance(p2));
}
}
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Point objects question
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Write a program that plays a point guessing game.
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When the user is wrong, the program hints which way to go.
Define (4, 2) as the correct answer for every game, for now.
I'm thinking of a point somewhere
between (1, 1) and (5, 5) ...
It is 4.47213595499958 from the origin.
guess x and
right up
guess x and
left up
guess x and
right down
guess x and
down
guess x and
you guessed
y? 1 1
y? 5 1
y? 3 4
y? 4 3
y? 4 2
it!
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Cumulative sum
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suggested reading: 4.1
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Adding many numbers
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Consider the following code to read three values from
the user and add them together:
Scanner console = new Scanner(System.in);
System.out.print("Type a number: ");
int num1 = console.nextInt();
System.out.print("Type a number: ");
int num2 = console.nextInt();
System.out.print("Type a number: ");
int num3 = console.nextInt();
int sum = num1 + num2 + num3;
System.out.println("The sum is " + sum);
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A cumulative sum
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You may have observed that the variables num1, num2, and num3
are unnecessary. The code can be improved:
Scanner console = new Scanner(System.in);
System.out.print("Type a number: ");
int sum = console.nextInt();
System.out.print("Type a number: ");
sum += console.nextInt();
System.out.print("Type a number: ");
sum += console.nextInt();
System.out.println("The sum is " + sum);
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cumulative sum: A sum variable that keeps a total-in-progress
and is updated many times until the task of summing is finished.
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The variable sum in the above code now represents a cumulative sum.
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Failed cumulative sum loop
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How would we modify the preceding code to sum 100
numbers?
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Creating 100 copies of the same code would be redundant.
A failed attempt to write a loop to add 100 numbers:
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The scope of sum is inside the for loop, so the last line of code
fails to compile.
Scanner console = new Scanner(System.in);
for (int i = 1; i <= 100; i++) {
int sum = 0;
System.out.print("Type a number: ");
sum += console.nextInt();
}
// sum is undefined here
System.out.println("The sum is " + sum);
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Fixed cumulative sum loop
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A corrected version of the sum loop code:
Scanner console = new Scanner(System.in);
int sum = 0;
for (int i = 1; i <= 100; i++) {
System.out.print("Type a number: ");
sum += console.nextInt();
}
System.out.println("The sum is " + sum);
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The key idea:
Cumulative sum variables must always be declared outside the
loops that update them, so that they will continue to live after
the loop is finished.
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User-guided cumulative sum
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The user's input can guide the number of times the cumulative
loop repeats:
Scanner console = new Scanner(System.in);
System.out.print("How many numbers to add? ");
int count = console.nextInt();
int sum = 0;
for (int i = 1; i <= count; i++) {
System.out.print("Type a number: ");
sum += console.nextInt();
}
System.out.println("The sum is " + sum);
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An example output:
How many numbers to add? 3
Type a number: 2
Type a number: 6
Type a number: 3
The sum is 11
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Variation: cumulative product
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The same idea can be used with other operators, such as
multiplication which produces a cumulative product:
Scanner console = new Scanner(System.in);
System.out.print("Raise 2 to what power? ");
int exponent = console.nextInt();
int product = 1;
for (int i = 1; i <= exponent; i++) {
product = product * 2;
}
System.out.println("2 to the " + exponent + " = " + product);
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Exercise: Change the above code so that it also prompts for the base,
instead of always using 2.
Exercise: Make the code to compute the powers into a method which
accepts a base a and exponent b as parameters and returns ab .
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Cumulative sum question
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Write a program that reads input of the number of
hours two employees have worked and displays each
employee's total and the overall total hours.
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The company doesn't pay overtime, so cap any day at 8 hours.
Example log of execution:
Employee 1: How many days? 3
Hours? 6
Hours? 12
Hours? 5
Employee 1's total paid hours = 19
Employee 2: How many days? 2
Hours? 11
Hours? 6
Employee 2's total hours = 14
Total paid hours for both employees = 33
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