Transcript Lecture Notes for Class 3

Chapter 5 Methods
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
1
Motivations
A method is a construct for grouping statements together to
perform a function. Using a method, you can write the
code once for performing the function in a program and
reuse it by many other programs. For example, often you
need to find the maximum between two numbers.
Whenever you need this function, you would have to write
the following code:
int result;
if (num1 > num2)
result = num1;
else
result = num2;
If you define this function for finding a
maximum number between any two
numbers in a method, you don’t have
to repeatedly write the same code. You
need to define it just once and reuse it
by any other programs.
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
2
Defining Methods
A method is a collection of statements that are
grouped together to perform an operation.
Define a method
modifier
method
header
return value
type
Invoke a method
method
name
formal
parameters
int z = max(x, y);
public static int max(int num1, int num2) {
actual parameters
(arguments)
int result;
method
body
if (num1 > num2)
result = num1;
else
result = num2;
return result;
parameter list
method
signature
return value
}
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
3
Method Signature
Method signature is the combination of the method name and the
parameter list.
Define a method
modifier
method
header
return value
type
Invoke a method
method
name
formal
parameters
int z = max(x, y);
public static int max(int num1, int num2) {
actual parameters
(arguments)
int result;
method
body
if (num1 > num2)
result = num1;
else
result = num2;
return result;
parameter list
method
signature
return value
}
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
4
Formal Parameters
The variables defined in the method header are known as
formal parameters.
Define a method
modifier
method
header
return value
type
Invoke a method
method
name
formal
parameters
int z = max(x, y);
public static int max(int num1, int num2) {
actual parameters
(arguments)
int result;
method
body
if (num1 > num2)
result = num1;
else
result = num2;
return result;
parameter list
method
signature
return value
}
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
5
Actual Parameters
When a method is invoked, you pass a value to the parameter. This
value is referred to as actual parameter or argument.
Define a method
modifier
method
header
return value
type
Invoke a method
method
name
formal
parameters
int z = max(x, y);
public static int max(int num1, int num2) {
actual parameters
(arguments)
int result;
method
body
if (num1 > num2)
result = num1;
else
result = num2;
return result;
parameter list
method
signature
return value
}
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
6
Return Value Type
A method may return a value. The returnValueType is the data type
of the value the method returns. If the method does not return a
value, the returnValueType is the keyword void. For example, the
returnValueType in the main method is void.
Define a method
modifier
method
header
return value
type
Invoke a method
method
name
formal
parameters
int z = max(x, y);
public static int max(int num1, int num2) {
actual parameters
(arguments)
int result;
method
body
if (num1 > num2)
result = num1;
else
result = num2;
return result;
parameter list
method
signature
return value
}
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
7
Calling Methods
Listing 5.1 Testing the max method
This program demonstrates calling a method max
to return the largest of the int values
TestMax
Run
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
8
animation
Calling Methods, cont.
pass the value of i
pass the value of j
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
9
animation
Trace Method Invocation
invoke max(i, j)
Pass the value of i to num1
Pass the value of j to num2
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
10
animation
Trace Method Invocation
result is now 5
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
11
animation
Trace Method Invocation
return result, which is 5
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
12
animation
Trace Method Invocation
return max(i, j) and assign the
return value to k
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
13
CAUTION
A return statement is required for a value-returning method. The
method shown below in (a) is logically correct, but it has a
compilation error because the Java compiler thinks it possible that
this method does not return any value.
public static int sign(int n) {
if (n > 0)
return 1;
else if (n == 0)
return 0;
else if (n < 0)
return –1;
}
(a)
Should be
public static int sign(int n) {
if (n > 0)
return 1;
else if (n == 0)
return 0;
else
return –1;
}
(b)
To fix this problem, delete if (n < 0) in (a), so that the compiler will
see a return statement to be reached regardless of how the if
statement is evaluated.
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
14
Reuse Methods from Other Classes
NOTE: One of the benefits of methods is for reuse. The max
method can be invoked from any class besides TestMax. If
you create a new class Test, you can invoke the max method
using ClassName.methodName (e.g., TestMax.max).
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
15
Call Stacks
Space required for
the max method
num2: 2
num1: 5
Space required for
the max method
result: 5
num2: 2
num1: 5
Space required for
the main method
k:
2
j:
5
i:
Space required for
the main method
k:
2
j:
5
i:
Space required for
the main method
k:
2
j:
5
i:
Space required for
the main method
5
k:
2
j:
5
i:
(a) The main
method is invoked.
(b) The max
method is invoked.
(c) The max method
is being executed.
(d) The max method is
finished and the return
value is sent to k.
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
Stack is empty
(e) The main
method is finished.
16
animation
Trace Call Stack
pass the values of i and j to num1
and num2
result:
num2: 2
num1: 5
Space required for the
main method
k:
j: 2
i: 5
The max method is
invoked.
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
17
animation
Trace Call Stack
(num1 > num2) is true
result:
num2: 2
num1: 5
Space required for the
main method
k:
j: 2
i: 5
The max method is
invoked.
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
18
animation
Trace Call Stack
Return result and assign it to k
Space required for the
max method
result: 5
num2: 2
num1: 5
Space required for the
main method
k:5
j: 2
i: 5
The max method is
invoked.
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
19
animation
Trace Call Stack
Execute print statement
Space required for the
main method
k:5
j: 2
i: 5
The main method
is invoked.
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
20
void Method Example
This type of method does not return a value. The method
performs some actions.
TestVoidMethod
Run
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
21
Passing Parameters
public static void nPrintln(String message, int n) {
for (int i = 0; i < n; i++)
System.out.println(message);
}
Suppose you invoke the method using
nPrintln(“Welcome to Java”, 5);
What is the output?
Suppose you invoke the method using
nPrintln(“Computer Science”, 15);
What is the output?
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
22
Pass by Value
Listing 5.2 Testing Pass by value
This program demonstrates passing values
to the methods.
TestPassByValue
Run
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
23
Pass by Value, cont.
The values of num1 and num2 are
passed to n1 and n2. Executing swap
does not affect num1 and num2.
Space required for the
swap method
temp:
n2: 2
n1: 1
Space required for the
main method
num2: 2
num1: 1
The main method
is invoked
Space required for the
main method
num2: 2
num1: 1
The swap method
is invoked
Space required for the
main method
num2: 2
num1: 1
The swap method
is finished
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
Stack is empty
The main method
is finished
24
Overloading Methods
Listing 5.3 Overloading the max Method
public static double max(double num1, double
num2) {
if (num1 > num2)
return num1;
else
return num2;
}
TestMethodOverloading
Run
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
25
Ambiguous Invocation
Sometimes there may be two or more
possible matches for an invocation of a
method, but the compiler cannot determine
the most specific match. This is referred to
as ambiguous invocation. Ambiguous
invocation is a compilation error.
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
26
Ambiguous Invocation
public class AmbiguousOverloading {
public static void main(String[] args) {
System.out.println(max(1, 2));
}
public static double max(int num1, double num2) {
if (num1 > num2)
return num1;
else
return num2;
}
public static double max(double num1, int num2) {
if (num1 > num2)
return num1;
else
return num2;
}
}
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
27
Scope of Local Variables
A local variable: a variable defined inside a
method.
Scope: the part of the program where the
variable can be referenced.
The scope of a local variable starts from its
declaration and continues to the end of the
block that contains the variable. A local
variable must be declared before it can be
used.
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
28
Scope of Local Variables, cont.
You can declare a local variable with the
same name multiple times in different nonnesting blocks in a method, but you cannot
declare a local variable twice in nested
blocks.
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
29
Scope of Local Variables, cont.
A variable declared in the initial action part of a for loop
header has its scope in the entire loop. But a variable
declared inside a for loop body has its scope limited in the
loop body from its declaration and to the end of the block
that contains the variable.
The scope of i
The scope of j
public static void method1() {
.
.
for (int i = 1; i < 10; i++) {
.
.
int j;
.
.
.
}
}
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
30
Scope of Local Variables, cont.
It is fine to declare i in two
non-nesting blocks
public static void method1() {
int x = 1;
int y = 1;
It is wrong to declare i in
two nesting blocks
public static void method2() {
int i = 1;
int sum = 0;
for (int i = 1; i < 10; i++) {
x += i;
}
for (int i = 1; i < 10; i++) {
y += i;
}
for (int i = 1; i < 10; i++) {
sum += i;
}
}
}
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
31
Method Abstraction
You can think of the method body as a black box
that contains the detailed implementation for the
method.
Optional arguments
for Input
Optional return
value
Method Header
Black Box
Method body
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
32
Benefits of Methods
• Write a method once and reuse it anywhere.
• Information hiding. Hide the implementation
from the user.
• Reduce complexity.
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
33
The Math Class
 Class
constants:
– PI
–E
 Class
methods:
– Trigonometric Methods
– Exponent Methods
– Rounding Methods
– min, max, abs, and random Methods
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
34
Trigonometric Methods

sin(double a)

cos(double a)

tan(double a)

acos(double a)

asin(double a)

atan(double a)
Radians
Examples:
Math.sin(0) returns 0.0
Math.sin(Math.PI / 6)
returns 0.5
Math.sin(Math.PI / 2)
returns 1.0
Math.cos(0) returns 1.0
Math.cos(Math.PI / 6)
returns 0.866
Math.cos(Math.PI / 2)
returns 0
toRadians(90)
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
35
Exponent Methods

exp(double a)
Returns e raised to the power of a.
Examples:

log(double a)
Returns the natural logarithm of a.

log10(double a)
Math.exp(1) returns 2.71
Math.log(2.71) returns 1.0
Math.pow(2, 3) returns 8.0
Math.pow(3, 2) returns 9.0
Math.pow(3.5, 2.5) returns
22.91765
Math.sqrt(4) returns 2.0
Math.sqrt(10.5) returns 3.24
Returns the 10-based logarithm of
a.

pow(double a, double b)
Returns a raised to the power of b.

sqrt(double a)
Returns the square root of a.
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
36
Rounding Methods

double ceil(double x)
x rounded up to its nearest integer. This integer is returned as a double
value.

double floor(double x)
x is rounded down to its nearest integer. This integer is returned as a
double value.

double rint(double x)
x is rounded to its nearest integer. If x is equally close to two integers,
the even one is returned as a double.

int round(float x)
Return (int)Math.floor(x+0.5).

long round(double x)
Return (long)Math.floor(x+0.5).
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
37
Rounding Methods Examples
Math.ceil(2.1) returns 3.0
Math.ceil(2.0) returns 2.0
Math.ceil(-2.0) returns –2.0
Math.ceil(-2.1) returns -2.0
Math.floor(2.1) returns 2.0
Math.floor(2.0) returns 2.0
Math.floor(-2.0) returns –2.0
Math.floor(-2.1) returns -3.0
Math.rint(2.1) returns 2.0
Math.rint(2.0) returns 2.0
Math.rint(-2.0) returns –2.0
Math.rint(-2.1) returns -2.0
Math.rint(2.5) returns 2.0
Math.rint(-2.5) returns -2.0
Math.round(2.6f) returns 3
Math.round(2.0) returns 2
Math.round(-2.0f) returns -2
Math.round(-2.6)
returns -3
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
38
min, max, and abs

max(a, b)and min(a, b)
Returns the maximum or
minimum of two parameters.

abs(a)
Returns the absolute value of the
parameter.

random()
Returns a random double value
in the range [0.0, 1.0).
Examples:
Math.max(2, 3) returns 3
Math.max(2.5, 3) returns
3.0
Math.min(2.5, 3.6)
returns 2.5
Math.abs(-2) returns 2
Math.abs(-2.1) returns
2.1
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
39
The random Method
Generates a random double value greater than or equal to 0.0 and less
than 1.0 (0 <= Math.random() < 1.0).
Examples:
(int)(Math.random() * 10)
Returns a random integer
between 0 and 9.
50 + (int)(Math.random() * 50)
Returns a random integer
between 50 and 99.
In general,
a + Math.random() * b
Returns a random number between
a and a + b, excluding a + b.
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
40
Stepwise Refinement (Optional)
The concept of method abstraction can be applied
to the process of developing programs. When
writing a large program, you can use the “divide
and conquer” strategy, also known as stepwise
refinement, to decompose it into subproblems. The
subproblems can be further decomposed into
smaller, more manageable problems.
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
41
PrintCalender Case Study
Let us use the PrintCalendar example to demonstrate the
stepwise refinement approach.
PrintCalendar
Run
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
42
Design Diagram
printCalendar
(main)
printMonth
readInput
printMonthTitle
getMonthName
printMonthBody
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
isLeapYear
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
43
Design Diagram
printCalendar
(main)
printMonth
readInput
printMonthTitle
getMonthName
printMonthBody
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
isLeapYear
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
44
Design Diagram
printCalendar
(main)
printMonth
readInput
printMonthTitle
getMonthName
printMonthBody
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
isLeapYear
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
45
Design Diagram
printCalendar
(main)
printMonth
readInput
printMonthTitle
getMonthName
printMonthBody
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
isLeapYear
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
46
Design Diagram
printCalendar
(main)
printMonth
readInput
printMonthTitle
getMonthName
printMonthBody
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
isLeapYear
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
47
Design Diagram
printCalendar
(main)
printMonth
readInput
printMonthTitle
getMonthName
printMonthBody
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
isLeapYear
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
48
Implementation: Top-Down
Top-down approach is to implement one method in the
structure chart at a time from the top to the bottom. Stubs
can be used for the methods waiting to be implemented. A
stub is a simple but incomplete version of a method. The
use of stubs enables you to test invoking the method from
a caller. Implement the main method first and then use a
stub for the printMonth method. For example, let
printMonth display the year and the month in the stub.
Thus, your program may begin like this:
A Skeleton for printCalendar
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
49
Implementation: Bottom-Up
Bottom-up approach is to implement one method in the
structure chart at a time from the bottom to the top. For
each method implemented, write a test program to test it.
Both top-down and bottom-up methods are fine. Both
approaches implement the methods incrementally and
help to isolate programming errors and makes debugging
easy. Sometimes, they can be used together.
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
50
Chapter 6 Arrays
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
51
Motivations
Often you will have to store a large number of values during
the execution of a program. Suppose, for instance, that you
need to read one hundred numbers, compute their average, and
find out how many numbers are above the average. Your
program first reads the numbers and computes their average,
and then compares each number with the average to determine
whether it is above the average. The numbers must all be stored
in variables in order to accomplish this task. You have to
declare one hundred variables and repeatedly write almost
identical code one hundred times. From the standpoint of
practicality, it is impossible to write a program this way. So,
how do you solve this problem?
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
52
Objectives















To describe why arrays are necessary in programming (§6.1).
To learn the steps involved in using arrays: declaring array reference variables
and creating arrays (§§6.2.1-6.2.2).
To initialize the values in an array (§6.2.3).
To access array elements using indexed variables (§6.2.4).
To simplify programming using the JDK 1.5 foreach loops (§6.2.5).
To declare, create, and initialize an array using an array initializer (§6.2.6).
To copy contents from one array to another (§6.3).
To develop and invoke methods with array arguments and return value (§6.46.5).
To declare a method with variable-length argument list (§6.6).
To search elements using the linear (§6.7.1) or binary (§6.7.2) search
algorithm.
To sort an array using the selection sort (§6.8.1)
To sort an array using the insertion sort algorithm (§6.8.2).
To use the methods in the Arrays class (§6.9).
To declare and create two-dimensional arrays to solve interesting problems
such as Sudoku (§6.10).
To declare and
create tomultidimensional
arrays
Liang, Introduction
Java Programming, Seventh Edition,
(c) 2009 (§6.11).
Pearson Education, Inc. All
53
rights reserved. 0136012671
Introducing Arrays
Array is a data structure that represents a collection of the
same types of data.
double[] myList = new double[10];
myList
reference
Array reference
variable
Array element at
index 5
myList[0]
5.6
myList[1]
4.5
myList[2]
3.3
myList[3]
13.2
myList[4]
4
myList[5]
34.33
myList[6]
34
myList[7]
45.45
myList[8]
99.993
myList[9]
11123
Element value
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
54
Declaring Array Variables

datatype[] arrayRefVar;
Example:
double[] myList;

datatype arrayRefVar[]; // This style is
allowed, but not preferred
Example:
double myList[];
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
55
Creating Arrays
arrayRefVar = new datatype[arraySize];
Example:
myList = new double[10];
references the first element in the array.
myList[9] references the last element in the array.
myList[0]
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
56
Declaring and Creating
in One Step

datatype[] arrayRefVar = new
datatype[arraySize];
double[] myList = new double[10];

datatype arrayRefVar[] = new
datatype[arraySize];
double myList[] = new double[10];
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
57
The Length of an Array
Once an array is created, its size is fixed. It cannot be
changed. You can find its size using
arrayRefVar.length
For example,
myList.length returns 10
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
58
Default Values
When an array is created, its elements are
assigned the default value of
0 for the numeric primitive data types,
'\u0000' for char types, and
false for boolean types.
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
59
Indexed Variables
The array elements are accessed through the index. The
array indices are 0-based, i.e., it starts from 0 to
arrayRefVar.length-1. In the example in Figure 6.1,
myList holds ten double values and the indices are
from 0 to 9.
Each element in the array is represented using the
following syntax, known as an indexed variable:
arrayRefVar[index];
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
60
Using Indexed Variables
After an array is created, an indexed variable can
be used in the same way as a regular variable.
For example, the following code adds the value
in myList[0] and myList[1] to myList[2].
myList[2] = myList[0] + myList[1];
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
61
Array Initializers
 Declaring,
creating, initializing in one step:
double[] myList = {1.9, 2.9, 3.4, 3.5};
This shorthand syntax must be in one
statement.
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
62
Declaring, creating, initializing
Using the Shorthand Notation
double[] myList = {1.9, 2.9, 3.4, 3.5};
This shorthand notation is equivalent to the
following statements:
double[] myList = new double[4];
myList[0] = 1.9;
myList[1] = 2.9;
myList[2] = 3.4;
myList[3] = 3.5;
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
63
CAUTION
Using the shorthand notation, you
have to declare, create, and initialize
the array all in one statement.
Splitting it would cause a syntax
error. For example, the following is
wrong:
double[] myList;
myList = {1.9, 2.9, 3.4, 3.5};
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
64
Canonical Form of the for Loop
double sum = 0;
for (int i = 0; i < 5; i++)
{ sum += myList[i]; }
System.out.println(sum);
Make sure that you do not access illegal indices
for (int i = 1; i <= 5;
{ sum += myList[i]; }
i++) // wrong!
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
65
animation
Trace Program with Arrays
double sum = 0;
for (int i = 0; I < 5; i++
double value: myList)
{ sum += value; }
System.out.println(sum);
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
66
Enhanced for Loop (for-each loop)
JDK 1.5 introduced a new for loop that enables you to traverse the complete array
sequentially without using an index variable. For example, the following code
displays all elements in the array myList:
for (double value: myList)
System.out.println(value);
In general, the syntax is
for (elementType value: arrayRefVar) {
// Process the value
}
int sum = 0;
for (double value: myList)
{ sum += value; }
System.out.println(sum);
You still have to use an index variable if you wish to traverse the array in a
different order or change the elements in the array.
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
67
Problem: Assigning Grades

Objective: read student scores (int), get the best
score, and then assign grades based on the
following scheme:
– Grade is A if score is >= best–10;
– Grade is B if score is >= best–20;
– Grade is C if score is >= best–30;
AssignGrade
– Grade is D if score is >= best–40;
Run
– Grade is F otherwise.
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
68
Copying Arrays
Often, in a program, you need to duplicate an array or a part of an
array. In such cases you could attempt to use the assignment statement
(=), as follows:
list2 = list1;
Before the assignment
list2 = list1;
list1
After the assignment
list2 = list1;
Contents
of list1
list2
list1
Contents
of list1
list2
Contents
of list2
Garbage
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
Contents
of list2
69
Copying Arrays
Using a loop:
int[] sourceArray = {2, 3, 1, 5, 10};
int[] targetArray = new
int[sourceArray.length];
for (int i = 0; i < sourceArrays.length; i++)
targetArray[i] = sourceArray[i];
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
70
The arraycopy Utility
arraycopy(sourceArray, src_pos,
targetArray, tar_pos, length);
Example:
System.arraycopy(sourceArray, 0,
targetArray, 0, sourceArray.length);
Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All
rights reserved. 0136012671
71