This page provides some extra notes on Java that may be helpful as you work on Lab 2. The sections are:
(top)
Some operations in Java are indicated by symbols, which we call operators; those include the arithmetic operators we have seen in class and the relational (comparison) operators.
A more general way of specifying operations is to give them names;
we refer to named operations as foo
that takes a String and an int for its
parameters, we could write a call as
Each parameter could be any expression that yields the right type of
value; so if we had a foo("something", 4)
String variable s, we
could make the call
Some method calls are used as statements, because they do something;
some are used as expressions, because they produce a value.
foo(s, 2+3*4)
System.out.println()(top)
Our programs will often need to display results or other information
for the person using the program. The simplest way to do this is to
"print" the data in Terminal window from which the program is run. In
Java, we do this by calling the built-in method named
System.out.println. This method is used as a statement;
it takes a single parameter, which should be either a
String or something that Java knows how to convert into a
String. What this will do is print the string, then
advance to the next line.
Sometimes you might want to print without advancing to the next
line. You can do that with the method System.out.print,
which is otherwise the same as println.
String operations(top)
The one operator we've seen so far for the type String
is the use of + for concatenation. A useful trick takes
advantage of the fact that if you try to concatenate a
non-String to a String, in
""+5
String into a
String, if it knows how. Conveniently, Java knows how to
convert each of its built-in types (including int) into strings.
The string type also has a lot of methods that you can use in expressions. Below are a few methods that might be especially useful in this lab, displayed as they appear in the (much-longer) Java documentation.
To explain the notation, consider the first entry, for
charAt. What it means is that if you have a
String variable named s, the call
s.charAt(0) gives you the character at position 0 in
s. The char at the beginning tells you that
the method produces a value of type char; the
int index in the parentheses tells you that there is
one parameter of type int.
| Method Summary | |
|---|---|
char |
charAt(int index)
Returns the |
boolean |
equals(String anotherString)
Compares this string to |
int |
length()
Returns the length of this string. |
String |
substring(int beginIndex)
Returns a new string that is a substring of this string. The substring begins with the character at the specified index and extends to the end of this string. Examples: "unhappy".substring(2) returns "happy" "Harbison".substring(3) returns "bison" "emptiness".substring(9) returns "" (an empty string) |
String |
substring(int beginIndex,
int endIndex)
Returns a new string that is a substring of this string. The
substring begins at the specified Examples: "hamburger".substring(4, 8) returns "urge" "smiles".substring(1, 5) returns "mile" |