Copyright © 1998-2009 Curt Hill Simple I/O Input and Output using the System and Scanner Objects.

Copyright © 1998-2009 Curt Hill

Simple I/OInput and Output using

the System and Scanner Objects


System Object

• The system object is available to every Java program

• No import statement is needed• It provides general and basic

services• Output is easiest so will be

considered first


I/O and System• System contains two objects that

give access to the console:– in– out

• The in object reads things from keyboard

• The out object writes things to screen


Simple Output• System.out.print(x);

– Takes any one standard type and writes it out– One argument only– Leaves the line open for further writing

• System.out.println(x);– Same as above but writes a newline afterward

• These methods are overloaded– There is one for each primitive type


Overloaded Methods• Many languages allow only one

function or method of a given name– FORTRAN, Pascal, VB

• Java and C++ allow overloaded function names

• The compiler defines the function to call based on the signature


Method Signature• The signature is the name and the

parameter types• Thus there may be two functions

or methods that have next as a name provided they have different parameter types– next with an int parameter – next with a double parameter


Output Example• Suppose:

int a,b;double d,e;

• More code … and thenSystem.out.print(a);System.out.print(“ “);System.out.println(d);– This would produce an output line

similar to:3 7.2depending on the values of a and d


Simple Input• Input is substantially more

complicated– Mostly because of error recovery

• The only thing that is real easy to read is a string– No possible errors

• Converting from a string to another type is more work


Input Errors• Reading a value to be placed into

integer variable is fraught with danger• How may the string “abcde” be

interpreted as an integer?• Many other types have the same

problem• Standard input requires error catching

and it is too early to discuss this now• Since Java has the extensibility of C++

there are alternatives

Scanner• A class that makes console input

easier• It makes I/O easier by doing most of

the error handling itself without showing any of it to the user

• Obtaining it will be discussed in a later screen

• It may be used to make System.in easier to use

• We will also see how objects workCopyright © 1998-2009 Curt Hill

What is needed?• An import statement

– This is how libraries are accessed• A declaration

– This declares a handle• An initialization• Usage by accessing methods of

the Scanner class


Import• The System class is available to

every Java program– It is very widely used

• There are many others that need an import– The import statement will be covered

in another presentation• After the package statement place

this:import java.util.Scanner;


Declaration• Like any declaration in Java the

type is given first and then the variable name:Scanner input;

• The convention is that class names start with capital letter and variables with a lower case

• The variable name input may be any name that you want– Scanner is the predefined class name


Initialization• Primitive types (int, float) are

initialized somewhat differently than object types like Scanner or String

• An object usually requires a new operator

• A typical initialization is:input = new Scanner(System.in);

• This could be combined in the declaration


Form of new• The new operator allocates and

initializes a new object• The form is:new Classname(parms)

• This is usually in an assignment• The Classname is actually a

constructor call



Input• The Scanner class has a number of

input routines that have the following kind of names:nextType()

• Type is the capitalized name of the type

• They always return the type they specify and take no parameters


Members for input• .nextInt() returns an int• .nextDouble() returns a double• Also .nextShort, .nextLong, .nextFl

oat and .nextByte • .next returns a string • Scanner has many more features

and is more complicated than needs to be considered here


A Simple Example Program import java.util.Scanner;

. . . int a; float f; double d; System.out.println ("Enter 3 values "); Scanner inp = new Scanner(System.in); a = inp.nextInt(); f = inp.nextFloat(); d = inp.nextDouble(); System.out.print(a); System.out.print(" "); System.out.print(f); System.out.print(" "); System.out.println(d);


Considering the Above• The data that is read is separated

by blanks• The results produced by the reads

could be used in other ways as well:– item = inp.nextInt() + inp.nextInt();

• Consider usage in simple program



Errors• What happens if the data on the

input line is not of an acceptable form– Not an integer for a nextInt

• An error with a traceback occurs



Flexibility


• The Scanner class is flexible in parsing input

• The items may be on one line or several

• Here is the same program with the three values placed on separate lines:


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Copyright © 1998-2009 Curt Hill Simple I/O Input and Output using the System and Scanner Objects.

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