2.2 Console Output
Key terms: console application, statement, package statement, standard output stream, string literal, syntax, syntax error, compile-time error, whitespace, logic error, bug, runtime error
2.2.1 Output Statements
A console application interacts with the user through a text interface: input is typed at a
keyboard, and output appears in a terminal window. Listing 2.2.1 consists of a main class and a
main method containing a single statement. In Java, statements play the same role as
imperative sentences in English: they are commands, and each ends with a semicolon.
Listing 2.2.1 - Greetings.java
package chap02.sect2;
public class Greetings {
public static void main(String[] args) {
System.out.println("A star shines on the hour of our meeting.");
}
}
Output 2.2.1
A star shines on the hour of our meeting.
The first line of the listing is a package statement. As noted in the previous chapter, a package is a collection of related classes. Package statements have no effect on execution, but they tell the compiler where the file is stored within the project or library to which it belongs. For example, the package name 'javabook.chap2.sect2' in Listing 2.2.1 corresponds to a directory path 'javabook/chap2/sect2' within the project’s source directory. By convention, only lowercase letters are used, with periods as separators.
Package names in the Java API start with 'java' or 'javax'; for example, the 'java.time' package contains classes representing dates, times, instants, and durations.
Packages are not required — in principle, all source files of a project could be placed directly within its source folder — but even for small projects it is considered best practice to organize its classes into one or more packages.
The 'main' method of Listing 2.2.1 contains a single output statement. Console I/O (input and output) is a complicated activity requiring specialized knowledge of the underlying system, but the details are made simple for Java programmers because the standard library provides convenient abstractions.
System.out refers to the standard output stream object created automatically. The println
method of this object prints text to the console and moves the cursor to the next line. The argument
passed to println is a string literal: a sequence of characters enclosed in double quotation
marks.
2.2.2 Syntax and Compile-Time Errors
Because Java is case-sensitive, a program will not compile if, for example, the keyword public is
capitalized. It will also fail to compile if a semicolon is omitted at the end of a statement,
square brackets are used where curly braces are required, or any other syntactic rule is violated.
The syntax of a programming language is the set of rules governing the structure of valid code,
and violations of these rules are syntax errors. A complete formal description of Java syntax
is provided in the Java Language Specification.
If you receive a message from a friend with spelling or grammatical errors, you can usually infer what was meant. A compiler cannot. Its task is mechanical: to translate valid source code into bytecode instructions for the JVM. When the code contains a syntax error, the compiler cannot complete this translation and instead reports an error, usually with an indication of the line where the problem was found.
Syntax errors form only one category of problems the compiler can detect. Even when a program satisfies all syntactic rules, the compiler performs additional checks that go beyond surface structure. The nature of these checks will become clear as new language features are introduced in later sections. A compile-time error is any problem detected by the compiler that prevents bytecode generation, whether due to a syntax error or a violation of one of the additional rules of the language.
Spacing in a Java program is not a matter of syntax: the compiler ignores whitespace (spaces, tabs, and line breaks) between tokens. Nevertheless, programmers follow widely accepted conventions for spacing to make their code easier to read. Coding style will be discussed more fully in Section 2.4.
2.2.3 Logic and Runtime Errors
A syntax error is immediately apparent because the compiler reports it, and the problem can be corrected by rewriting the offending code according to the rules of the language. Another class of errors arises when the code is syntactically correct but does not produce the intended result. These are called logic errors, or bugs. For example, suppose the average of two numbers is calculated like this:
(16 + 64) / 2
This evaluates to 40, which is correct, but now suppose the parentheses are omitted:
16 + 64 / 2
Now following the order of operations as in ordinary arithmetic, 64/2 is evaluated first, producing 48 as the final result.
In large and complex applications, logic errors can go unnoticed for years, and in mission-critical software they can lead to catastrophic results. Rigorous testing to detect and correct logic errors is an essential part of professional software development.
A separate category of problems occurs when a program is syntactically correct and logically sound but fails during execution. These are called runtime errors. They cannot be anticipated by the compiler and are detected only during execution, often causing abrupt termination. Specific causes will be discussed later.