Summary: A First Look at Java

In this lesson, we will summarize the main ideas presented in this chapter.

  • An identifier is a word composed of letters, digits, the underscore (_), and/or the dollar sign ($). It cannot begin with a digit. An uppercase letter is distinct from its lowercase counterpart.
  • A reserved word is an identifier that has a special meaning in Java.
  • We use a Java statement of the form System.out.println(. . .) to display text. The text is placed between the parentheses and enclosed in double quotes. Such text is called a string.
  • When we use println, text is displayed and then an advance is made to the next line. If we use print instead of println, no advance to the next line occurs. A subsequent print or println places text on the same line.
  • Comments are remarks that we place in a program to describe its purpose and methodology, or to clarify aspects of its logic. We can begin a comment with two slashes or enclose it between /* and */. A third form that enables a program called javadoc to create HTML documentation, is to enclose the comment between /** and */.
  • Java’s data types are organized into two categories: primitive types and reference types. For now, we will focus on the primitive types int, double, char, and boolean, although other primitive types exist.
  • A variable represents a piece of data stored in memory. An identifier names the variable.
  • Before the first use of a variable, we declare its data type, but we do so only once. We then give it a value either by using an assignment statement or by reading data from the keyboard.
  • When a variable appears within a println or print statement, its value is converted to a string that is displayed.
  • The + operator joins, or concatenates, one string with another.
  • An unnamed constant, or literal, has a value that is self-defined and does not change. A named constant, like a variable, has a declared data type and is assigned a value. However, it is tagged with final to ensure that its value does not change.
  • We can read a value typed at the keyboard into a variable. The class Scanner provides the methods nextInt and nextDouble to read integers and real numbers.

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