Character Literals in Java

Java programs contain many values written directly in the source code. These written values are called literals. Two of the most common are the word true and the text true inside double quotes. They look almost identical on the page, yet Java treats them as different kinds of things. Before reading further, try the question below and commit to an answer.

A useful habit is to read a literal from left to right and notice three things: whether it starts with a quotation mark, which kind of quotation mark it uses, and whether any special characters appear inside it. A bare word with no quotes points to a boolean literal. A piece of text wrapped in double quotes points to a String literal. Text wrapped in single quotes belongs to a different category, the char literal. Each of these checks is a clue about what Java will do with the value.

Because true and false are written without quotes, Java recognizes them as boolean values rather than as text. The quotes are what turn a word into text, so the same letters inside double quotes no longer mean the boolean value. A common first guess is that true and false are simply ordinary words that Java somehow figures out. That guess leads to mistakes, because the spelling and punctuation of the literal are what carry its type.

The quotation marks are part of Java's notation, but they do not become characters in the String. A String literal such as "Hello" holds five characters, H, e, l, l, and o, and nothing else. The empty String is written as two double quotes with nothing between them. It contains zero characters, which is different from a String containing a single space.

Single quotes and double quotes indicate different literal categories. A letter in single quotes, such as 'a', is a char literal. The same letter in double quotes, such as "a", is a String literal. The letters look the same, but the quotation marks tell Java which category the literal belongs to. Mixing them up is one of the most common sources of surprising results.

Some characters cannot be placed directly inside a String literal. A double quote is the clearest case. If a double quote appeared unchanged in the middle of a String literal, Java would read it as the closing quote and the literal would end early. The backslash solves this. It introduces a special representation, so that the character following it is treated as part of the String rather than as notation. The same mechanism covers the other special characters.

The source example combines several of these ideas in one literal. The text is ordinary words, then escaped double quotes surround the question, and a linefeed notation appears at the end. Read left to right, the literal is three parts: ordinary text, escaped quotes around the question, and a trailing linefeed. The backslashes are instructions in the notation, and they allow the special characters to appear in the resulting String.

Consider a literal with three different escape notations and one doubled backslash. Each notation is read as a unit, and the question is how many characters the String finally holds. Work through it before checking the result.

When a literal produces an unexpected value, the cause usually falls into one of three groups: the literal's type, its quotation marks, or its special-character notation. The flowchart below shows a sequence of checks that narrows down which one is responsible. Start from the unexpected result and move through the questions in order.

The flowchart reflects the source's advice to inspect a literal from left to right. A boolean problem usually diverges at the spelling or quotation marks. A String problem often diverges at the opening or closing quote, at an accidental single quote, or at a special character written without backslash notation.