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10.8: Operators for Strings

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  • In general, you cannot perform mathematical operations on strings, even if the strings look like numbers. The following expressions are illegal:

    "Hello" - 1     "World" / 123     "Hello" * "World"

    The + operator works with strings, but it might not do what you expect. For strings, the + operator performs concatenation, which means joining end-to-end. So "Hello, " + "World!" yields the string "Hello, World!".

    Or if you have a variable called name that has type String, the expression "Hello, " + name appends the value of name to the hello string, which creates a personalized greeting.

    Since addition is defined for both numbers and strings, Java performs automatic conversions you may not expect:

    System.out.println(1 + 2 + "Hello"); // the output is 3Hello
    System.out.println("Hello" + 1 + 2); // the output is Hello12

    Java executes these operations from left to right. In the first line, 1 + 2 is 3, and 3 + "Hello" is "3Hello". But in the second line, "Hello" + 1 is "Hello1", and "Hello1" + 2 is "Hello12".

    When more than one operator appears in an expression, they are evaluated according to order of operations. Generally speaking, Java evaluates operators from left to right (as we saw in the previous section). But for numeric operators, Java follows mathematical conventions:

    • Multiplication and division take “precedence” over addition and subtraction, which means they happen first. So 1 + 2 * 3 yields 7, not 9, and 2 + 4 / 2 yields 4, not 3.
    • If the operators have the same precedence, they are evaluated from left to right. So in the expression minute * 100 / 60, the multiplication happens first; if the value of minute is 59, we get 5900 / 60, which yields 98. If these same operations had gone from right to left, the result would have been 59 * 1, which is incorrect.
    • Any time you want to override the order of operations (or you are not sure what it is) you can use parentheses. Expressions in parentheses are evaluated first, so (1 + 2) * 3 is 9. You can also use parentheses to make an expression easier to read, as in (minute * 100) / 60, even though it doesn’t change the result.

    Don’t work too hard to remember the order of operations, especially for other operators. If it’s not obvious by looking at the expression, use parentheses to make it clear.

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