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17.8: Wrapper Classes

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  • Primitive values (like ints, doubles, and chars) do not provide methods. For example, you can’t call equals on an int:

    int i = 5;
    System.out.println(i.equals(5));  // compiler error

    But for each primitive type, there is a corresponding class in the Java library, called a wrapper class. The wrapper class for char is called Character; for int it’s called Integer. Other wrapper classes include Boolean, Long, and Double. They are in the java.lang package, so you can use them without importing them.

    Each wrapper class defines constants MIN_VALUE and MAX_VALUE. For example, Integer.MIN_VALUE is -2147483648, and Integer.MAX_VALUE is 2147483647. Because these constants are available in wrapper classes, you don’t have to remember them, and you don’t have to include them in your programs.

    Wrapper classes provide methods for converting strings to other types. For example, Integer.parseInt converts a string to (you guessed it) an integer:

    String str = "12345";
    int num = Integer.parseInt(str);

    In this context, parse means something like “read and translate”.

    The other wrapper classes provide similar methods, like Double.parseDouble and Boolean.parseBoolean. They also provide toString, which returns a string representation of a value:

    int num = 12345;
    String str = Integer.toString(num);

    The result is the string "12345".