By Peter van der Linden
E-book do conceal all of the themes yet not likely intimately. It was once my direction requirement.
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Extra info for Just Java(TM) 2 (6th Edition)
You cannot directly assign a floating-point variable to an integer variable. It is reasonable, however, to convert between closely related types. That is what a cast does. 142; int i = (int) f; // a cast Some numeric conversions don't need a cast. You are allowed to directly assign from a smaller-range numeric type into a larger range—a byte to int, or int to long, or long to float—without a cast. Assigning to a more capacious type is called a widening primitive conversion, and it does not lose any information about the overall magnitude of the numeric value.
That line of code calls the fillTimes() method on the fields of the object that t points to. If t is not currently pointing at an object a run-time error occurs, so silly mistakes are caught. Reference variables always point at a valid object or contain the value null. They can never hold a pointer to random memory as their contents. In this example, the object t has to belong to a class type that has a fillTimes() method, or else the compiler will flag it as an error. You can't call one of Timestamp's methods on a Calendar object.
You'll probably want to define a whole set of allowable operations and legal values to be part of your new type. Picking up on that second point, the operations for a-type-that-you-define probably won't be as simple as the arithmetic operations on numeric primitive types. For example, say you define a composite type Timestamp. It contains three fields (hh, mm, ss). You'll want a method to update those fields with the current time. You can imagine other operations on Timestamps, too: clear, set to a known time, etc.