BlackBerry OS
The Java Virtual Machine
Machine language consists of very simple instructions that can be executed directly by the CPU of a computer. Almost all programs, though, are written in high-level programming languages such as Java, Pascal, or C++. A program written in a high-level language cannot be run directly on any computer. First, it has to be translated into machine language. This translation can be done by a program called a compiler. A compiler takes a high-level-language program and translates it into an executable machine-language program. Once the translation is done, the machine-language program can be run any number of times, but of course it can only be run on one type of computer (since each type of computer has its own individual machine language). If the ...
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Why, you might wonder, use the intermediate Java bytecode at all? Why not just distribute the original Java program and let each person compile it into the machine language of whatever computer they want to run it on? There are many reasons. First of all, a compiler has to understand Java, a complex high-level language. The compiler is itself a complex program. A Java bytecode interpreter, on the other hand, is a fairly small, simple program. This makes it easy to write a bytecode interpreter for a new type of computer; once that is done, that computer can run any compiled Java program. It would be much harder to write a Java compiler for the same computer.
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Furthermore, many Java programs are meant to be downloaded over a network....
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Furthermore, many Java programs are meant to be downloaded over a network....
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Java Keywords
The 50 Java keywords have specific meanings within the language, so you can’t use the keywords in this table for anything other than their function within Java. And, you can't make up new meanings for the words false, null, and true, either. But for technical reasons, those three words aren't called keywords. Whatever!
Note: The boldface words in the “What It Does” column are other keywords.
Keyword What It Does
abstract Indicates that the details of a class, a method, or an interface are given elsewhere in the code.
assert Tests the truth of a condition that the programmer believes is true.
boolean Indicates that a value is either true or false, in the Java sense.
break Jumps out of a loop or switc...
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Java Keywords
The 50 Java keywords have specific meanings within the language, so you can’t use the keywords in this table for anything other than their function within Java. And, you can't make up new meanings for the words false, null, and true, either. But for technical reasons, those three words aren't called keywords. Whatever!
Note: The boldface words in the “What It Does” column are other keywords.
Keyword What It Does
abstract Indicates that the details of a class, a method, or an interface are given elsewhere in the code.
assert Tests the truth of a condition that the programmer believes is true.
boolean Indicates that a value is either true or false, in the Java sense.
break Jumps out of a loop or switc...
(continues)
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