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Glossary of Handy Words

Here are a few vocabulary words that might help you translate the craziness Allen says in lecture into something resembling English.

Variable:
A container that holds a value. All variables have a name, a value, and a type.
Type:
A type is a set of values. For example, the type int is the set of integers; the type double is the set of floating-point numbers. NOTE: the integers are not a subset of the floating-point numbers. The value 32.0 has type double even though it is integral (it's fraction part is zero).

Statement:
A sentence of Java code, usually ending with a semi-colon. There are several kinds of statements; so far we have seen declarations, assignments and method invocations.

Declaration:
A statement that creates a variable, specifying its name, type and sometimes its initial value. For example int x; declares a variable named x with type int; int x = 5; declares x and sets its initial value to 5.

Assignment:
A statement that changes the value of a variable. For example, x = 5; assigns the value 5 to the variable x. NOTE: Assignment is an operation that has a one-time effect on one variable. It is not a statement of equality!

Expression:
A collection of operators, variables and literals that can be evaluated. For example, x+5, Math.sqrt(y), and Math.ceil((length-9) * 3.0/2.0) + 15.0. Expressions, all by themselves, are not statements; they are like nouns without verbs.

Operator:
A symbol indicating a mathematical or logical calculation. For example + (addition), * (multiplication), % (modulus), and & (logical bitwise AND).

Literal:
A simple value, like 7, 32.6, or ``Hello!''. Like variables, literals have types (the examples above are an int, a double, and a String), but unlike variables, they do not have names and they cannot be modified. So it makes no sense to write 7 = x + 2;, because you cannot assign a new value to the literal 7.

Cast:
Sometimes you can convert an expression from one type to another by casting. For example, the assignment statement int i = (int) x + 0.5; adds 0.5 to the value of the variable x and then casts the resulting value (which is a double) to be an integer.

Class:
For now, a class is just a named collection of related methods. For example, the Math class contains all the methods that perform mathematical operations. Also, the programs we are writing consist of a single class definition that contains all the methods.

Later on, we will have another definition for a class: it is a blueprint for creating new objects.

Method:
In English, a method is a way of doing something. In Java a method is a named set of statements that specify how to do something. Methods take zero or more inputs (called arguments) and return zero or one outputs (called the return value). main is a special method -- it indicates where the execution of your program begins.

Prototype:
A way of describing a method, specifying the types of its arguments and the type of the return value. For example, double pow (double a, double b); indicates that the method named pow takes two doubles as arguments and returns a double as a return value. NOTE: the arguments you pass to this method do not have to be named a and b.

Invocation:
When you use a method, you provide arguments of the appropriate types, and then invoke the method. The method executes, does whatever it is supposed to do, and then returns a value of the promised type. For example, System.out.println (``Hello.''); invokes the println method, giving it the String literal ``Hello.'' as an argument. As another example, c = Math.cos (pi/2.0); evaluates the expression pi/2.0 and sends it as an argument to the cos method. It then assigns the return value to the variable c.

NOTE: when you invoke a method, you do not have to specify the types of the arguments or the return value.



 
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Allen B. Downey
2/7/1998